Muscle Concepts

Functional Characteristics

Irritability — Muscle tissue is responsive to neurons. Neurons control functional muscle contraction.

Contractility — Due to the interaction of critical proteins, muscle is able to actively shorten its resting length. Generally, muscle can shorten to approximately half its resting length.

Extensibility — Generally, muscle can be lengthened up to one half its resting length.

Elasticity — When a lengthening force is relieved muscle passively returns to its resting length. The force of elasticity is directly proportional to the extent of lengthening. The force of contractility and the force of elasticity summate to produce forcible movement.

Descriptive Terms

Excursion — The distance between maximum lengthening and maximum contraction. The excursion ratio from maximal lengthening to maximum shortening is typically 2:1. In muscles that only cross one joint there is typically more excursion than range of motion, allowing for full joint range of motion.

Passive Insufficiency — In muscles that cross multiple joints there is typically not enough excursion for the muscle to be simultaneously stretched over all crossed joints.

Active Insufficiency — This occurs when a multi-joint muscle shortens across all joints simultaneously, reducing its ability to generate optimal tension and force.

Muscle Activity

Isotonic (concentric) Activity — This activity is characterized by muscle contracting across a joint resulting in joint motion.

Isometric Activity — This occurs as the muscle actively builds tension but its length does not change and there is no net joint movement. Isometric activity occurs when a muscle is unable to overcome an internal or external force.

Eccentric Activity — This occurs when the muscle is acting as the resistor of motion such as when resisting gravity of the forearm moving from full flexion to extension in standing. Eccentric activity is a controlled relaxation of contractile force.