text: I would like to throw in my two cents to this question. First, in order for actin and myosin to attract, calcium is required. Where does it come from? Well to start a muscle contraction, the neurotransmitter ach must be present at the myoneural junction. Now an action potential will travel down the t-tubules and open gates that allows calcium to leave the sarcoplasmic reticulum . The calcium stored there simply diffuses down its own concentration gradient. Now for the muscle to relax the calcium must be re-stored in the sarcoplasmic reticulum, now it is against its concentration gradient, and it takes ATP energy to put it back. So to answer your question it takes ATP energy to cause muscle contraction and it takes ATP energy to cause muscles to relax. In fact as ATP stores deplete, it takes muscles a longer period of time to relax. I have actually measured this.