Muscles work in pairs, with one contracting while the opposing muscle relaxes again.

The longer version

Skeletal muscle can only pull, never push, so every movement needs at least two opposing groups. When one contracts and shortens, the other relaxes and lengthens, and the bone between them rotates around a joint.

The contracting side is called the agonist and its partner the antagonist, and the two swap roles depending on the direction of the movement. Signals from the spinal cord and brain coordinate the timing so the resistance does not fight the motion. This arrangement also gives joints a braking system, since the opposing group can slow the motion as it lengthens. Other muscles often join in as stabilisers, holding nearby joints steady while the main pair does the work.

Where this entry sits

These facts look at how the human body works in ordinary moments, from breathing and blinking to balance and sleep. They focus on anatomy, senses, and small reactions that happen constantly without any conscious effort. The full list sits on the Human Body page, where every entry is listed in order.