A muscle in a state of contraction that opposes the action of another muscle termed the antagonist. It is the prime mover or muscle directly responsible for a change in position of a part of the body. It can also be defined as one member of a group of muscles whose line of application produces a given moment at a joint.
See Antagonist muscle, Central pattern generator (CPG), Isotonic contraction, Moment of force, Monosynaptic and polysynaptic reflexes, Primary motor cortex, Reflex