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. Muscle contraction is often followed by muscle relaxation, when contracted muscles return to their normal state. Renowned expert Dr David Behm explains Myofilament Cross-bridge Kinetics, force-length and force-velocity relationships, electromechanical delay, and the stretch-shortening cycle.