Why do the biceps and triceps have to work as a pair to bend and straighten your arm?
A muscle can only actively shorten and pull — it cannot push.
A muscle can only actively shorten and pull — it cannot push. So to move a joint both ways, muscles are arranged in antagonistic pairs. To bend (flex) the elbow, the biceps contracts while the triceps relaxes; to straighten (extend) it, the triceps contracts while the biceps relaxes. One muscle undoes the other's action, which is why you need at least two opposing muscles per movable joint.
Key point
Muscles only pull, so antagonistic pairs (biceps/triceps) move a joint in opposite directions.
Common mistake
thinking a single muscle can both flex and extend a limb.
Memory tip
muscles pull, never push — so they come in opposing pairs.
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