Antagonistic muscles

Skeletal muscles usually work in pairs. When one contracts the other relaxes and vice versa. Pairs of muscles that work like this are called antagonistic muscles. For example the muscles in the upper forearm are the biceps and triceps (see diagram 7.3). Together they bend the elbow.

When the biceps contracts (and the triceps relaxes) the lower forearm is raised and the angle of the joint is reduced. This kind of movement is called flexion. When the triceps is contracted (and the biceps relaxes), the angle of the elbow increases. The term for this movement is extension.

When you or animals contract skeletal muscle it is a voluntary action. For example, you make a conscious decision to walk across the room, raise the spoon to your mouth or smile. There is however, another way in which contraction of muscles attached to the skeleton happens that is not under voluntary control. This is during a reflex action, such as jerking your hand away from the hot stove you have touched by accident.