Question

When a muscle cell contracts, the intracellular calcium concentration decreases the myosin heads produce a single power stroke the position of each myosin head is identical at any give moment some myosin heads?

Answer 1

When a muscle cell contracts some myosin heads bind to actin molecules as other myosin heads release from actin molecules. 
Muscle contraction occurs when the thick filament and the thin filaments slide over one another in a series of repetitive events.