Muscles and Bones · Muscles
Because muscles only pull, they must work in opposition.
Bend your elbow: the biceps pulls. Straighten it: the biceps can't push you back, so the triceps on the other side pulls instead. Neither muscle can undo its own work — they take turns.
There's more: because muscles only pull, they must work in opposition. The full explanation — and the interactive version you can tap and quiz yourself on — lives in the app.
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