antagonistic muscles

Even simple movements such as bending and straightening the arm require muscle pairs to contract and relax synchronously.

In the body, a pair of muscles working together to allow coordinated movement of the skeletal joints. Muscles can only exert a force and do work by contracting. An example is the antagonistic pair of muscles used in bending and straightening the arm. To bend the arm requires one set of muscles - the biceps - to contract, while another set - the triceps - relaxes. If the arm is to be straightened, the reverse happens. The individual components of antagonistic pairs can be classified into extensors (muscles that straighten a limb) and flexors (muscles that bend a limb).