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.

answer; ///(b) it is composed of four protein chains and four heme groups;

enzyme active site and substrate specificity

enzymes bind with chemical reactants called substrates. there may be one or more substrates for each type of enzyme, depending on the particular chemical reaction. in some reactions, a single-reactant substrate is broken down into multiple products