What happens to one muscle when the other relaxes?

When one muscle relaxes, the opposite muscle contracts. This is known as reciprocal inhibition. It occurs when the motor neurons (nerve cells) that innervate one muscle send inhibitory signals to the motor neurons that innervate the opposing muscle. This causes the opposing muscle to relax.

For example, when you flex your bicep muscle, the tricep muscle relaxes. This is because the motor neurons that innervate the bicep muscle send inhibitory signals to the motor neurons that innervate the tricep muscle. This causes the tricep muscle to relax and allows the bicep muscle to contract.

Reciprocal inhibition is important for coordinated movement. It ensures that muscles work in pairs to produce smooth and efficient movements.