Understanding the physical properties of muscle is of great importance for maintaining optimal muscle function and treating soft tissue dysfunction. Muscle tone depends on two things: (1) the basic viscoelastic properties of the soft tissues associated with the muscle and (2) the degree of activation of the contractile mechanisms of the muscle. Viscoelasticity is related to the biomechanical properties of connective tissues, whereas contractility depends either on physiologic contraction, which is controlled by motor neurons in the spinal cord, or on pathologic endogenous contraction, which is not. This endogenous contraction, defined as contracture, is often the pathologic cause of a reduction in the level and quality of muscle performance.
The concept of muscle tone means the elastic or viscoelastic stiffness, or both, in the absence of contractile activity (motor unit and contracture).2 In some pathologic conditions, muscle tone may increase for any reason. This hypertonia includes a variety of conditions, such as spasticity, rigidity, dystonia, and muscular contracture. The causes and mechanisms of different types of hypertonia may be quite different. Hypotonia, on the other hand, is the condition of loss of normal elastic stiffness of the muscle.
Muscle contractile activity may occur in three forms2:
1. Electrogenic contraction or stiffness, which is muscle tension resulting from electrogenic muscle contraction (determined on the basis of observable electromyographic activity) in normal muscles that are not completely relaxed. Under these conditions the a-motor neurons and the neuromuscular junctions are physiologically active.
2. Electrogenic spasm, which is specifically a pathologic involuntary electrogenic contraction.
3. Contracture, which arises endogenously within the muscle fibers independently of electromyo-graphic activity. This concept is of critical importance in the treatment of soft tissue dysfunction in soft tissue pain management, both in sports and in ordinary activity.
Electromyographic recordings can identify elec-trogenic contraction arising from electrical activity of the motor nerves and muscle fibers, but it cannot identify endogenous contracture of skeletal muscle, because the latter does not depend on the propagation of action potentials in the muscle fiber. Contracture indicates the endogenous shortening of some muscle fibers in the absence of elec-tromyography-initiated a-motor neurons of the spinal cord. The molecular mechanisms of this are explained by Mense and Simons.2
When muscle is suddenly mobilized, its stiffness can be reduced immediately without activating any electromyogenic process. This physiologic process is referred to as the thixotropic property of skeletal muscle. Human postural muscles do not show sustained electromyographic activity except for occasional minimal corrective bursts to maintain balance. This indicates that the tension in postural muscles that is necessary to maintain posture for long periods is produced by the mechanical properties of muscles and related soft tissues. This property also explains how muscle stiffness is reduced when needling therapy is applied to stiff muscles.
The sudden insertion of the needles may reduce the thixotropic viscosity of the muscles, so that static muscles can perform less restricted motion of the muscles and joints. This property may be related to changes in myoplasmic viscosity and connectin filaments.3
Muscle spasm is an involuntary contraction of a muscle or a segment within a muscle that can be caused by irritation of a nerve root, plexus, or peripheral nerve branch. Spasm caused by irritation of the nerve endings within a muscle may be limited to the muscle involved or may spread to other muscles because of reflex pain mechanisms.
Protective spasm may occur as a consequence of injury to an underlying structure, such as a ligament of bone. This type of spasm, which often occurs after a back injury, prevents movement and further irritation of the injured structure.
Segmental muscle spasm is the involuntary contraction of an uninjured segment of a muscle as a result of an injury to the muscle. The contraction of this segment creates tension for the injured part and strains the muscle. Pain associated with tension within the muscle may be limited to that muscle or be more widespread because of reflex or referred pain mechanisms.
Diseased viscera cause muscle spasm in particular locations. This reflex muscle spasm can be observed in patients with cardiac infarction, appendicitis, renal inflammation, and acute pancreatitis. Needling therapy is very effective in relaxing these spasms.
Contracture is the physiologic condition in which all or some of the fibers in a muscle are in a state of contractile activity unaccompanied by electrical activity.4 Some contractures cause slight or moderate shortening of the muscle, which results in restriction of range of motion. Some cases of contracture are remedied by proper treatment, especially needling therapy. Stretching movements may damage the tissue structures if done improperly. A period of several weeks is usually needed for restoration of mobility in muscle that shows moderate tightness, but with needling treatments, this restoration can be achieved in a few days.
Individuals who spend most of the day in sedentary postures may develop adaptive shortening in the one-joint hip flexors (iliopsoas). Prolonged sitting with the knees partially extended places the foot in a position of plantar flexion and may result in adaptive shortening of the soleus. Wearing high-heeled shoes much of the time may also lead to the development of adaptive shortening of the soleus. Adaptive shortening of postural muscles can affect both balance and alignment. When adaptive shortening continues too long, it may become an irreversible histologic contracture.
Endogenous contracture is the commonest symptom that affects performance in sports, but it is ignored by clinicians in sports medicine. Needling is the most effective therapy for preventing and treating contracture. Detailed knowledge of the molecular mechanisms of contracture and its contribution to trigger-point formation is indispensable for clinicians, and readers are referred to a review and other new materials.2
Stiffness is another form of physiologic tension of skeletal muscle. A stiff muscle resists passive movement. Stiffness can be measured by examining the distance moved between origin and insertion during passive movement.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.