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.
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.
Sarcomeres are added and subtracted to muscles depending on how frequently they're used, so improving overall flexibility requires a comprehensive stretching regimen.
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.
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.