Skeletal muscles produce motion by pulling on the bones. Although a few motions can be related to a single muscle, the majority of bodily movements are accomplished by several muscles working in concert. The primer mover, or agonist, is the major muscle which creates the impetus and power of the movement. Subsidiary muscles that assist and fine tune the motion are called synergists. When the agonist and its synergists contract, the opposing muscle, or antagonist, has to relax and stretch to allow the joint to move. Once the motion has occurred, it can be reversed by reversing the roles of the muscles; the same muscle that was the antagonist now contracts and acts as the agonist, while the original agonist relaxes and acts as the antagonist.
This system allow the joint to move in both directions. A straightforward example is the biceps and triceps in the upper arm. When the bicep contracts and the tricep relaxes, the arm flexes (bends) at the elbow. When the bicep relaxes and the tricep contracts the elbow straightens.