The anatomy and physiology of human muscle is fascinating. It is very complex, yet simple. An example of this is Sherrington’s law of reciprocal inhibition. This law states that when a muscle is activated, or contracting, it opposing muscle will relax.
Imagine you are doing a bicep curl, or simple lifting a water glass to your mouth. If the bicep is contracting, trying to give you elbow flexion, the opposing muscle (triceps) must relax the same amount. If the triceps did not relax, the elbow joint would be in a tug-of-war with one muscle trying to flex and the other muscle trying to extend the joint.
This is all carried out by the central nervous system and is done without thought or intent. It allows for smooth and coordinated movement in the human body.