Muscle Physiology, Kinetics, Assessment, and Rehabilitation
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.