Ever wonder exactly
what happens inside your muscles when you stretch? Does the muscle
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.
Read more about microscopic view of stretching ...
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