Microscopic View of Stretching

Microscopic View of Stretching

Ever wonder exactly what happens inside your muscles when you stretch? Does the muscle tissue simply stretch like a rubber band, or is something more involved? To better understand what happens, consider the basic functional unit of muscle, the sarcomere.

Muscle fibrils can change length because they are constructed of overlapping strands of protein polymers called actin (the thin strands) and myosin (the thicker strands). The "boundaries" of the sarcomere are called "Z discs," and that is where the actin filaments are attached. In the center of the sarcomere are the myosin strands, which during contraction essentially "pull" the Z discs closer together by attaching to the actin filaments with specialized heads called "cross bridges." These cross bridges function much like oars as they reach out, attach, and pull on the actin filaments, causing the Z discs to move toward one another.

When you stretch a muscle, the opposite occurs. During the stretch, the fibers elongate as each sarcomere extends to the point where no overlap between the thick and thin filaments exists at all (specialized elastic filaments called titin keep the sarcomere together in the absence of overlap). At this point, the remaining stress is taken up by the surrounding connective tissue (sarcoplasmic reticulum, sarcolemma and endomesium).

If the stretch tension continues to escalate beyond this point, microscopic tears develop both in the connective tissues and within the sarcomere itself. Such micro traumatic injuries eventually heal, but at the cost of "scarring" and micro-adhesions that may leave the muscle fiber less capable of maximal contraction as well as full-range movement.

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