Page 4 - CH-8-8.5 MUSCLES

of a pair contracts, then the other muscle of a pair is relaxed. A

contracted muscle can come back to its original position only when the
other muscle of the pair pulls it by contracting itself.
 For example, when you raise your lower arm, some muscles in the
upper arm contract and pull the lower arm upwards. At the same time,
other muscles of the upper arm relax. When you straighten the lower
arm again, the opposite happens.
 Thus, the muscles and bones work together to help in the movement of
the body.

Working of muscles in pairs