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10 Articles in Volume 8, Issue #8
Botulinum Toxin Type-A in Pain Management
Chronic Migraine: An Interactive Case History
Consistent Documentation Drives Compliance
Muscle Physiology, Kinetics, Assessment, and Rehabilitation
Non-surgical Decompression Treatment for Carpal Tunnel Syndrome
The Pseudo-RSD Pain Patient
Therapeutic Laser Evolution: Part 1
TMJ Pain and Temporal Tendonitis with Autonomic Features
Topical Use of Morphine
Toward a Neuroethics of Pain Medicine

Muscle Physiology, Kinetics, Assessment, and Rehabilitation

A review of some basic science, homeostasis, functional specialization, resting and activity tonus, objective methodology in assessment, and rehabilitation considerations.

The article below is partly a com-pendium of previous articles published in this journal and partly a list of clinical, anatomical, and physiological pearls—all related to muscle function and dysfunction. The more I have studied muscles in the past nineteen years, the more I felt like one of the proverbial eight blind men who endeavored to touch and describe an elephant. This article is directed to those with an ongoing interest in muscle physiology, kinetics, pathology, etc.

I would very much welcome hearing from interested readers and learning from their pearls of knowledge and wisdom on the subject.

A homeostatic truism is that muscles function best in equilibrium when they exert bilateral action involving the least individual effort while exerting optimal group effort along with relaxation for an optimal time period needed for energy regeneration.

Most commonly we think of muscles primarily as our means of locomotion. However, they participate very much in multiple functions, including homeostasis. This includes buffering the internal organs and tissues against direct or indirect noxious factors, maintenance of body turgidity, diaphragmatic vital functions such as breathing, peristaltic actions, defecation, urination, sexual functions including sexual acts, and pregnancy and fetal delivery, to name but a few.

Temperature regulation is a prime homeostatic activity. Muscles play a direct role in cooling the body by modulating the vasodilation of the vascular and muscular system and helping the sweating activity, on the one hand, and increasing the temperature of the cold body by directing a shivering action.

While the heart is the largest muscle and is essential to life, most muscles of the body may participate in moving the blood by exerting recurrent pressure on the veins and lymphatic channels to propel the blood back to the heart.

CNS and Muscular Activity

Engram and neuroplasticity are two concepts that encompass very relevant factors of understanding of muscle function. The engram is a central nervous system (CNS) neuromotor event. It is a fairly autonomous motor component and simultaneously a component of a much larger motoneuron memory. A repeated muscular activity that requires some precision enhances the motoneuron pathway and memory loop. The more the action is repeated, the more vivid its memory and the less likely that the motor cortex and connections will ‘forget it.’ Examples of such behavior abound and include sports or ergonomic functions, e.g., pitching a baseball or shooting at a target. There are two aspects involved in the formation of the engram: (1) learning the activity itself and (2) optimizing it in time and space (i.e., the ‘bulls-eye’ component). The former, once learned, is rarely forgotten. The latter, on the other hand, needs to be ‘rehearsed’ in an optimal temporal rhythm in order to be maintained. The old adage “Bones forget, Muscles remember” may refer to engram memory.

Neuroplasticity is a rather new concept and is the antithesis of the generational learning that the mature nervous system is a fixed entity with very little chance of growth and re-growth.1 The reality of the central nervous system and the peripheral nervous system (PNS) is that neural maturity and re-growth, or neural regeneration, is possible in conditions of neural injury. The body of evidence pointing that way is constantly growing. When nerves grow back, muscles may regain function with appropriate and patient re-learning.

Myofascial Mantle: The ‘Leotard’ Concept

The shape and turgidity of the human (and mammalian) body is maintained in large measure by the presence of a strong connective tissue mantle—composed of several layers—and is close to being the vertebrate equivalent of the invertebrate exoskeleton. All muscles of the body are vested within the different layers of this mantle: the myofascial complex.2,3

The main structural component of the fascia is collagen, a substance that has the tensile strength of steel. Fascia is live tissue, with ability to heal from injury and form scars. It derives from the mesoderm, just as muscles do.

When thinking of a muscle, clinicians must take into consideration not only the location of the tendon insertions but also the presence of the fascia around the muscle and around all the muscles of a salient primary myofascial unit. A functional understanding of muscle action, injury, pain, and dysfunction has to include the fact that any particular muscle is connected to other muscles of the body via the fascial mantle. Just like with a ‘leotard’ costume, if one pulls on any area or region, the pull is transferred to all the other regions. Fascial scarring, just like that of the leotard material, may result in decreased ability to stretch and move optimally. Various descriptions have been given to the fascial layers according to their direct anatomic connections.

The myofascial entity functions best when there is contra-lateral equilibrium.4 Prolonged disruption of the equilibrium produces dysfunction, fatigue, and pain. The pain memory is transmitted to the CNS via neurotransmitters such as the muscle spindles. The longer the memory lingers, the longer it takes to rehabilitate the affected muscles.

The Myotatic Unit

By definition, all muscles that insert their tendons in the same joints or bones form one functional unit, the myotatic unit.2,3 The definition may be extended when there are muscles in a group that insert partially in the same joints and partially in contiguous joints.

Primary Myotatic Unit

The primary myotatic unit refers to the number of muscles that insert in the same joint terminals, even if one or more of the muscles may also insert in an adjacent joint. They function as a group in joint action. As discussed below, some muscles in the unit function in a synergistic (agonistic) mode while others function as antagonists. This concept is a functional not an anatomic one.3,4

Secondary Myotatic Units

The concept of secondary myotatic units refers to the presence of contiguous myotatic units on proximal and distal joints. Except for the distal joints of the digits, all other joints have proximal and distal relationships. The functional rele-vance of this concept is that any muscle action is subserved directly by the primary myotatic unit of the salient joint action, as well as supported and modulated by the action of the myotatic units of the proximal and distal joints.3,4

“The myofascial entity functions best when there is contra-lateral equilibrium.4 Prolonged disruption of the equilibrium produces dysfunction, fatigue, and pain.”

It can be shown with SEMG studies that during any given motion, electric potential activity is found not only in the primary myotatic unit but also in the secondary units and in farther contiguous proximal units.

The relevance of this concept is found not only in ‘classic’ tests of joints ROM but in ergonomic activity including athletics, rehabilitation, instrument playing, etc.

“Different Strokes for Different Folks”

A common image that one gets when considering “muscle” is that of contracting a muscle against a given resistance. In the body, the resistance may be postural, diaphragmatic, ballistic, or mixed. This allows for a functional classification of muscles into postural muscles, dia-phragms, ballistic muscles, and muscles of expression. This classification is by no means absolute, since muscles can be trained to fulfill several functions.


It is relevant to remember that the body functions very much like a series of pneumatic and hydraulic (blood, in this case) pumps. Muscular diaphragmatic action is quintessential to life.

The main diaphragms are (1) the larynx, (2) the thoracic diaphragm, and (3) the pelvic floor diaphragm.

The larynx is essential to life not only because of the part it plays in opening to allow breathing but also in closing to work with the diaphragm to change the pneumatic pressure in the lung, e.g., for coughing and the volume of venous blood in the mediastinum.

The thoracic diaphragm is one of the largest muscles of the body. It divides the thoracic from the abdominal cavity and functions in inhalation and exhalation during the breathing cycle. It also functions, together with the larynx, in a number of thoracic/laryngeal activities and with the pelvic diaphragm in abdominal/pelvic activities. It helps the intestinal peristalsis and propulsion in the act of defecation. It has an essential role in parturition.

The pelvic diaphragm also has essential functions in the acts of urination and defecation as well as in parturition. Concerted action with other muscles and with the diaphragm is necessary to maintain the venous and lymphatic pressure in the abdomen and help propel the blood to the thoracic cavity. The Valsalva maneuver involves simultaneous and concerted action of the three main diaphragms.

Postural Muscles

The muscles of the trunk are generally referred to as postural muscles. The paraspinal groups, including the para-spinal muscles of the neck, function within several anatomical strata and have a rather limited individual range of motion or contractile span. They subserve posture—both static or dynamic. They help modulate and stabilize the muscles of the limbs, neck, and pelvis.

The muscles of the neck have a dual function. They maintain posture and may also move widely in ballistic mode (e.g., ‘head throw’ in soccer games). Those muscles also serve to maintain the myofascial relationships of the muscles of the head and face, many of which insert with their tendons only in the contiguous fascia and not in bones.

Ballistic Muscles

The muscles of the limbs have generally ballistic properties. They can be taught to function as a chain to impart momentum necessary for rapid locomotion. They can be taught to assist and produce real ballistic activity such as in throwing objects— balls, weapons, etc.—at large distances with great propulsive energy. They need to rely on the postural muscles for support in the direction of motion and minimal recoil, as necessary. The evolutionary change of the human body from the quadruped to the biped position and motion contributed greatly to the new abilities of the limb muscles to acquire new modes of operation and new neuromotor engrams.

Muscles of Facial Expression

These muscles, as a rule, derive embryologically from the second branchial arch, which generated the respiratory muscles of the (fish) gills. They comprise the muscles of the face and neck and include the trapezius muscle. While they allow for emotional expression and verbal and non-verbal communication, they also maintain a proximal and direct relationship with visceral functions such as breathing. A prime example found by the author is that if quiet breathing is followed suddenly by the act of frowning, one becomes unable to breathe until one becomes aware of it, and then breathing is labored until the frowning action is terminated.5,6

It is easy to understand why these muscles can function readily as accessory muscles of respiration and emotional tension. This visceral connection is most probably also related to ‘expressions’ originally geared towards fight and flight activities and group protection.7

Muscular Agonism and Antagonism

Anatomic and kinesiologic studies clearly show that muscles function in groups. For many years, it was hypothesized that muscles that insert on the same side of a bone/joint functioned in synergistic (agonistic) fashion while muscles inserting on the opposite side functioned in an ‘antagonistic’ manner. It was further considered that during a given motion, the agonists were active while the antagonists relaxed. While the whole concept is interesting, there is one essential problem: there is no bone or joint in the human body that has only two muscles that would fit the theory by exhibiting joint insertion on opposite sides of a bone, or by contraction or extension activity.

Dynamic SEMG studies were conducted on over 6000 muscles.4,8 Regression analysis of the electric activity potentials of the muscles of any primary joint segments of motion show positive and negative correlations between any two muscles of a unit. The statistical regression may be positive or negative and, in a number of cases, very close to the ‘zero’ point. Similar regression statistics can be conducted among the various segments of motion of a complete joint ROM.

Therefore, on a statistical functional basis (i.e., joint ROM), muscles can relate in a synergistic or antagonistic basis (ie. positive or negative, respectively). The structural position of the muscles is not necessarily relevant, since it has been shown that the regression sign may change when SEMG dynamic testing is done for an adjacent joint in which the same muscles participate, e.g., testing of shoulder muscles during the joint ROM of the elbow, etc.

Thus, while muscular agonism and antagonism does exist on a functional, dynamic basis, it is not a fixed structural entity.

The same studies demonstrated that in all cases all the muscles of a myotatic unit are active to various degrees during any and all segments of motion tested. The concept of ‘rest of the antagonist while the agonist is active’ cannot stand experimental evidence in normal muscles having normal innervation.

Objective Methodology In Muscular Functional Assessment

There are several objective methodologies aimed at the assessment of different components of muscle function: joint range of motion (ROM); dynamometry, dolorimetry, and pressure perception analysis; nerve conduction studies (NCS) and needle electromyography (EMG); and surface EMG (SEMG) testing for frequency domain and amplitude domain muscular characteristics. These methodologies are described in the following sections.

Joint Range of Motion (ROM). This encompasses inclinometry and goniometry. In normal individuals, joints have structural limitations in their range of motion. Conditions such as the Danlos syndrome show that it is actually possible to overcome ‘normal’ structural restrictions. Otherwise, genetics, stretch training, and restrictions of the myofascial mantle comprise the ‘borders’ of the range of motion of any joint. Various studies have documented the actual range of motion of all joints. Accuracy of methodology and instrumentation is paramount in the measurement of the joints ROM.9

Joints that cannot maintain a normal ROM either because of intrinsic or muscular restrictions may contribute to fatigue and overload related to the protective guarding of the contra-lateral joints and muscles. Pain may ensue if the situation is not redressed within a short period of time.

Dynamometry. The strength of muscles in any joint motion against a given resistance varies widely. A large number of factors contribute to this variability. Among others, they include genetic factors, conditioning, age, nutritional factors, and gender differences. Strength may be increased by gradual training or conditioning. Loss of strength may occur in deconditioned muscles, even in the absence of disease.

Various instruments may be utilized to measure the strength of groups of muscles or of the body as a whole against given resistances. Wrist muscles strength is commonly assessed with hand-held gripometers.10,11 Various databases can be established for the ergonomic needs related to various athletic, rehabilitative, or other specific requirements.

Muscles that cannot sustain the dynamometric requirements may fatigue earlier than expected and pain may be the signal related to the fatigue and overload.

Dolorimetry and Pressure Perception Analysis. These are related methodologies aimed at assessing the amount of tension required to elicit a pain response. The skin above a tender area is pressed with a calibrated gage. The initiation of the perception of pain is measured (in Kg/unit pressure) in the affected area and compared to other salient areas. The concept is interesting, however the reality is that there are several variables that need to be overcome to establish forensic validity and lack of confounding.12 It is, of course, very relevant to be able to assess not only the presence of pain perception but also its intensity at a given point of time.

Nerve Conduction Studies (NCS) and Needle Electromyography (EMG). These are time-tested methodologies that aim at testing nerve conduction velocity and amplitude in peripheral nerves and evidence of neuromuscular junctional pathology. The techniques involve, respectively, surface electric stimulation to the salient nerves and electric needle insertion in the salient muscles.

It is of relevance to note that needle EMG (n-EMG) is not assessing muscle directly, just the motor point neuromuscular junctional units. Thus the name is somewhat a misnomer.

While both methodologies are valid for what they test, they provide very little information on the presence of dysfunction of C-fibers associated with pain. There are several other methodologies such as neurometry that purport to measure C-fiber function and dysfunction.

Surface EMG (SEMG) Testing for Frequency Domain and Amplitude Domain Muscular Characteristics. SEMG may be used to assess two domains of muscular activity: the frequency and amplitude domains. The technique utilizes surface electrodes placed in a standardized manner on the skin above the muscles to be tested. Testing can be done in conditions of rest, activity, as well as sequences of activity and rest.13,14

“Muscles that are not allowed to regain optimal energy will signal their inability to function properly by early signs of fatigue and pain.”

The frequency domain refers to the muscle characteristic of utilizing the electrical energy sources of multiple unit action potentials (MUAPS) at different frequencies (Hz). Until the muscle is fatigued, normal muscular activity occurs with the muscle utilizing a high frequency range, usually between 100-140 HZ, though variability is considerable. As the muscle uses the readily available energy, the frequency range diminishes below 100 Hz. Normal muscle needs only a matter of seconds or minutes of rest in order to regenerate the ability to function at optimal frequencies. Muscle affected by neuropathy functions at low frequencies of firing MUAPS and takes a long time to rest and restore ‘optimal’ action. The amount of time it can function at the best frequency range may be measured in seconds.14

Fatigue and ensuing pain are readily perceived in such muscle, thus the frequency domain may become a valuable methodology in the assessment of subjective muscular fatigue and pain.

The amplitude domain refers to the muscular characteristic of utilizing electric action potentials in proportion to the amount of effort a muscle has to exert for a determined motion in space relative to gravity and resistance factors. The amount of energy utilized varies in proportion to the effort and depends on a variety of factors, including muscle size, vascularity, nutrition, and conditioning.15 SEMG dynamic studies have shown that, for any segment of joint motion, different muscles of the primary myotatic unit consume different amounts of energy.4,13 This observation is relevant in rehabilitation and in ergonomic fields since understanding the stratification of energy requirements of different muscles can lead to optimization of training. As shown below, dysfunctional muscles have patterns of SEMG amplitude domain characteristics that objectively document the presence of abnormal function and pain.4 This principle is of relevance both in muscular investigation and neuromuscular rehabilitation/ re-education.16

The Resting Tonus

This is the tonus of a muscle at rest as determined with static and dynamic SEMG studies done on over 4000 muscles or against gravity.7 Resting potentials vary between ≤1µV in well trained muscles to ≤3µV. In general, postural and ballistic muscles average ±2.5µV. These results appear rather uniform over a wide array of muscles tested (>6000) through all the joints ROM. The only exception is that of the muscles of expression. Their resting tonus is higher and varies between 3 to 6µV. The above discussion on muscles of expression may give a clue to the relationship of muscles ‘staying at attention’ and their propensity to fatigue and pain. Muscles that are not allowed to regain optimal energy will signal their inability to function properly by early signs of fatigue and pain.

The Activity Tonus

This refers to the magnitude of the action potentials during any given muscular activity—with or against gravity and with or without resistance. In general, the larger the muscular effort, the higher the action potentials amplitude required to subserve that effort. In states of motion performed with the least perceivable effort (minimal voluntary contraction, MinVC), there is relatively low variability in the amplitude potentials for any given joint ROM. This was observed while testing more than 6000 adult muscles.4,7 The variability is much greater in any other states of activity, as related to overall strength, conditioning, etc.

Muscular maximal voluntary contraction (MVC) is a subjective perception of strength and effort. It can be shown with dynamic SEMG studies that the amplitude of contraction during MVC is less than that required for motion against 20-30 lbs of resistance, even in healthy individuals. This just serves to show the difference between the actual muscular effort of the subjective ‘MVC’ and actual activation needed to contract against a moderate weight/ resistance.

A database has been constructed for more that 180 different muscles acting in the condition of MinVC through the primary and secondary joints ROM. This has been based on the study of more than 6000 muscles described above.4

Two findings of interest in muscle studies are as follows:

  1. symmetry of bilateral muscle activity, and
  2. the mirror image activity

Homologous contralateral muscles involved in the same joint or region ROM show amplitudes of contraction that may vary within approximately 1-18%. This level of symmetry is remarkably similar among all the muscles tested—whether at rest or during activity. Laterality, gender, or conditioning play no part. In summary, asymptomatic contralateral muscles show a remarkable symmetry in tonus during activity and rest. The mirror image activity is a singular condition involving contralateral muscles of the axial skeleton. The mirror image property can be assessed during both rotational or bending motions. Thus, the right SCM (sternocleidomastoid) will show a similar amplitude of contraction with the left when they are exposed to the same lateral or rotational motions.

Muscle fatigue and pain may be evidenced by a number of SEMG parameters of dysfunction, however the definitions of resting and activity tonus described above may not apply to dysfunctional muscles.

Various conditions of muscular dys-function may show in SEMG signals. For example, muscular fatigue due to faulty neural innervation, e.g., brachial plexopathy, shows a low frequency signal that may decrease further during muscular activity.4,17

The SEMG amplitude domain signals pertinent to muscular dysfunction are the following:

  • Disruption of symmetry of bilateral activity
  • Disruption of mirror image activity
  • Spasm
  • Hypertonus
  • Hypotonus
  • Co-contraction/ co-activation
  • Contracture
  • Myokimia

Respective definitions of the parameters of dysfunction listed above are found in textbooks and other articles. Within the scope of the present work, they present as specific abnormalities of the resting potential signal, the activity potential signal, or both. Clinically, the affected muscle shows pain and/or fatigue and inability to perform well in conjunction with the rest of the primary myotatic unit.

Muscular Rehabilitation Considerations

The person affected by muscular pain and fatigue of any etiology expects a thorough physical examination and objective investigation conducive to the relief of symp-toms and optimization of function.18,19General rehabilitation considerations include the following:

  • relief of pain
  • improved muscular agility
  • normalization of strength
  • normalization of the joint ROM
  • normalization of endurance

It has been a clinical paradigm that pain relief follows improved function and not vice-versa. This has to be explained clearly and often repeatedly to the sufferer. A general rehabilitation program may take a few months, no matter how cooperative the relationship between the therapist and the patient. The physiology of muscle is such that strengthening, improved agility, endurance, and optimal stretching necessary for normal joint ROM take time and only after the muscle functions within acceptable limits does pain relief occur. Centrally, it is an interplay between the new or modified neuromuscular engram and the neuroplasticity process that leads to normalized function. It has been a long time observation that many pain sufferers are motivated to get relief instantly and, if possible, with opioids. They really don’t care about the rehabilitation process and long-term results. It takes a lot of patience on the part of the therapist (or team) to educate and overcome the initial motivation of the pain sufferer. Only once the motivation is addressed in the right direction can the rehabilitation process begin.

A few principles apply: therapy may have active and passive components and one type does not exclude the other. The passive components include drug therapies, injections, massage, and other passive therapeutic modalities. The active components include neuromuscular reeducation with SEMG, a process which reestablishes the engram, and exercise therapy aimed at optimizing muscle length, strength, and endurance.


A combination of therapies in the motivated patient is probably the best way to go. Realities of interfering external factors, such as insurance and administrative bodies, need to be dealt with as they hardly ever speed the process of recovery.

Finally, the end point of the rehabilitation process needs to be a final evaluation, such that no factor of dysfunction is left behind. The therapist (or team) and the patient need to view this like a pilot’s safety checklist before takeoff to ensure a safe flight—or, in this case, safe activity without risk of re-injury.

Last updated on: June 7, 2016
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