It has something to do with a sliding interaction between actin and myosin. In 1954, scientists published two groundbreaking papers describing the molecular basis of muscle contraction.

seen as a cross-bridge between actin and myosin filaments in muscle, acts as the motor domain of the molecule and pulls actin filaments along by repetitive conformational changes coupled to ATP ...

This occurs through a physiologic process that leads to an increased number of contractile proteins (actin and myosin) in each muscle fiber. With the right training regimen, you can catalyze this ...

When stimulated by the autonomic nervous system to contract from the release of hormones or other chemical signals, smooth muscle cells shorten via the movement of actin and myosin myofilaments.