antagonist.
ATP
The bicep muscle. The tricep contracts to straighten the arm as the bicep relaxes.
Relaxes
contract
the smooth muscle
gdg
I think what your after is called an antagonistic pair of muscles, for example the bicep and the tricep. When the bicep is contracting the tricep relaxes and when the tricep is contracting the bicep relaxes.
It Contracts
relaxes
It relaxes
When one muscle contracts, its opposing muscle relaxes in a process known as reciprocal inhibition. For example, when the biceps muscle contracts to flex the elbow, the triceps muscle relaxes to allow that movement to occur smoothly. This coordination between agonist and antagonist muscles helps maintain balance and control in movement.
A muscle that contracts shortens whereas a muscle that relaxes lengthens.