Muscle pairs: Muscles are grouped together in pairs on your skeleton
Relaxed or contracted: When one muscle of a pair contracts, the other relaxes
Skeletal muscles only pull in one direction. For this reason they always come in pairs. When one muscle in a pair contracts, to bend a joint for example, its counterpart then contracts and pulls in the opposite direction to straighten the joint out again. Without this arrangement you wouldn't be able to straighten your legs when you walk or bend your fingers to grip something.
When your biceps muscle in your upper arm contracts, it pulls your lower arm in towards your shoulder. However, when it relaxes, your biceps cannot push your arm back out. To do this, your triceps muscle, on the underside of your upper arm, contracts and straightens your arm out. If your triceps muscle wasn't there, your arm would stay drawn in permanently.
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