The contraction of a muscle fiber in response to a single nerve impulse is called a twitch. Under a given set of starting conditions, the force of a single fiber’s twitch is fixed and the strength of a muscular contraction is therefore determined by the number of muscle fibers contracting at the same time. Most skeletal muscle contraction is under voluntary control of the central nervous system.

Muscle contraction thus results from the simultaneous shortening of all the sarcomeres in all the activated muscle fibers. It is brought about by the increase in overlap between the thick and thin filaments within each sarcomere. The increase in overlap is accomplished by a cycle of making and breaking crossbridge linkages between the thick and thin filaments.

The head groups of the myosin molecules alternately flex and extend to interact with successive actin subunits on the thin filaments, which are brought progressively closer to the opposite Z band.

This “rowing” action slides the thin filaments past the thick filaments, narrowing the I band. As the ends of the actin filaments get closer to the M band, the I band appears denser and the H zone becomes narrower. The force of the contraction depends on the number of crossbridges linking the thick and the thin filaments at the same time.

Muscle relaxation occurs when the crossbridge linkages are broken, allowing the thick and thin filaments to slide in the reverse direction. The elastic properties of the muscle and the tension on the ends of the muscle (e.g., due to the weight of the limb) determine the muscle length during relaxation.