They're divided lengthwise into segments called sarcomeres, which contain two even tinier strands of protein — two different kinds of myofilaments called actin and myosin.
Sarcomeres are added and subtracted to muscles depending on how frequently they're used, so improving overall flexibility requires a comprehensive stretching regimen.
A sarcomere contains both thin filaments, made up mostly of two light and twisty actin strands, and thick filaments, composed of thicker, lumpy-looking myosin strands.
Each sarcomere is separated by what's known as a Z line at either end, which is just a border formed by alternating thin filaments in a kind of zig zag pattern.
Sarcomere's long, fibrous proteins can relax to elongate muscle fibers or they can contract to shorten them— pulling on tendons and protective tissue to create the force propelling our athlete's body.
Immediately after a muscle fiber is stimulated by a nerve — when calcium ions are flooding into the sarcomeres to pull away those two protein bodyguards of tropomyosin and troponin from the actin — that's called the latent period.