A muscle contraction (also known as a muscle twitch or simply twitch) occurs when a muscle cell (called a muscle fiber) lengthens or shortens. Locomotion in most higher animals is possible only through the repeated contraction of many muscles at the correct times. Contraction is controlled by the central nervous system comprised of brain and spinal cord. The brain controls voluntary muscle contractions, while the spine controls involuntary reflexes.
Though the term 'contraction' implies a shortening or reduction, when used as a scientific term referring to the muscular system, contraction refers to the generation of force by muscle fibers. During muscular contraction, a muscle's length may decrease, increase or remain the same.