Muscle contraction is the tightening, shortening, or lengthening of muscles when you do some activity. It can happen when you hold or pick up something, or when you stretch or exercise with weights.

slow-twitch muscle fibers, which move more slowly but help to keep you moving longer fast-twitch muscle fibers, which help you move faster, but for shorter periods “Twitch” refers to the contraction, ...

Human movement is reciprocal in nature: Opposing muscle groups must coordinate for proper movement. Muscle length and strength between opposing muscle groups need to be in balance for normal movement ...