Giovanni Aldini , Galvani's nephew, continued his uncle's work after Luigi Galvani died in 1798. [ 12 ]  In 1803, Aldini performed a famous public demonstration of the electro-stimulation technique of deceased limbs on the corpse of an executed criminal  George Foster  at  Newgate  in  London . [ 13 ] [ 14 ]  The Newgate Calendar describes what happened when the galvanic process was used on the body:


On the first application of the process to the face, the jaws of the deceased criminal began to quiver, and the adjoining muscles were horribly contorted, and one eye was actually opened. In the subsequent part of the process the right hand was raised and clenched, and the legs and thighs were set in motion. [ 15 ]


Galvani has been called the father of  electrophysiology . The debate between Galvani and Volta "would result in the creation of electrophysiology, electromagnetism, electrochemistry and the electric battery." [ 16 ]


Mary Shelley's   Frankenstein , wherein a man stitches together a human body from corpses and brings it to life, was inspired in part by the theory and demonstrations of Galvanism which may have been conducted by  James Lind . [ 17 ] [ 18 ]  Although the Creature was described in later works as a composite of whole body parts grafted together from cadavers and reanimated by the use of electricity, this description is not consistent with Shelley's work; [ 19 ]  both the use of electricity and the cobbled-together image of Frankenstein's monster were more the result of James Whale's popular  1931 film adaptation of the story .


Galvanism influenced metaphysical thought in the domain of  abiogenesis , the underlying process of the generation of living forms. In 1836,  Andrew Crosse  recorded what he referred to as "the perfect insect, standing erect on a few bristles which formed its tail," as having appeared during an experiment wherein he used electricity to produce mineral crystals. While Crosse himself never claimed to have generated the insects, even in private, the scientific world at the time viewed the connection between life and electricity to be sufficiently clear that he received threats against his life for this "blasphemy." [ 20 ]


Giovanni Aldini is claimed to have applied Galvanic principles (application of electricity to biological organisms) in successfully alleviating the symptoms of "several cases of insanity", and with "complete success". [ 21 ]  Today,  electroconvulsive therapy  is used as a treatment option for severely depressed pregnant mothers [ 22 ]  (as it is the least harmful for the developing fetus) and people suffering treatment-resistant  major depressive disorder . It is found to be effective for half of those who receive treatment while the other half may relapse within 12 months. [ 23 ]


The modern application of electricity to the human body for medical diagnostics and treatments is practiced under the term  electrophysiology . This includes the monitoring of the electric activity of the heart, muscles, and even the brain, respectively termed  electrocardiography ,  electromyography , and  electrocorticography .


Halo-gravity traction  (HGT) is a type of  traction  device utilized to treat spinal deformities such as  scoliosis , [ 1 ] [ 2 ]   congenital  spine deformities,  cervical instability ,  basilar invagination , and  kyphosis . [ 3 ]  It is used prior to surgical treatment to reduce the difficulty of the following surgery and the need for a more dangerous surgery. [ 4 ] [ 5 ] [ 6 ]  The device works by applying weight to the spine in order to stretch and straighten it. Patients are capable of remaining somewhat active using a  wheelchair  or a walker whilst undergoing treatment. Most of the research suggests that HGT is a safe treatment, and it can even improve patients'  nutrition  or  respiratory functioning . However, some patients may experience side effects such as  headaches  or neurological complications.  The halo device itself was invented in the 1960s by doctors working at the  Rancho Los Amigos hospital . [ 7 ] [ 8 ] [ 9 ]  Their work was published in a paper entitled "The Halo: A Spinal Skeletal Traction Fixation Device." [ 10 ]  The clinician  Pierre Stagnara  utilized the device to develop Halo-Gravity traction. [ 11 ] [ 12 ] [ 13 ]


Halo-gravity traction works by straightening and stretching the compressed  spine . [ 14 ] [ 15 ]  It relies on the  viscoelastic  properties of  vertebrae . This means vertebrae can stretch over time. Doctors will apply weight to the spine, and gradually increase it over time, slowly straightening and stretching it. [ 16 ]  Patients undergoing the procedure will typically spend the entire course of the treatment, which is usually three to eight weeks, in a  hospital . Usually, halo-gravity traction is the first step in the treatment plan for a child with severe spinal deformity. Following the procedure, it is common for a surgical operation such as  spinal fusion surgery  to be performed afterward to permanently mend the issue. [ 14 ] [ 17 ]  It is utilized before the operation to reduce the need for a more dangerous surgery and to reduce the risk of damage to the soft  tissues  or  nerves  that surround the spine during the surgery. [ 15 ] [ 18 ] [ 19 ]  In addition, HGT has also been found to reduce the risk of complications during the following surgery. [ 20 ] [ 21 ] [ 22 ]


To perform halo-traction therapy a surgeon will use six to ten small pins to attach a "halo" made of a metal ring to the patient's skull. [ 23 ] [ 24 ] [ 25 ]  Doctors will typically leave one to two centimeters of distance between the halo and the patient's head. It is common for older patients to be given eight pins while younger patients are given 10. Prior to pin placement, some patients may undergo hair removal. It is not required for successful treatment, but it can help to reduce the risk of pin infection from hair getting caught in a pin or  scalp necrosis . [ 7 ]  The pins will be placed into the  forehead   bones  to prevent the head from moving. Pin placement is determined using a  CT scan . The chosen area will be cleaned with  betadine  swabs. Usually, two to three pins are placed in the  frontal  and  occipital  areas. Pins placed on the occipital area will have to be placed one centimeter above and behind the  auricle  of the ear. Pins placed on the  anterior  of the head will likely be placed one centimeter to above the sides of the eyebrow to avert potential damage to the  supraorbital  and the  supratrochlear nerves , and potential  muscle  damage.  Parietal  placements are generally avoided as the skull around this area is generally softer, which risks the pins puncturing the  temporal artery . Typically, the pins will tightened to a  torque  equivalent to the age of the child using a  torque wrench . Adults can withstand tighter torques than children can. [ 26 ]  Whilst this operation is being performed the child will be given general  anesthesia . [ 7 ]  In  infant  children, significantly less torque is required to tighten the pins. This allows for the pins to be placed in more areas than they could be placed in older patients. [ 27 ]


Afterward, the halo will be attached to a  pulley system  which is attached to the patient's bed,  walker , or  wheelchair . [ 28 ]  Spring-type pulleys are typically used as they allow for the patient to self-regulate the weight applied to the pulley, which improves the safety of the device. [ 29 ] : 390   Spring-based HGT devices are also cheaper and easier to build than other methods of construction. [ 30 ]  It is common for patients to begin the procedure with 5-10 pounds of weight on the pulley system. [ 29 ] : 387   Over the next few weeks, clinicians will add weight to the pulley, which will slowly straighten the patient's spine over time. Eventually, a weight greater than 50% of the patient's body weight may be achieved. [ 23 ] [ 31 ]  Doctors will monitor the movements and strength and will take  x-rays  of the patient to track their progress. They will adjust the amount of weight on the pulley system based on the results. All patients will undergo  cranial nerve  testing during the procedure. [ 23 ]  After the spine has reached its optimal position, spinal fusion surgery will be performed on the patient. [ 15 ]


Whilst undergoing the procedure, patients are encouraged to remain as active as possible. Activities such as low-impact play, walking, or standing can all increase the benefits of halo-traction therapy. However, patients are limited to leaving the traction for only a short time span. They can leave for activities such as repositioning, changing clothes, daily medical care,  showering , or using the  toilet .  Baby shampoo  is required to be used for bathing purposes as other shampoos could contain chemicals that react negatively with the metal halo. Patients will be required to utilize a special bed for  sleeping  whilst in the traction. After ending treatment the patient is required to avoid strenuous activities for a few months as their spine and muscles will still need to recover. Some patients may wear an orthopedic vest or a  halo vest . [ 15 ]


Halo-gravity traction has been found to be almost completely safe. [ 29 ] : 385  [ 32 ]  Patients who have undergone the procedure report that they have a greater ability to stand upright, an increased appetite, and an improved  body image . [ 33 ]  It can improve  respiratory  functioning by relieving pressure on the lungs caused by a deformed spine. [ 13 ] [ 34 ] [ 35 ]  Patients often  gain weight  and have improved  nutrition  following HGT. [ 36 ]  This may occur since HGT can correct issues associated with spinal deformities, such as  exercise ,  comorbid   metabolic disorders , and  gastrointestinal  malformations. These issues are associated with malnutrition and low weight, and HGT can lead to weight gain by correcting them. [ 37 ]  When in combination with surgical release, HGT may improve the  flexibility  of the spine and lead to more spinal correction. [ 38 ] [ 39 ]


Patients may experience pain from the pins, which is usually caused by the loosening of the pins. This can be remedied by tightening them. Up to 20% of patients may experience  infections  at the site where the pins were applied. These infections are typically treated with  antibiotics . One rare, but serious complication of the procedure can be the development of intra-cranial  abscesses  due to  septic  contamination of the pin site. [ 13 ]  Some patients experience  headaches  around the area where the pins were applied for a short while after the halo is attached. It is common for patients to recover from this pain in less than 24 hours. [ 29 ] : 384   Halo therapy will leave small  lesions  in the skin when the pins are first removed. Typically, they will turn into  scabs  after a few days. Patients who have undergone the procedure will also have small  scars  on their foreheads. These scars will typically fade over time. [ 15 ]


If the traction that is applied is greater than the tolerable amount, the patient may feel  cervical pain ,  cranial nerve lesions ,  nausea ,  vertigo , [ 40 ]  or  dizziness . These side effects are treated by lowering the level of weight applied. [ 41 ]  Some patients may suffer from  motor paresis  after the application of the device. Typically it is present in patients with preexisting spinal cord abnormalities. [ 29 ] : 389   Generally, HGT does not cause neurological side effects due to the slow progression of traction. The spine adjusts slowly over time, and as a result, consequences are generally limited. Children are less likely than adolescents or adults to experience neurological side effects, due to the softness and flexibility of their spine, as well as their low weight. [ 42 ]


Erb's palsy  has been identified as a rare neurological side effect of HGT. [ 43 ]  One 2006 study published in the journal "Studies in Health Technology and Informatics" found that in extremely rare cases HGT could induce  Erb's palsy , ulnar nerve paralysis, and  median nerve palsy  in cases. In the seven cases identified by the study, all patients had fully recovered within a few months of treatment. The likelihood of developing Erb's palsy due to HGT is associated with the weight of the traction. [ 44 ]  Another study published in the  Journal of Spinal Disorders & Techniques  found that patients may experience Erb's palsy or  sensory loss  during or after treatment. However, none of the patients who had experienced these side effects reported in this study had permanent neurological loss. [ 45 ]


One 2016 clinical study published in  BioMed Research International  found that HGT resulted in reduced bone density among patients with kyphoscoliosis. [ 46 ]  However, little other research has investigated this potential side effect or found any evidence to support this claim. [ 37 ]


Patients with bone conditions such as  fibrous dysplasia  or  osteogenesis imperfecta  may be unsuitable for treatment if the pins are not capable of safely being applied to the bone. [ 29 ] : 385    Osteoporosis  is considered a contraindication that sometimes may prevent treatment,  however, doctors may avert complications by utilizing more pins with a lower torque. [ 40 ]   Absolute contraindications  for halo-gravity traction include the presence of a  stenotic segment , an  intradural or extradural lesion , lesions in the skull by the sites of pin application, [ 12 ]  any lesion or  tumor  by the spine cord, [ 23 ]  severe skull deformity, and  spine instability . [ 12 ]


Most of the research conducted on HGT found that it is mostly a safe, [ 47 ]  reliable, [ 47 ]  and effective treatment. [ 48 ] [ 49 ] [ 50 ]  The average correction rate of HGT has been shown to be 19.4% for  sagittal curves  and 24.1%  coronal  curvature. [ 12 ]  One 2013 study on 33 patients published in the journal  Spine Deformity  found an average correction rate of 33% for coronal curves and 35% for sagittal. [ 51 ]  According to a  cohort study  conducted on 75 subjects investigating the efficacy of Halo traction therapy found an improvement rate of 31% to 66% for the spine. They found a  coronal  curvature improvement of 19.6% for adolescents, and 12% for adults. Kyphosis had improved at a rate of 23.9% for adolescents. Afterward, spinal surgery performed on people who had undergone the procedure had a greater than 50% chance of success. [ 52 ]


One study conducted on 20 patients with either scoliosis, kyphosis, or kyphoscoliosis found that the most improvement occurred within the first 3 weeks of treatment. According to this study, the spinal curve had improved by 63.7% during the first two weeks, which decreased to 24.3% at 3 weeks, and to 15.9% at 4 weeks. [ 38 ]  Other studies have found similar results. One study conducted on 21 patients found that 45% of improvement occurred within the first 3 weeks. [ 53 ]  Another study on 24 patients found that a mean improvement of 82% occurred during the first three weeks. [ 54 ]


Much of the research utilized as evidence of the efficacy of HGT has been criticized for a lack of a  control group  and a small  sample size . [ 55 ]  Some research has suggested that HGT leads to  statistically insignificant  improvement. [ 56 ]   Paul Sponseller, an  Orthopedic surgeon  at  Johns Hopkins University , claims in his study "The use of traction in the treatment of severe spinal deformity" that his research found "no statistically significant difference in main coronal curve correction (62% vs. 59%), operative time, blood loss, and total complication rate (27% vs. 52%)." However, his data did showcase that people who had not undergone HGT required  surgical resection  30% more often. [ 55 ]  In a study on 25 patients with severe spinal deformities who had been treated with spinal surgery, a mean correction of 44 degrees was found in patients who had not undergone HGT prior to the operation, and a mean correction of 52 was found in patients who had been treated with HGT. The researchers concluded that this difference was not statistically significant, and therefore HGT should not be used as the general treatment for these issues, and should be reserved for specific cases. [ 57 ]


Some research suggests that HGT may be less effective than other forms of traction, such as Halo-femoral traction or Halo-pelvic traction. [ 58 ] [ 59 ]  HGT also has been found to require lengthy hospital stays, which many patients dislike about the treatment. [ 60 ]  In the study by Paul Sponsellor, he found that patients who had undergone HGT spent almost twice the amount of time hospitalized as those who had not received the treatment. [ 55 ]  HGT is significantly safer than other forms of traction. It is less likely to produce significant complications such as  blood loss , [ 59 ] [ 61 ]  neurological side effects, and spine stiffness or degeneration. [ 62 ]  HGT also allows patients to remain social and active, whilst other forms of traction severely restrict movement. [ 63 ]  These reasons have led to HGT becoming the standard preoperative treatment for patients with severe spinal deformities. [ 64 ]


Henneman’s size principle  describes relationships between properties of  motor neurons  and the  muscle fibers  they innervate and thus control, which together are called  motor units . Motor neurons with large cell bodies tend to innervate fast-twitch, high-force, less fatigue-resistant  muscle fibers , whereas motor neurons with small cell bodies tend to innervate  slow-twitch , low-force, fatigue-resistant muscle fibers. In order to contract a particular muscle, motor neurons with small cell bodies are recruited (i.e. begin to fire action potentials) before motor neurons with large cell bodies. It was proposed by  Elwood Henneman .


At the time of Henneman’s initial study of motor neuron recruitment, [ 1 ]  it was known that neurons varied greatly in size, that is in the diameter and extent of the dendritic arbor, size of the soma, and diameter of axon. However, the functional significance of neuron size was not yet known. In 1965, Henneman and colleagues published five papers describing the firing patterns of motor neurons innervating two muscles in a cat's leg, the  soleus muscle  and the  gastrocnemius muscle  (the "calf" in a cat's hindleg). [ 2 ] [ 3 ] [ 4 ] [ 5 ] [ 6 ]


The  soleus muscle  is composed of "red" muscle which was revealed to indicate that muscle fibers were fatigue-resistant but created small forces when contracting. The gastrocnemius muscle is heterogeneous, composed of both "red" and "pale" muscle, and thus containing fast-twitch high force fibers. Henneman's and colleagues took advantage of the differences between the soleus and gastrocnemius muscles to show that the neurons innervating the soleus muscle:


Together, these relationship were termed the "size principle". Decades of research elaborated on these initial finding on motor neuron properties and recruitment of motor units (neuron + muscle fibers), [ 7 ]  and the relationship between neuron excitability and its size became a central focus of neurophysiology. [ 8 ]


A neuron's size is related to its electrical excitability, and so it was hypothesized that neuron size was the causal mechanism for the recruitment order. An alternative hypothesis is that the structure of spinal circuits and inputs to motor neurons controls recruitment. Both likely contribute and reflect the astounding coordinated development of neural circuit and cellular properties in motor neurons and muscle. [ 9 ]


A relationship between force production and recruitment order is a common feature across motor systems (vertebrates examples: human, cat, zebrafish; invertebrate examples: stick insect, drosophila, crayfish). This proposed to confer a number of computational and energetic advantages. Recruitment of additional motor units increases force nonlinearly, overcoming suppressive nonlinearities in spike rates and muscle force production. Furthermore, the relative increase in force does not decrease with successive recruitment, as it would if all motor units produced similar amounts of force. Thus, much like  Weber’s law  describes the constant sensitivity to relative stimulus intensity, a recruitment hierarchy maximizes the resolution of motor unit force while also simplifying the dimensionality of the motor system. [ 10 ]


The size principle states that as more force is needed, motor units are recruited in a precise order according to the magnitude of their force output, with small units being recruited first, thus exhibiting task-appropriate recruitment. This has two very important physiological benefits. First, it minimizes the amount of fatigue an organism experiences by using fatigue-resistant muscle fibers first and only using fatigable fibers when high forces are needed. Secondly, the relative change in force produced by additional recruitment remains relatively constant. For counterexample, if all motor units produced similar force, then recruiting an additional unit might increase force by 10% when only 10 motor units are active, but produce only 1% increase when 100 are active. [ 11 ]


From the time of Henneman and his discovery of size principle, many studies have been done to see if his theory holds up to the results of multiple experiments. An experiment of the  quadriceps femoris  found that motor units are in fact recruited in an orderly manner according to the size principle. [ 12 ]  The study looked at average motor unit size and firing rate in relationships with force productions of the quadriceps femoris by using a clinical  electromyograph  (EMG). [ 12 ]  Results showed the size of motor units increased linearly with increased force production, and firing rate remained constant to 30% maximum force and then increased with greater generation of force. [ 12 ]  When viewing motor unit potential during muscle contraction on an EMG, as the force generated increases, the amplitude (strength) and frequency (firing rate) increases. [ 13 ]  The motor units are being recruited in an order from slow, low force to fast, high force.


The concept of size principle can be applied to therapeutic techniques. It was shown that the use of electrical stimulation of muscles for motor control would stimulate large, fatigable motor unit first. [ 14 ]  For many years it has been believed that the use of  electromyostimulation  (EMS) to stimulate muscle contraction creates a reversal of the general size principle recruitment order, due to the larger motor unit axons having a lower resistance to  electric current . [ 15 ]  Recently, however, the results of the studies purporting this theory have come under some minor contention. In an article titled “Recruitment Patterns in Human Skeletal Muscle During Electrical Stimulation”, Professors Chris M. Gregory and C. Scott Bickel propose instead that the muscle fiber recruitment induced by EMS is non-selective pattern that is both spatially fixed and temporally synchronous. [ 15 ]  They back this claim with physiological data, metabolic data, mechanical data, and even by re-examining the results of other studies which claimed the reverse size principle paradigm.


Despite the debate, orderly recruitment of motor units can be achieved under optical control in vivo. Thus, the use of optical control with  microbial opsins  has been shown to promote normal physiological order of recruitment. [ 14 ]


In 1986, a study comparing factors such as conduction velocity, twitch torque, twitch rise time, and half-relaxation of stimulated tibial muscle found evidence that the conduction velocity of individual muscle fibers types may be another parameter to include in the size principle. [ 16 ]  The data from the experiments showed a high degree of correlation between the four factors, which were consistent with a similar study performed several years prior. In that study, an increase in muscle fiber conduction velocity was observed when there was a higher level of voluntary muscle contraction, which agrees with the gradual recruitment of higher-force muscle types. [ 16 ]


In Wistar rats, it was found that cell size is the crucial property in determining neuronal recruitment. [ 17 ]  Motor neurons of different sizes have similar voltage thresholds. Smaller neurons have higher membrane resistance and require lower depolarizing current to reach spike threshold. The cell size contribution to recruitment in motor neurons during postnatal development is investigated in this experiment. Experiments were done on 1- to 7-day-old Wistar rats and 20- to 30-day-old Wistar rats as well. The 1- to 7-day-old Wistar rats were selected because early after birth, the rats show an increase in cell size. In 20- to 30-day-old Wistar rats, the physiological and anatomical features of oculomotor nucleus motor neurons remain unchanged. Rat oculomotor nucleus motor neurons were intracellularly labelled and tested using electrophysical properties. The size principle applies to the recruitment order in neonatal motor neurons and also in the adult oculomotor nucleus. The increase in size of motor neurons led to a decrease in input resistance with a strong linear relationship in both age groups.


Hypertrophy  is the increase in the volume of an organ or tissue due to the enlargement of its component  cells . [ 1 ]  It is distinguished from  hyperplasia , in which the cells remain approximately the same size but increase in number. [ 2 ]  Although hypertrophy and hyperplasia are two distinct processes, they frequently occur together, such as in the case of the  hormonally  induced proliferation and enlargement of the cells of the  uterus  during  pregnancy .


Eccentric hypertrophy  is a type of hypertrophy where the walls and chamber of a hollow organ undergo growth in which the overall size and volume are enlarged. It is applied especially to the  left ventricle  of heart. [ 3 ]   Sarcomeres  are added in series, as for example in  dilated cardiomyopathy  (in contrast to  hypertrophic cardiomyopathy , a type of  concentric hypertrophy , where sarcomeres are added in parallel).


The  hypothenar muscles  are a group of three muscles of the  palm  that control the motion of the  little finger .


The three muscles are: [ 1 ]


The muscles of hypothenar eminence are from medial to lateral:


The intrinsic muscles of hand can be remembered using the  mnemonic ,  "A OF A OF A"  for,  A bductor pollicis brevis,  O pponens pollicis,  F lexor pollicis brevis (the three thenar muscles),  A dductor pollicis, and the three hypothenar muscles,  O pponens digiti minimi,  F lexor digiti minimi brevis,  A bductor digiti minimi. [ 2 ]


"Hypothenar atrophy" is associated with the lesion of the  ulnar nerve , which supplies the three hypothenar muscles.


Hypothenar hammer syndrome  is a vascular occlusion of this region. [ 3 ]


Hysterical strength  refers to a display of extreme  physical strength  by  humans , beyond what is believed to be within their capacity, usually occurring when people are in — or perceive themselves, or others, to be in —  life-or-death situations . [ 1 ] [ 2 ]  It was also reported to be present during situations of  altered states of consciousness , such as  trance  and alleged  possession . Its description is mostly based on  anecdotal evidence .


The name refers to  hysteria , a  nosological  category that included bouts of  superhuman strength  as one of the possible  symptoms , but in  Europe  this had also been an attribution in previous cases of alleged  demonic possession . [ 3 ] [ 4 ] [ 5 ]   Charcot  imputed to the phase of hysterical attacks called  clownism  the presence of strength and agility not consistent with the age and sex of the person, which before in the Catholic ritual of  exorcism  was attributed to demonic force. Thus, the cause of the phenomenon began at that time to be addressed by the investigation of  insanity . [ 5 ]  During that period in the 19th century, the term  hysterical strength  could also be found in the intersection of such fields, scientific and religious, for instance appearing in a statement by a physician for the  Society for Psychical Research . [ 6 ]


It was also described in reports of  trance  or  possession  in several other cultures, as for example in the  New Testament  ( Mark 5 :4) or in  shamanic  practices. [ 7 ] [ 8 ]


Unexpected strength is claimed to occur during  excited delirium . [ 9 ] [ 10 ]


The most common  anecdotal  examples based on  hearsay  are of parents lifting vehicles to rescue their children, and when people are in life-and-death situations. Periods of increased strength are short-lived, usually no longer than a few minutes, and might lead to muscle injuries and exhaustion later. It is not known if there are any reliable examples of this  phenomenon . [ citation needed ]


Early experiments showed that  adrenaline  increases twitch, but not  tetanic force  and  rate of force development  in muscles. [ 38 ]


One proposed explanation is  Tim Noakes ' " central governor " theory, which states that higher instances in the  central nervous system  dynamically and subconsciously control the number of active  motor units  in the muscle. Normally, in order to guarantee  homeostasis , the entire motor neural capacity is not activated and, therefore, the total capacity of the muscle during performances outside of an emergency situation remains inaccessible: this would lead to exhaustion of energy resources and even physical injuries. However, in life-threatening situations, it is adaptive for the central governor limits to be removed or modified. [ 1 ]  People in high load  weightlifting  training are able to activate more motor units, which ensures more strength and efficiency in muscle contraction, even though they had the same amount of  muscle mass  compared to people in low load training. [ 39 ]


Exercise physiologist Robert Girandola has pointed out that most cars have a 60/40  weight distribution , as the engine block puts the center of mass slightly towards the front of the car. In most instances, the individual is lifting one or two wheels of the car from the back. Therefore, they are only actually lifting a small fraction of the vehicle's weight. While the fight or flight response allows for increased lifting capacity, it would be hundreds of pounds rather than thousands. [ 40 ] [ 41 ]


In vitro  muscle testing  is a method used to characterize properties of living  muscle  tissue after removing it from an organism, which allows more extensive and precise quantification of its properties than  in vivo  testing.  In vitro  muscle testing has provided the bulk of scientific knowledge of muscle structure and physiology, and how both relate to organismal performance. Stem cell research relies on  in vitro  muscle testing to establish sole muscle cell function and its individual behavior apart from muscle cells in the presence of nonmuscle cells seen in  in vitro  studies. [ 1 ]


Once an appropriate animal has been selected—whether for a specific locomotor function (i.e.  frogs  for jumping); or a specific animal strain, to answer a research question—a specific muscle is identified based on its  in vivo  function and fibre type distribution. Following ethical approval, and if necessary, government approval, the animal is humanely euthanised. Humane methods differ by country, with the most appropriate based on ethical approval and researcher skill level. A number of further criteria should be followed to ensure the animal is completely dead without the possibility of recovery, which includes cessation of blood flow via the removal of the heart from the circulatory system and/or complete destruction of the brain and spinal column. Following this, common measures of animal morphology are usually rapidly obtained, such as animal length, body mass, and other biomechanical markers that may be of importance. The animal is then prepared for harvesting of the target muscle. In isolated muscles, these tend to be muscles of the hind limbs, such as the  soleus  or EDL of mammals, or the  plantaris  or iliotibialis of amphibians. Other muscles that have been examined  in vitro  include the diaphragm and the  papillary muscle .


For the successful isolation of skeletal muscles, specific conditions are required. The tissue should be isolated in frequently changed, chilled  Ringer's solution  or  Krebs-Henseleit solution  to ensure metabolic conditions are slowed down, hence the need for chilled dissecting medium, and to prevent the tissue from dying due to lack of substrates within the medium, hence the requirement for the solutions to be changed frequently. The dissecting solution should be continually oxygenated with the appropriate concentration of oxygen and carbon dioxide for the tissue that is being prepared. Typically, non-mammalian tissues are prepared in a gaseous solution bubbled through with 98% oxygen, 2% carbon dioxide whilst mammalian tissues in a solution bubbled through with 95% oxygen, 5% carbon dioxide. A microscope with an appropriate magnification level is required due to the dexterity required for isolation of muscles. An external, fibre optic light source is also beneficial to provide sufficient light without the emission of heat.


There is no correct approach for the preparation of muscles for testing, as long as the muscle is not damaged during preparation, the muscle-tendon unit is intact and there is something that can be used to anchor the muscle within the testing rig. Pieces of bone can be left at the proximal and/or distal end of skeletal muscles to allow for anchoring. In addition, silk sutures or aluminium T-foil clips can be used to wrap around the tendon of the muscle to provide both support at the tendon and to be used for anchoring in the mechanics rig.


In vitro  muscle testing typically requires a dual-mode servomotor, which can both control and detect changes in force and length. Should a dual-mode system be unavailable, then an independent force transducer and motor arm can be used. One end of the sample tissue is anchored in place, via a needle if sutured or crocodile clip if prepared with aluminium T-foil clips, while the other end is attached to the servomotor. The entire muscle is bathed in  Ringer's solution  or  Krebs-Henseleit solution  with oxygen bubbling through in order to keep the tissue alive and metabolically active. The solution is heated, usually via an external heater/cooler water bath, to an appropriate test temperature for the muscle that is being tested. Muscles are stimulated to contract by applying electric current to either the nerve which innervates the muscle or via platinum electrodes placed in the circulating solution to evoke a response of the entire muscle. The servomotor detects changes in force and/or length due to  muscle contraction . Stimulation level is often set to the level which ensures maximal  motor unit  recruitment. The servomotor can be programmed to maintain a given force while allowing the muscle to change length, vice versa, or the muscle may be subject to more complex testing, such as in  work loops . When  pennate muscles  are used,  sonomicrometry  is often used to accurately determine fiber length during the test.


In vitro  muscle testing can be done on any scale of muscle organization - entire groups of  muscles  (provided they share a common insertion or origin, as in the human quadriceps), a single muscle, a "bundle" of  muscle fibers , a single muscle fiber, a single myofibril, a single  sarcomere , a  cardiomyocyte  or even a half-sarcomere.  Muscle fibers may be intact, or may be "skinned", a process which removes the cell membrane, sarcoplasmic reticulum, and cytoplasm, allowing greater access to the contractile components of the sarcomere.


Several properties are commonly tested, and a given experiment will often use a subset of these properties, including twitch times, tetanic force, force-length relationship, force velocity relationship,  work loops , fatigue trials, fusion frequency, and energetic cost.


A hybrid approach between  in vitro  and  in vivo  has recently been used, called  in situ , in which the organism is put under terminal anesthesia, and  in vivo  tests are performed with the muscle still attached to the organism. This ensures the muscle is kept at the right temperature and amply supplied with nutrients and oxygen by the blood, but the procedure is more difficult and some tests may not be possible. [ 2 ]


In vitro  muscle testing is almost never used in humans, with the exception of small sections of muscle removed via biopsy or while undergoing surgery for other ailments.  Testing is generally more difficult in mammals and birds because of the high temperature and oxygen requirements of the muscle, leading to rapid cell death once muscle tissue is removed from the organism. Mammalian skeletal muscles are commonly tested at ~25°C to prolong the test protocol for as long as reasonably possible. A test temperature of ~37°C can also been used during testing of whole isolated mammalian skeletal muscles to better replicate the temperature found in  in vivo . Moreover, it is important to consider the thermal specialisation of skeletal muscles, with core muscles more susceptible to changes in mechanical performance with small temperature changes than peripheral muscles. [ 3 ]  In ectotherms (reptiles, amphibians, fish, and invertebrates), the muscle tissue can survive outside of the organism for hours or even days, depending on the temperature and organism. Many experiments are conducted at or near 0°C to prolong the usable life of the muscle.  Additionally, in fish and amphibians, it is possible to separate out a single muscle fiber while keeping it intact, but in other species, this is usually not possible.


Isolating muscle tissue  in vitro  allows individual data of muscle cell function without the presence of signaling nonmuscle cells nearby. [ 1 ]   In vitro  testing allows for exact stimulation of the muscle, providing precise data on innate tissue behavior. [ 4 ]  Isolated muscle testing limits other factors on the environment around the tissue such as substrates.  In vitro  isolated muscle testing is a beneficial procedure based on its ideal accuracy, precision, and reproducibly. [ 5 ]


Lateral to the  conjoint tendon , previously known as the inguinal aponeurotic falx, there is a ligamentous band originating from the lower margin of the  transversalis fascia [ 1 ]  and extending down in front of the  inferior epigastric artery  to the  superior ramus  of the  pubis ; it is termed the  interfoveolar ligament of Hesselbach  and sometimes contains a few muscular fibers.


It is named for  Franz Kaspar Hesselbach . [ 2 ]


This article incorporates text in the  public domain  from  page 416  of the 20th edition of   Gray's Anatomy   (1918)


This  ligament -related article is a  stub . You can help Wikipedia by  expanding it .


Intermediate fibers , also known as  fast oxidative-glycolytic fibers , are  fast twitch muscle fibers  which have been converted via endurance training. These fibers are slightly larger in diameter, have more  mitochondria  as well as a greater blood supply and more endurance than typical fast twitch fibers. Most of the body's muscles are composed of these intermediate fibers.


Intermuscular coordination  describes the coordination within different  muscles  and groups of muscles. These are used for sceletoral movement, stabilisation of joints, as well as stabilisation of body positioning. [ 1 ]


central nervous system  is controlling positioning of joints via anticipatory and correcting adaptions of posture, that work against occurring intersegmental forces. [ 2 ]


The specific role and hierarchy of certain muscles and their meaning for certain movements is further differentiated within literature. [ 3 ] [ 4 ]


Joints are stabilised by interacting muscles, so called  synergist muscle . Different synergists feature partial similar functions. Therefore, a certain movement can be formed out of different combinations and participations of muscles acting on a certain joint.


Even muscles not being in a direct connection towards a certain joint can fulfill a stabilising function for that very joint. For a clear specification of any muscles function it is necessary to measure precisely muscular function of not directly involved muscles within certain movements via  elktromyografia . [ 5 ]


Complex movement structures are coordinated within the  cerebellum  via  somatosensoric feedback  via  psychomotoric learning .


This  anatomy  article is a  stub . You can help Wikipedia by  expanding it .


Interossei  refer to muscles between certain bones.  There are many interossei in a human body. Specific interossei include:


Intrafusal muscle fibers  are  skeletal muscle fibers  that serve as specialized  sensory organs  ( proprioceptors ). [ 1 ] [ 2 ]  They detect the amount and rate of change in length of a  muscle . [ 1 ]  They constitute the  muscle spindle , [ 2 ]  and are innervated by both sensory (afferent) and motor (efferent) fibers. [ 3 ]


Intrafusal muscle fibers are not to be confused with  extrafusal muscle fibers , which contract, generating skeletal movement and are innervated by  alpha motor neurons . [ 4 ]


There are two types of intrafusal muscle fibers:  nuclear bag fibers  and  nuclear chain fibers . [ 3 ]  They bear two types of sensory ending, known as annulospiral and flower-spray endings. Both ends of these fibers contract, but the central region only stretches and does not contract.


Intrafusal muscle fibers are walled off from the rest of the muscle by an outer  connective tissue  sheath consisting of flattened  fibroblasts  and  collagen . [ 5 ]  This sheath has a spindle or "fusiform" shape, hence the name "intrafusal". [ 2 ]


They are innervated by  gamma motor neurons  and  beta motor neurons . [ 3 ]  Gamma efferents from small  multipolar neurons  from anterior gray column innervate it. These form a part of  neuromuscular spindles .


Intrafusal muscle fibers detect the amount and rate of change in muscle length. [ 1 ]  It is by the sensory information from gamma motor neurons and beta motor neurons that an individual is able to judge the position of their muscles. [ 4 ]


Intramuscular injection , often abbreviated  IM , is the  injection  of a substance into a  muscle . In  medicine , it is one of several methods for  parenteral administration  of medications. Intramuscular injection may be preferred because muscles have larger and more numerous  blood vessels  than subcutaneous tissue, leading to faster absorption than  subcutaneous  or  intradermal injections . Medication administered via intramuscular injection is not subject to the  first-pass metabolism  effect which affects oral medications.


Common sites for intramuscular injections include the  deltoid muscle  of the upper arm and the  gluteal muscle  of the buttock. In infants, the  vastus lateralis muscle  of the thigh is commonly used. The injection site must be cleaned before administering the injection, and the injection is then administered in a fast, darting motion to decrease the discomfort to the individual. The volume to be injected in the muscle is usually limited to 2–5  milliliters , depending on injection site. A site with signs of infection or muscle atrophy should not be chosen. Intramuscular injections should not be used in people with  myopathies  or those with trouble clotting.


Intramuscular injections commonly result in pain, redness, and swelling or  inflammation  around the injection site. These side effects are generally mild and last no more than a few days at most. Rarely,  nerves  or blood vessels around the injection site can be damaged, resulting in severe pain or  paralysis . If proper technique is not followed, intramuscular injections can result in localized infections such as  abscesses  and  gangrene . While historically aspiration, or pulling back on the syringe before injection, was recommended to prevent inadvertent administration into a vein, it is no longer recommended for most injection sites by some countries.


Intramuscular injection is commonly used for medication administration. Medication administered in the muscle is generally quickly absorbed in the bloodstream, and avoids the  first pass metabolism  which occurs with oral administration. [ 1 ]  The medication may not be considered 100%  bioavailable  as it must still be absorbed from the muscle, which occurs over time. [ 2 ] : 102–103   An intramuscular injection is less invasive than an intravenous injection and also generally takes less time, as the site of injection (a muscle versus a vein) is much larger. Medications administered in the muscle may also be administered as  depot injections , which provide slow, continuous release of medicine over a longer period of time. [ 3 ]  Certain substances, including  ketamine , may be injected intramuscularly for  recreational  purposes. [ 4 ]  Disadvantages of intramuscular administration include skill and technique required, pain from injection, anxiety or fear (especially in children), and difficulty in self-administration which limits its use in  outpatient medicine . [ 5 ]


Vaccines , especially  inactivated vaccines , are commonly administered via intramuscular injection. [ 6 ]  However, it has been estimated that for every vaccine injected intramuscularly, 20 injections are given to administer drugs or other therapy. [ 6 ]  This can include medications such as  antibiotics ,  immunoglobulin , and  hormones  such as testosterone and  medroxyprogesterone . [ 5 ]  In a case of severe allergic reaction, or  anaphylaxis , a person may use an  epinephrine   autoinjector  to self-administer epinephrine into the muscle. [ 7 ]


Because an intramuscular injection can be used to administer many types of medications, specific  contraindications  depend in large part on the medication being administered. [ 8 ]  Injections of medications are necessarily more invasive than other forms of administration such as by mouth or topical and require training to perform appropriately, without which complications can arise regardless of the medication being administered. For this reason, unless there are desired differences in rate of absorption, time to onset, or other pharmacokinetic parameters in the specific situation, a less invasive form of drug administration (usually by mouth) is preferred. [ 8 ]


Intramuscular injections are generally avoided in people with  low platelet count  or clotting problems, to prevent harm due to potential damage to blood vessels during the injection. They are also not recommended in people who are in  hypovolemic shock , or have myopathy or  muscle atrophy , as these conditions may alter the absorption of the medication. [ 5 ]  The damage to the muscle caused by an intramuscular injections may interfere with the accuracy of certain cardiac tests for people with suspected  myocardial infarction  and for this reason other methods of administration are preferred in such instances. [ 5 ]  In people with an active myocardial infarction, the decrease in circulation may result in slower absorption from an IM injection. [ 9 ] : 368–369   Specific sites of administration may also be contraindicated if the desired injection site has an infection, swelling, or inflammation. [ 9 ] : 368–369   Within a specific site of administration, the injection should not be given directly over irritation or redness, birthmarks or moles, or areas with scar tissue. [ 9 ] : 368–369


As an injection necessitates piercing the skin, there is a risk of infection from  bacteria  or other organisms present in the environment or on the skin before the injection. This risk is minimized by using proper  aseptic technique  in preparing the injection and sanitizing the injection site before administration. [ 9 ] : 369   Intramuscular injections may also cause an abscess or gangrene at the injection site, depending on the specific medication and amount administered. There is also a risk of nerve or vascular injury if a nerve or blood vessel is inadvertently hit during injection. If single-use or sterilized equipment is not used, there is the risk of transmission of infectious disease between users, or to a practitioner who inadvertently injures themselves with a used needle, termed a  needlestick injury . [ 5 ] [ 9 ] : 372


Injections into the deltoid site in the arm can result in unintentional damage to the  radial  and  axillary nerves . In rare cases when not performed properly, the injection may result in shoulder dysfunction. [ 10 ]  The most frequent complications of a deltoid injection include pain, redness, and inflammation around the injection site, which are almost always mild and last only a few days at most. [ 11 ]


The dorsogluteal site of injection is associated with a higher risk of skin and tissue trauma, muscle  fibrosis  or  contracture ,  hematoma , nerve  palsy ,  paralysis , and infections such as  abscesses  and  gangrene . [ 12 ]  Furthermore, injection in the gluteal muscle poses a risk for damage to the  sciatic nerve , which may cause shooting pain or a sensation of burning. Sciatic nerve damage can also affect a person's ability to move their foot on the affected side, and other parts of the body controlled by the nerve. Damage to the sciatic nerve can be prevented by using the ventrogluteal site instead, and by selecting an appropriate size and length of needle for the injection. [ 13 ]


An intramuscular injection can be administered in multiple different muscles of the body. Common sites for intramuscular injection include:  deltoid ,  dorsogluteal ,  rectus femoris ,  vastus lateralis  and  ventrogluteal  muscles. [ 12 ] [ 14 ]  Sites that are bruised, tender, red, swollen, inflamed or scarred are generally avoided. [ 15 ]  The specific medication and amount being administered will influence the decision of the specific muscle chosen for injection.


The injection site is first cleaned using an antimicrobial and allowed to dry. The injection is performed in a quick, darting motion perpendicular to the skin, at an angle between 72 and 90 degrees. The practitioner will stabilize the needle with one hand while using their other hand to depress the plunger to slowly inject the medication – a rapid injection causes more discomfort. The needle is withdrawn at the same angle inserted. Gentle pressure may be applied with gauze if bleeding occurs. [ 16 ]  Pressure or gentle massage of the muscle following injection may reduce the risk of pain. [ 17 ]


Aspirating for blood to rule out injecting into a blood vessel is not recommended by the US  CDC ,  Public Health Agency of Canada , or  Norway Institute of Public Health , as the injection sites do not contain large blood vessels and aspiration results in greater pain. [ 18 ] [ 19 ] [ 20 ]  There is no evidence that aspiration is useful to increase safety of intramuscular injections when injecting in a site other than the dorsogluteal site. [ 6 ]


Aspiration was recommended by the  Danish Health Authority  for  COVID-19 vaccines  for a time to investigate the potential rare risk of blood clotting and bleeding, but it is no longer a recommendation. [ 21 ] [ 22 ]


The Z-track method is a method of administering an IM injection that prevents the medication being tracked through the subcutaneous tissue, sealing the medication in the muscle, and minimizing irritation from the medication. Using the Z-track technique, the skin is pulled laterally, away from the injection site, before the injection; then the medication is injected, the needle is withdrawn, and the skin is released. This method can be used if the overlying tissue can be displaced. [ 23 ]


The deltoid muscle in the outer portion of the upper arm is used for injections of small volume, usually equal to or less than 1 mL. This includes most intramuscular vaccinations. [ 12 ]  It is not recommended to use the deltoid for repeated injections due to its small area, which makes it difficult to space out injections from each other. [ 12 ]  The deltoid site is located by locating the lower edge of the  acromion process , and injecting in the area which forms an upside down triangle with its base at the acromion process and its midpoint in line with the  armpit . [ 15 ]  An injection into the deltoid muscle is commonly administered using a 1-inch long needle, but may use a  5 ⁄ 8 -inch long needle for younger people or very frail elderly people. [ 11 ]


The ventrogluteal site on the hip is used for injections which require a larger volume to be administered, greater than 1 mL, and for medications which are known to be irritating, viscous, or oily. It is also used to administer  narcotic  medications,  antibiotics ,  sedatives  and  anti-emetics . [ 12 ]  The ventrogluteal site is located in a triangle formed by the  anterior superior iliac spine  and the  iliac crest , and may be located using a hand as a guide. [ 15 ]  The ventrogluteal site is less painful for injection than other sites such as the deltoid site. [ 17 ]


The  vastus lateralis  site is used for infants less than 7 months old and people who are unable to walk or who have loss of muscular tone. [ 12 ]  The site is located by dividing the front  thigh  into thirds vertically and horizontally to form nine squares; the injection is administered in the outer middle square. [ 15 ]  This site is also the usual site of administration for  epinephrine autoinjectors , which are used in the outer thigh, corresponding to the location of the vastus lateralis muscle. [ 24 ]


The dorsogluteal site of the buttock site is not routinely used due to its location near major  blood vessels  and  nerves , as well as having inconsistent depth of  adipose tissue . [ 25 ]  Many injections in this site do not penetrate deep enough under the skin to be correctly administered in the muscle. [ 12 ] [ 26 ]  While current  evidence-based practice  recommends against using this site, many healthcare providers still use this site, often due to a lack of knowledge about alternative sites for injection. [ 27 ]
This site is located by dividing the buttock into four using a cross shape, and administering the injection in the upper outer quadrant. This is the only intramuscular injection site for which aspiration is recommended of the syringe before injection, due to higher likelihood of accidental  intravenous administration  in this area. [ 12 ]  However, aspiration is not recommended by the  Centers for Disease Control and Prevention , which considers it outdated for any intramuscular injection. [ 16 ]


Some populations require a different injection site, needle length, or technique. In very young or weak elderly patients, a normal-length needle may be too long to inject properly. In these patients, a shorter needle is indicated to avoid injecting too deeply. [ 28 ]  It is also recommended to consider using the anterolateral thigh as an injection site in infants under one year old. [ 28 ]


To help infants and children cooperate with injection administration, the  Advisory Committee on Immunization Practices  in the United States recommends using distractions, giving something sweet, and rocking the baby side to side. In people who are overweight, a 1.5-inch needle may be used to ensure the injection is given below the  subcutaneous  layer of skin, while a  5 ⁄ 8 -inch needle may be used for people who weigh under 60 kilograms (130 lb). In any case, the skin does not need to be pinched up before injecting when the appropriate length needle is used. [ 29 ]


Injections into muscular tissue may have taken place as early as the year 500 AD. Beginning in the late 1800s, the procedure began to be described in more detail and techniques began to be developed by physicians. In the early days of intramuscular injections, the procedure was performed almost exclusively by physicians. [ 8 ]  After the introduction of  antibiotics  in the middle of the 20th century, nurses began preparing equipment for intramuscular injections as part of their delegated duties from physicians, and by 1961 they had "essentially taken over the procedure". [ 8 ]  Until this delegation became virtually universal, there were no uniform procedures or education for nurses in proper administration of intramuscular injections, and complications from improper injection were common. [ 8 ]


Intramuscular injections began to be used for administration of vaccines for  diphtheria  in 1923,  whooping cough  in 1926, and  tetanus  in 1927. [ 30 ]  By the 1970s, researchers and instructors began forming guidance on injection site and technique to reduce the risk of injection complications and side effects such as pain. [ 8 ]  Also in the early 1970s,  botulinum toxin  began to be injected into muscles to intentionally paralyze them for therapeutic reasons, and later for cosmetic reasons. [ 31 ]  Until the 2000s, aspiration after inserting the needle was recommended as a safety measure, to ensure the injection was being administered in a muscle and not inadvertently in a vein. However, this is no longer recommended as evidence shows no safety benefit and it lengthens the time taken for injection, which causes more pain. [ 29 ]


In animals common sites for intramuscular injection include the  quadriceps , the lumbodorsal muscles, and the triceps muscle. [ 32 ]


The  iris dilator muscle  ( pupil dilator muscle ,  pupillary dilator ,  radial muscle of iris ,  radiating fibers ), is a  smooth muscle [ 2 ]  of the  eye , running radially in the  iris  and therefore fit as a dilator. The pupillary dilator consists of a spokelike arrangement of modified contractile cells called  myoepithelial cells . These cells are stimulated by the sympathetic nervous system. [ 3 ]  When stimulated, the cells contract, widening the pupil and allowing more light to enter the eye.


The  ciliary muscle ,  pupillary sphincter muscle  and  pupillary dilator muscle  sometimes are called  intrinsic ocular muscles [ 4 ]  or  intraocular muscles . [ 5 ]


It is innervated by the sympathetic system, which acts by releasing  noradrenaline , which acts on  α1-receptors . [ 6 ]  Thus, when presented with a threatening stimulus that activates the  fight-or-flight response , this innervation contracts the muscle and dilates the  pupil , thus temporarily letting more light reach the  retina .


The dilator muscle is innervated more specifically by  postganglionic  sympathetic nerves arising from the  superior cervical ganglion  as the  sympathetic root of ciliary ganglion . From there, they travel via the  internal carotid artery  through the  carotid canal  to  foramen lacerum . They then enter the  middle cranial fossa  above foramen lacerum, travel through the  cavernous sinus  in the middle cranial fossa and then travel with the  ophthalmic artery  in the  optic canal  or on the  ophthalmic nerve  through the  superior orbital fissure . From there, they travel with the  nasociliary nerve  and then the  long ciliary nerve . They then pierce the  sclera , travel between sclera and  choroid  to reach the iris dilator muscle. They will also pass through ciliary ganglion and travel in  short ciliary nerves  to reach the iris dilator muscle.


The pupillary dilator acts to increase the size of the pupil to allow more light to enter the eye. It works in opposition to the pupillary constrictor. [ 7 ]  Pupil dilation occurs when there is insufficient light for the normal function of the eye, and during heightened sympathetic activity, for example in the "fight-or-flight reflex". [ 7 ] [ 8 ]


The English name  dilator pupillae muscle [ 9 ]  as currently used in the list of English equivalents of the  Terminologia Anatomica , the reference-work of the official anatomic nomenclature, [ 10 ]  can be considered as a corruption [ 11 ]  of the full Latin expression  musculus dilatator pupillae . [ 12 ]  The full Latin expression exhibits three words that each can be traced back to  Roman antiquity . The  Classical Latin  name  musculus  is actually a diminutive of the Classical Latin name  mus , [ 13 ]  and can be translated as  little mouse . [ 13 ]  In the medical writings of  Aulus Cornelius Celsus  we can also find this specific name to refer to a muscle instead of its literal meaning. [ 13 ]  Latin  musculus  can be explained by the fact that a muscle looks like a little mouse that moves under the skin. [ 14 ]  In the writings of Greek  philosopher   Aristotle  the Ancient Greek word for  mouse , i.e. μῦς [ 15 ]  is also used to refer to a muscle. [ 15 ]


Dilatator  in the Latin expression  musculus dilatator pupillae  is derived from the classical Latin verb  dilatare , [ 16 ]  to dilate, to spread out. [ 13 ]  Two possible explanations exist concerning the etymological derivation of this verb. The first explanation considers  dilatare  as frequentative of  differere . [ 13 ]  The Latin verb  differe  can mean,  to carry different ways ,  to spread abroad ,  to scatter , [ 13 ]  but also  to delay . [ 13 ]  The other explanation [ 14 ]  considers  dilatare  as a compound from  di-  and  latus , with the latter word meaning,  broad  or  wide , [ 13 ]  hence the German name  Erweiterer  for Latin  dilatator . [ 16 ]


The expression  dilator pupillae muscle , as used in the list of English equivalents of the  Terminologia Anatomica , is actually partly Latin, i.e.  dilator pupillae , with  pupillae  (=of the pupil [ 13 ] ), a noun in the genitive case modifying  dilator , a noun in the nominative case, and partly English, i.e.  muscle . In previous editions ( Nomina Anatomica ) this muscle was officially called the  musculus dilator pupillae , [ 17 ] [ 18 ] [ 19 ] [ 20 ] [ 21 ]  The  Nomina Anatomica  as authorized in 1895 in  Basel [ 12 ]  and in 1935 in  Jena [ 22 ] [ 23 ]  used the full Latin expression.


The  iris sphincter muscle  ( pupillary sphincter ,  pupillary constrictor ,  circular muscle of iris ,  circular fibers ) is a  muscle  in the part of the  eye  called the  iris . It encircles the  pupil  of the iris, appropriate to its function as a constrictor of the pupil.


The  ciliary muscle ,  pupillary sphincter muscle  and  pupillary dilator muscle  sometimes are called  intrinsic ocular muscles [ 2 ]  or  intraocular muscles . [ 3 ]


This structure is found in  vertebrates  and in some  cephalopods . [ citation needed ]


All the  myocytes  are of the  smooth muscle  type. [ 4 ]


Its dimensions are about 0.75 mm wide by 0.15 mm thick. [ citation needed ]


In humans, it functions to constrict the  pupil  in bright light ( pupillary light reflex ) or during  accommodation . [ citation needed ]  In lower animals, the muscle cells themselves are photosensitive causing iris action without brain input. [ 5 ]


It is controlled by  parasympathetic  postganglionic fibers releasing acetylcholine acting primarily on the  muscarinic acetylcholine receptor  (M3) of iris sphincter muscle. [ 6 ]  Preganglionic fibers originate from the  Edinger–Westphal nucleus , travel along the  oculomotor nerve  (CN III), and make nicotinic cholinergic synapses on neurons in the  ciliary ganglion . [ 7 ]  Those neurons' postganglionic parasympathetic fibers then enter the eye through the  short ciliary nerves . The short ciliary nerves then run forward and pierce the  sclera  at the back of the eye, traveling between the sclera and the  choroid  to innervate the iris sphincter muscle.


Isopotential muscle  refers to  muscle fibers  which are fewer than two times the  length constant .


In an  isotonic contraction , tension remains the same, whilst the muscle's length changes. Isotonic contractions differ from  isokinetic  contractions in that in isokinetic contractions the muscle speed remains constant.  While superficially identical, as the muscle's force changes via the  length-tension relationship  during a contraction, an isotonic contraction will keep force constant while velocity changes, but an isokinetic contraction will keep velocity constant while force changes.  A near isotonic contraction is known as Auxotonic contraction.


There are two types of isotonic contractions: (1) concentric and (2) eccentric. In a concentric contraction, the muscle tension rises to meet the resistance, then remains the same as the muscle shortens.  In eccentric, the muscle lengthens due to the resistance being greater than the force the muscle is producing. [ citation needed ]


This type is typical of most exercise. The external force on the muscle is less than the force the muscle is generating - a  shortening  contraction. The effect is not visible during the classic biceps curl, which is in fact auxotonic because the resistance ( torque  due to the weight being lifted) does not remain the same through the exercise. Tension is highest at a parallel to the floor level, and eases off above and below this point. Therefore, tension changes as well as muscle length. [ citation needed ]


There are two main features to note regarding eccentric contractions. First, the absolute tensions achieved can be very high relative to the muscle's maximum tetanic tension generating capacity (you can set down a much heavier object than you can lift [ 1 ] ). Second, the absolute tension is relatively independent of lengthening velocity.


Muscle injury and soreness are selectively associated with eccentric contraction. [ 2 ] [ 3 ] [ 4 ]  Muscle strengthening using exercises that involve eccentric contractions is lower than using concentric exercises. [ 5 ]  However because higher levels of tension are easier to attain during exercises that involve eccentric contractions it may be that, by generating higher signals for muscle strengthening,  muscle hypertrophy is better than exercises that involve concentric contractions, albeit at a higher level of resistance. [ citation needed ]


This is almost an isotonic contraction because there is some fluctuation towards the end of the contraction. For example, the heart's ventricles contract to expel blood into the pulmonary artery and aorta. As the blood flows out, the previous built-up load is decreased and hence less force is required to expel the rest of the blood. Thus the tension is reduced. [ citation needed ]


In  physiology ,  isotropic bands  (better known as  I bands ) are the lighter bands of skeletal muscle cells (a.k.a.  muscle fibers ). Isotropic bands contain only  actin -containing  thin filaments . [ 1 ]  The thin filaments are placed between 2 myosin filaments and contain only the  actin filaments  of neighboring  sarcomeres . Bisecting the I band and serving as an anchoring point for the two adjacent actin filaments is the Z disc. During muscle contraction, the I band will shorten, while an A band will maintain its width. [ 2 ]


The darker bands within skeletal muscle, known as anisotropic bands (A bands), encompass both thick and thin filaments and constitute the central region of the sarcomere, extending across the H-zone. Collectively, the A bands and the I bands create the distinctive striped appearance of skeletal muscle tissue. [ 3 ]  Tropomyosin, a protein, shields the myosin-binding sites, hindering actin from binding to myosin. It attaches to  troponin , which secures it in place. During muscle relaxation, the troponin-tropomyosin complex inhibits  myosin  heads from binding to the active sites on actin microfilaments. Troponin also possesses a calcium ion binding site. These two regulatory proteins cooperate in response to calcium levels, overseeing sarcomere contraction. During muscle contraction,  tropomyosin  shifts to expose the myosin-binding site on an actin filament, allowing the interaction between actin and myosin microfilaments to occur. The initiation of contraction involves calcium ions binding to troponin, prompting a reaction that displaces tropomyosin from the actin filament binding sites. Consequently, myosin heads can attach to these exposed sites, forming cross-bridges and initiating muscle contraction. [ 4 ]


A key component in  lateral force transmission in skeletal muscle  is the  extracellular matrix  (ECM).   Skeletal muscle  is a complex biological material that is composed of muscle fibers and an ECM consisting of the  epimysium ,  perimysium , and  endomysium .  It can be described as a  collagen   fiber-reinforced composite .  The ECM has at least three functions: (1) to provide a framework binding muscle fibers together and ensure their proper alignment, (2) to transmit the forces, either from active muscle contraction or ones passively imposed on it, and (3) providing lubricated surfaces between muscle fibers and bundles enabling the muscle to change shape. [ 1 ]   The mechanical properties of skeletal muscle depend on both the properties of muscle fibers and the ECM, and the interaction between the two.  Contractile forces are transmitted laterally within intramuscular connective tissue to the epimysium and then to the  tendon .  Due to the nature of skeletal muscle, direct measurements are not possible, but many indirect studies and analyses have shown that the ECM is an important part of force transmission during muscle contraction. [ 2 ] [ 3 ] [ 4 ] [ 5 ] [ 6 ] [ 7 ] [ 8 ]


The muscle fibers within a fascicle are surrounded by the  endomysium .  The structure of the endomysium is generally equivalent for all skeletal muscle from different muscles and species.  Many of the muscle fibers end within the muscle and do not connect to the  tendon  directly, thus necessitating a force transmission pathway via the endomysium.  The planar network of the  collagen  fibers appears to be randomly distributed at first glance, but detailed analysis has shown that the network is not truly random and that there is a preferred direction that changes with muscle length. [ 4 ]


Any linkage transmitting force from the muscle fibers to the tendon must be non-compliant (i.e. high  stiffness ) in order to be efficient.  Experimental force-length measurements showed that the passive tensile properties of relaxed muscle were mainly from the elastic resistance of the myofibrils and not the connective tissue. [ 9 ]   Also, analysis of the non-linear tensile properties of the endomysium showed that under normal physiological conditions the collagen networks were too compliant in tension to act as efficient force transmitters in tension. [ 4 ]  Therefore, the concept of shear linkage between adjacent muscle fibers in the endomysium was introduced. [ 10 ]   The figure to the right is a model consisting of two parallel muscle cells with the endomysium sandwiched between them.  Assuming the simplest linear models, the  shear stress  (F/LW) and the shear strain (ΔL/H) in the endomysium can be related to the  shear modulus  (G) by the following equation:


G
 =

 F

 Δ
 L

 H

 L
 W

 {\displaystyle G={\frac {F}{\Delta L}}{\frac {H}{LW}}}


where


If the change in length (ΔL) is approximated as the longitudinal tensile displacement, the apparent tensile modulus (E app ) becomes:


E

 a
 p
 p

 =
 G

 (

 L
 H

 )

 2

 {\displaystyle E_{app}=G\left({\frac {L}{H}}\right)^{2}}


The length of the muscle fiber (L) is orders of magnitude larger than the thickness (H), making the term (L/H) 2  very large.  Therefore, the apparent longitudinal modulus much greater than the true shear modulus.  Because the endomysium is so thin, even large shear strains would only result in minute tensile elongations.  If the apparent longitudinal modulus is represented in series with the fibers, the modulus of the composite structure (E c ) is:


1

 E

 c

 =

 1

 E

 f

 +

 1

 E

 a
 p
 p

 {\displaystyle {\frac {1}{E_{c}}}={\frac {1}{E_{f}}}+{\frac {1}{E_{app}}}}


and is dictated by the fiber modulus E f , because E app  is many orders of magnitude higher.  This shear force transmission pathway from one muscle cell to its neighbors is highly efficient and because it has low tensile stiffness, it does not restrict changes in muscle fiber length and diameter during muscle contraction. [ 11 ]


Unlike the  endomysium , the  perimysium  has large variations in quantity and organization from one muscle group to another. [ 12 ]  Muscles contain far more perimysial than endomysial connective tissue, and it has also been observed that the ratio of the dry mass of perimysium to that of endomysium ranges between 2.8-1 and 64–1. [ 13 ]   The anatomical arrangement of the connective tissue at each level of organization influences the function of the muscle.


The  collagen  fibers in the perimysium usually orient between 45 and 60 degrees to the long axis of the muscle fibers in their relaxed state. [ 1 ]   Well defined contact regions between the endomysium and perimysium were observed and coined the perimysial junctional plate (PJP). [ 14 ]   These sites were hypothesized to be a focal region for delivery of tension during muscle contraction.  To test the possibility of tensile force transmission via the perimysium, it was experimentally shown that cutting of the  aponeurosis  in a pennate muscle did not prevent tension generation further along towards the  tendon . [ 6 ]   Also, in a separate study it was clearly demonstrated that the perimysium could transmit force if tendons normally transmitting force from distinct parts of the  extensor digitorum longus muscle  were cut. [ 5 ]


Although a lot of evidence may seem to point to lateral force transmission via the perimysium in tension, the experiments were conducted at very high loads.  Under sufficient stress, the collagen fibers of the perimysium begin to orient parallel to the stress direction.  The stretching and reorientation of the perimysium makes it much stiffer and able to transmit tensile force.   Scanning electron microscope  images have shown that the perimysium has an organized crimped structure. [ 1 ]   The crimped structure of the perimysium makes it very compliant in tension under normal physiological conditions allowing the muscle to change shape, thus rendering it unfeasible for tensile force transmission. [ 11 ]   Although strong efforts have been made to demonstrate the mechanical role of the perimysium as a force transmission pathway during active contraction of the muscle, an accepted model has yet to be derived.


It can also be suggested that the perimysium could transmit force generated in fascicles to neighboring fascicles by shear, similar to the endomysium described above.  The perimysium is significantly thicker than the endomysium.  Even if the  shear modulus  of the perimysium were within an order of magnitude of the endomysium, the perimysium would still be a lot more compliant in shear than the endomysium, also making it an inefficient force transmission pathway. [ 15 ]


There are alternate theories on the role of the perimysium being strictly for distributing passive forces imposed on the muscle and that the perimysial network's main purpose is to prevent over-stretching of the muscle fascicles. [ 16 ]


This is a table of  skeletal muscles  of the  human anatomy , with muscle counts and other information.


sensory :  cervical nerves   C3  and  C4


medial head does not function at  shoulder .


medial belly :  muscular branches of ulnar nerve


lateral head : lateral sesamoid bone of  metatarsophalangeal joint , proximal phalanx of great toe


transverse head :  metatarsophalangeal joints , ligaments of lateral 3 toes


The muscles are described using  anatomical terminology . The columns are as follows:


For Origin, Insertion and Action please name a specific Rib, Thoracic vertebrae or Cervical vertebrae, by using C1-7, T1-12 or R1-12.


There does not appear to be a definitive source counting all skeletal muscles. Different sources group muscles differently, regarding physical features as different parts of a single muscle or as several muscles. There are also  vestigial muscles  that are present in some people but absent in others, such as the  palmaris longus muscle . [ 14 ] [ 15 ]  There are between 600 and 840 muscles within the typical human body, depending on how they are counted. [ 16 ] [ 17 ] [ 18 ]  In the present table, using statistical counts of the instances of each muscle, and ignoring gender-specific muscles, there are 753 skeletal muscles. Sometimes male and females have the same muscle but with different purposes [ 19 ]


Muscles are often paired as agonistic and antagonistic muscles. [ 20 ]  This can be a bit misleading as, in general, it is groups of muscles working together to either make or cancel a movement. [ 21 ]  The present table lists some well-known relationships but is not at all complete.


Lombard's paradox  describes a paradoxical  muscular contraction  in humans. When rising to stand from a  sitting  or  squatting position , both the  hamstrings  and  quadriceps  contract at the same time, despite them being  antagonists  to each other.


The  rectus femoris   biarticular muscle  acting over the hip has a smaller hip  moment arm  than the hamstrings. However, the rectus femoris moment arm is greater over the knee than the hamstring knee moment. This means that contraction from both rectus femoris and hamstrings will result in hip and knee extension. Hip extension also adds a passive stretch component to rectus femoris, which results in a knee extension force. This paradox allows for efficient movement, especially during  gait .


In facial anatomy, the  modiolus  is a dense, compact, mobile, fibromuscular tissue mass of facial muscles formed by the interlacing of a number of muscles just lateral to the angle of the mouth opposite the second upper premolar tooth.


There are no precise histological boundaries because the modiolus is an irregular zone where dense, compact, interlacing tissue grades into the stems of individually recognizable muscles. It is contributed to by at least nine muscles:  orbicularis oris ,  buccinator ,  levator anguli oris ,  depressor anguli oris ,  zygomaticus major ,  zygomaticus minor ,  risorius ,  quadratus labii superioris ,  quadratus labii inferioris . [ 1 ] [ 2 ]


Its position and movements are important in moving the  mouth ,  facial expression  and in  prosthetic dentistry . It is extremely important in relation to stability of lower denture, because of the strength and variability of movement of the area. It derives its motor nerve supply from the  facial nerve , and its blood supply from labial branches of the  facial artery . [ 3 ]


2.Al-Hoqail RA, Abdel Meguid EM. An anatomical and analytical study of the modiolus: enlightening its relevance to plastic surgery. Aesthetic Plast Surg. 2009;33(2):147–152. doi:10.1007/s00266-008-9187-x


3.Pélissier P, Pistre V, Bustamante K, Martin D, Baudet J. Le modiolus. Anatomie comparée, rappels embryologique et physiologique, intérêt chirurgical [The modiolus. Comparative anatomy, embryological and physiological review, surgical importance]. Ann Chir Plast Esthet. 2000;45(1):41–47.


4.Yu SK, Lee MH, Kim HS, Park JT, Kim HJ, Kim HJ. Histomorphologic approach for the modiolus with reference to reconstructive and aesthetic surgery. J Craniofac Surg. 2013;24(4):1414–1417. doi:10.1097/SCS.0b013e318292c939


Muscle  is a  soft tissue , one of the four basic types of  animal tissue . Muscle tissue gives  skeletal muscles  the ability to  contract . Muscle is formed during  embryonic development , in a process known as  myogenesis . Muscle tissue contains special  contractile proteins  called  actin  and  myosin  which interact to cause movement. Among many other muscle proteins, present are two  regulatory proteins ,  troponin  and  tropomyosin . [ 1 ]


Muscle tissue varies with function and location in the body.


In  vertebrates , the three types are:


Skeletal muscle tissue  consists of elongated, multinucleate  muscle cells  called  muscle fibers , and is responsible for movements of the body. Other tissues in skeletal muscle include  tendons  and  perimysium . [ 3 ]  Smooth and cardiac muscle contract involuntarily, without conscious intervention. These muscle types may be activated both through the interaction of the  central nervous system  as well as by receiving innervation from peripheral  plexus  or  endocrine  (hormonal) activation. Striated or skeletal muscle only contracts voluntarily, upon the influence of the central nervous system.  Reflexes  are a form of non-conscious activation of skeletal muscles, but nonetheless arise through activation of the central nervous system, albeit not engaging  cortical  structures until after the contraction has occurred. [ citation needed ]


The different muscle types vary in their response to  neurotransmitters  and  hormones  such as  acetylcholine ,  noradrenaline ,  adrenaline , and  nitric oxide  depending on muscle type and the exact location of the muscle. [ citation needed ]


Sub-categorization of muscle tissue is also possible, depending on among other things the content of  myoglobin ,  mitochondria , and  myosin ATPase  etc. [ citation needed ]


The word  muscle  comes from Latin  musculus , diminutive of  mus  meaning  mouse , because the appearance of the flexed biceps resembles the back of a mouse.


The same phenomenon occurred in  Greek , in which μῦς,  mȳs , means both "mouse" and "muscle".


There are three types of muscle tissue in vertebrates:  skeletal ,  cardiac , and  smooth . Skeletal and cardiac muscle are types of  striated muscle tissue . [ 2 ]  Smooth muscle is non-striated.


There are three types of muscle tissue in  invertebrates  that are based on their pattern of striation: transversely striated, obliquely striated, and smooth muscle. In arthropods there is no smooth muscle. The transversely striated type is the most similar to the skeletal muscle in vertebrates. [ 4 ]


Vertebrate skeletal muscle tissue is an elongated, striated muscle tissue, with the fibres ranging from 3-8 micrometers in width and from 18 to 200 micrometers in breadth. In the uterine wall, during pregnancy, they enlarge in length from 70 to 500 micrometers. [ 5 ]  Skeletal striated muscle tissue is arranged in regular, parallel bundles of  myofibrils , which contain many contractile units known as  sarcomeres , which give the tissue its striated (striped) appearance. Skeletal muscle is voluntary muscle, anchored by  tendons  or sometimes by  aponeuroses  to  bones , and is used to effect  skeletal  movement such as  locomotion  and to  maintain posture . Postural control is generally maintained as an unconscious reflex, but the responsible muscles can also react to conscious control. The body mass of an average adult man is made up of 42% of skeletal muscle, and an average adult woman is made up of 36%. [ 6 ]


Cardiac muscle tissue is found only in the walls of the  heart  as  myocardium , and it is an involuntary muscle controlled by the  autonomic nervous system . Cardiac muscle tissue is striated like skeletal muscle, containing sarcomeres in highly regular arrangements of bundles. While skeletal muscles are arranged in regular, parallel bundles, cardiac muscle connects at branching, irregular angles known as  intercalated discs .


Smooth muscle tissue is non-striated and involuntary. Smooth muscle is found within the walls of organs and structures such as the  esophagus ,  stomach ,  intestines ,  bronchi ,  uterus ,  urethra ,  bladder ,  blood vessels , and the  arrector pili  in the skin that control the erection of body hair.


Skeletal muscle is broadly classified into two fiber types:  type I  (slow-twitch) and  type II  (fast-twitch).


The  density  of mammalian skeletal muscle tissue is about 1.06 kg/liter. [ 11 ]  This can be contrasted with the density of  adipose tissue  (fat), which is 0.9196 kg/liter. [ 12 ]  This makes muscle tissue approximately 15% denser than fat tissue.


Skeletal muscle  is a highly oxygen-consuming tissue, and  oxidative DNA damage  that is induced by  reactive oxygen species  tends to accumulate with  age . [ 13 ]   The oxidative DNA damage  8-OHdG  accumulates in  heart  and skeletal muscle of both mouse and rat with age. [ 14 ]   Also, DNA double-strand breaks accumulate with age in the skeletal muscle of mice. [ 15 ]


Smooth muscle  is involuntary and non-striated. It is divided into two subgroups: the  single-unit  (unitary) and  multiunit smooth muscle . Within single-unit cells, the whole bundle or sheet contracts as a  syncytium  (i.e. a  multinucleate  mass of  cytoplasm  that is not separated into cells). Multiunit smooth muscle tissues  innervate  individual cells; as such, they allow for fine control and gradual responses, much like  motor unit recruitment  in skeletal muscle.


Smooth muscle is found within the walls of  blood vessels  (such smooth muscle specifically being termed  vascular smooth muscle ) such as in the  tunica media  layer of the large ( aorta ) and small  arteries ,  arterioles  and  veins . Smooth muscle is also found in lymphatic vessels, the  urinary bladder ,  uterus  (termed  uterine smooth muscle ), male and female  reproductive tracts , the  gastrointestinal tract , the  respiratory tract , the arrector pili of  skin , the  ciliary muscle , and the  iris of the eye . The structure and function is basically the same in smooth muscle cells in different organs, but the inducing stimuli differ substantially, in order to perform individual actions in the body at individual times. In addition, the  glomeruli  of the kidneys contain smooth muscle-like cells called  mesangial cells .


Cardiac muscle is involuntary,  striated muscle  that is found in the walls and the  histological  foundation of the  heart , specifically the myocardium. The  cardiac muscle cells , (also called cardiomyocytes or myocardiocytes), predominantly contain only one nucleus, although populations with two to four nuclei do exist. [ 16 ] [ 17 ] [ page needed ]  The  myocardium  is the muscle tissue of the heart and forms a thick middle layer between the outer  epicardium  layer and the inner  endocardium  layer.


Coordinated  contractions  of cardiac muscle cells in the heart propel  blood  out of the  atria  and  ventricles  to the blood vessels of the left/body/systemic and right/lungs/pulmonary  circulatory systems . This complex mechanism illustrates  systole  of the heart.


Cardiac muscle cells, unlike most other tissues in the body, rely on an available blood and electrical supply to deliver oxygen and nutrients and to remove waste products such as  carbon dioxide . The  coronary arteries  help fulfill this function.


All muscles are derived from  paraxial mesoderm . The paraxial mesoderm is divided along the  embryo 's length into  somites , corresponding to the  segmentation  of the body (most obviously seen in the  vertebral column . [ 18 ]  Each somite has three divisions,  sclerotome  (which forms  vertebrae ),  dermatome  (which forms skin), and  myotome  (which forms muscle). The myotome is divided into two sections, the epimere and hypomere, which form  epaxial and hypaxial muscles , respectively. The only epaxial muscles in humans are the  erector spinae  and small intervertebral muscles, and are innervated by the dorsal rami of the  spinal nerves . All other muscles, including those of the limbs are hypaxial, and innervated by the  ventral rami  of the spinal nerves. [ 18 ]


During development,  myoblasts  (muscle progenitor cells) either remain in the somite to form muscles associated with the vertebral column or migrate out into the body to form all other muscles. Myoblast migration is preceded by the formation of  connective tissue  frameworks, usually formed from the somatic  lateral plate mesoderm . Myoblasts follow chemical signals to the appropriate locations, where they fuse into elongate skeletal muscle cells. [ 18 ]


The primary function of muscle tissue is  contraction . The three types of muscle tissue (skeletal, cardiac and smooth) have significant differences. However, all three use the movement of  actin  against  myosin  to create contraction.


In skeletal muscle, contraction is stimulated by  electrical impulses  transmitted by the  motor nerves . Cardiac and smooth muscle contractions are stimulated by internal pacemaker cells which regularly contract, and propagate contractions to other muscle cells they are in contact with. All skeletal muscle and many smooth muscle contractions are facilitated by the  neurotransmitter   acetylcholine . [ 19 ]


Smooth muscle is found in almost all  organ systems  such as  hollow organs  including the  stomach , and  bladder ; in tubular structures such as  blood  and  lymph vessels , and  bile ducts ; in sphincters such as in the uterus, and the eye. In addition, it plays an important role in the ducts of exocrine glands. It fulfills various tasks such as sealing orifices (e.g. pylorus, uterine os) or the transport of the chyme through wavelike contractions of the intestinal tube. Smooth muscle cells contract more slowly than skeletal muscle cells, but they are stronger, more sustained and require less energy. Smooth muscle is also involuntary, unlike skeletal muscle, which requires a stimulus.


Cardiac muscle is the muscle of the heart. It is self-contracting,  autonomically regulated  and must continue to contract in a rhythmic fashion for the whole life of the organism. Hence it has special features. [ 20 ]


There are three types of muscle tissue in  invertebrates  that are based on their pattern of  striation : transversely striated, obliquely striated, and smooth muscle. In arthropods there is no smooth muscle. The transversely striated type is the most similar to the skeletal muscle in vertebrates. [ 4 ]


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Muscle architecture  is the physical arrangement of  muscle fibers  at the macroscopic level that determines a  muscle's  mechanical function. There are several different muscle architecture types including: parallel, pennate and hydrostats. Force production and  gearing  vary depending on the different muscle parameters such as muscle length, fiber length, pennation angle, and the  physiological cross-sectional area  (PCSA). [ 1 ]


Parallel and  pennate  (also known as pinnate) are two main types of muscle architecture. A third subcategory,  muscular hydrostats , can also be considered. Architecture type is determined by the direction in which the muscle fibers are oriented relative to the  force-generating axis . The force produced by a given muscle is proportional to the cross-sectional area, or the number of parallel sarcomeres present. [ 2 ]


The parallel muscle architecture is found in muscles where the fibers are parallel to the force-generating axis. [ 1 ]  These muscles are often used for fast or extensive movements and can be measured by the anatomical cross-sectional area (ACSA). [ 3 ]  Parallel muscles can be further defined into three main categories: strap, fusiform, or fan-shaped.


Strap muscles are shaped like a strap or belt and have fibers that run longitudinally to the contraction direction. [ 4 ]  These muscles have broad attachments compared to other muscle types and can shorten to about 40–60% of their resting length. [ 3 ] [ 4 ]  Strap muscles, such as the  laryngeal muscles , have been thought to control the fundamental frequency used in speech production, as well as singing. [ 5 ]  Another example of this muscle is the longest muscle in the human body, the  sartorius .


Fusiform muscles  are wider and cylindrically shaped in the center and taper off at the ends. This overall shape of fusiform muscles is often referred to as a spindle. The line of action in this muscle type runs in a straight line between the attachment points which are often tendons. Due to the shape, the force produced by fusiform muscles is concentrated into a small area. [ 3 ]  An example of this architecture type is the  biceps brachii  in humans.


The fibers in convergent, or triangular muscles converge at one end (typically at a tendon) and spread over a broad area at the other end in a fan-shape. [ 3 ] [ 6 ]
Convergent muscles, such as the  pectoralis major  in humans, have a weaker pull on the
attachment site compared to other parallel fibers due to their broad nature. These
muscles are considered versatile because of their ability to change the direction of pull
depending on how the fibers are contracting. [ 3 ]


Typically, convergent muscles experience varying degrees of fiber strain. This is largely due to the different lengths and varying insertion points of the muscle fibers. Studies on ratfish have looked at the strain on these muscles that have a twisted tendon. It has been found that strain becomes uniform over the face of a convergent muscle with the presence of a twisted tendon. [ 7 ]


Unlike in parallel muscles, fibers in  pennate muscles  are at an angle to the force-generating axis (pennation angle) and usually insert into a central tendon. [ 3 ] [ 8 ]  Because of this structure, fewer  sarcomeres  can be found in series, resulting in a shorter fiber length. [ 2 ] [ 3 ]  This further allows for more fibers to be present in a given muscle; however, a trade-off exists between the number of fibers present and force transmission. [ 3 ] [ 8 ]  The force produced by pennate muscles is greater than the force produced by parallel muscles. [ 3 ]  Since pennate fibers insert at an angle, the anatomical cross-sectional area cannot be used as in parallel fibered muscles. Instead, the  physiological cross-sectional area  (PCSA) must be used for pennate muscles. Pennate muscles can be further divided into uni-, bi- or multipennate.


Unipennate muscles are those where the muscle fibers are oriented at one fiber angle to the force-generating axis and are all on the same side of a tendon. [ 1 ]  The pennation angle in unipennate muscles has been measured at a variety of resting length and typically varies from 0° to 30°. [ 1 ]  The  lateral gastrocnemius  is an example of this muscle architecture.


Muscles that have fibers on two sides of a tendon are considered bipennate. [ 1 ]  The  stapedius  in the middle ear of humans, as well as the  rectus femoris  of the quadriceps are examples of bipennate muscles.


The third type of pennate subgroup is known as the multipennate architecture. These muscles, such as the  deltoid  muscle in the shoulder of humans, have fibers that are oriented at multiple angles along the force-generating axis. [ 1 ]


Muscular hydrostats  function independently of a hardened skeletal system. Muscular hydrostats are typically supported by a membrane of connective tissue which holds the volume constant. Retaining a constant volume enables the fibers to stabilize the muscle's structure that would otherwise require skeletal support. [ 9 ]  Muscle fibers change the shape of the muscle by contracting along three general lines of action relative to the long axis: parallel, perpendicular and helical. These contractions can apply or resist compressive forces to the overall structure. [ 10 ]  A balance of synchronized, compressive and resistive forces along the  three lines of action, enable the muscle to move in diverse and complex ways. [ 10 ]


Contraction of helical fibers causes elongation and shortening of the hydrostat.  Unilateral contraction of these muscles can cause a bending movement. Helical fibers can oriented into either left or right-handed arrangements. Contraction of orthogonal fibers causes torsion or twisting of the hydrostat.


Muscle architecture directly influences force production via muscle volume, fiber length, fiber type and pennation angle.


Muscle volume is determined by the cross-sectional area. Anatomical cross-sectional area is


C
 S
 A
 =

 V
 l

 {\displaystyle CSA={\frac {V}{l}}}


In muscles, a more accurate measurement of CSA is physiological CSA (PCSA) which takes into account fiber angle.


P
 C
 S
 A
 =

 m
 ⋅
 cos

 θ

 l
 ⋅
 ρ

 {\displaystyle PCSA={\frac {m\cdot \cos \theta }{l\cdot \rho }}}


PCSA relates the force produced by the muscle to the summation of the forces produced along the force generating axis of each muscle fiber and is largely determined by the pennation angle. [ 3 ] [ 8 ]


Fiber length is also a key variable in muscle anatomy. Fiber length is the product of both the number of sarcomeres in series in the fiber and their individual lengths.  As a fiber changes length, the individual sarcomeres shorten or lengthen, but the total number does not change (except on long timescales following exercise and conditioning). To standardize fiber length, length is measured at the peak of the length-tension relationship (L0), ensuring all sarcomeres are at the same length. Fiber length (at L0) does not affect force generation, much as the strength of a chain is unaffected by the length.  Similarly, increased fiber cross-section or multiple fibers increase the force, like having multiple chains in parallel.  Velocity is affected in the reverse manner –  because sarcomeres shorten at a certain percentage per second under a certain force, fibers with more sarcomeres will have higher absolute (but not relative) velocities. [ 11 ]  Muscles with short fibers will have higher PCSA per unit muscle mass, thus greater force production, while muscle with long fibers will have lower PCSA per unit muscle mass, thus lower force production. However, muscles with longer fibers will shorten at greater absolute speeds than a similar muscle with shorter fibers. [ 2 ]


The type of muscle fiber correlates to force production. Type I fibers are slow oxidative with a slow rise in force and an overall low force production. The type I fibers have a smaller fiber diameter and exhibit a slow contraction. Type IIa fibers are fast oxidative which exhibit fast contraction and a fast rise in force. These fibers have fast contraction times and maintain some, though not a great amount of their force production with repeated activity due to being moderately fatigue resistant. Type IIb fibers are fast glycolytic which also exhibit fast contraction and fast rise in force. These fibers display extremely large force production, but are easily fatigued and therefore unable to maintain force for more than a few contractions without rest.


The  pennation angle  is the angle between the longitudinal axis of the entire muscle and its fibers. The longitudinal axis is the force generating axis of the muscle and pennate fibers lie at an oblique angle. As tension increases in the muscle fibers, the pennation angle also increases. A greater pennation angle results in a smaller force being transmitted to the tendon. [ 9 ]


Muscle architecture affects the force-velocity relationship. Components of this relationship are fiber length, number of sarcomeres and pennation angle. In pennate muscles, for example, as the fibers shorten, the pennation angle increases as the fibers pivot which effects the amount of force generated. [ 2 ]