Which process must occur for a muscle to be able to relax?

The process of muscle relaxation begins when signaling from the motor neuron stops. This event causes repolarization of the sarcolemma and T-tubules and closes the voltage-gated calcium channels in the sarcoplasmic reticulum (SR). As a result, Ca+ ions are then pumped back into the SR, which causes the tropomyosin to re-cover the binding sites on the actin strands.  

When the amount of ATP runs out, the muscle also relaxes.

but there is no

i guess you forgot putting it.