Sherrington's law of reciprocal innervation

Sherrington's law of reciprocal innervation, also called Sherrington's law II explains how a muscle will relax when its opposite muscle (e.g., biceps/triceps) is activated. René Descartes had hypothesized as much in 1626.

Sherrington's law of reciprocal innervation states that: When a muscle contracts, its direct antagonist relaxes to an equal extent allowing smooth movement.

...
Wikipedia