Muscles contract or relax when they receive signals from

motor neuron or the nervous system

The contraction and relaxation of our skeletal muscles which move our body are caused by the messages or signals from our nervous system.

The whole process  called the mechanism of muscle contraction involves three steps:

(1) the nervous system sends a message to the muscular system (resulting in chemical reactions).

(2) muscle fibers contracts

(3) muscle relaxes , when there is no more signal from  nervous system.