Complex sentence: In skeletal muscle, adenosine triphosphate (ATP) must attach to the myosin heads for them to disassociate from the actin and allow relaxation; the absence of ATP in sufficient quantities means that the myosin heads remains attached to actin.
Split into simpler sentences: In skeletal muscle, adenosine triphosphate (ATP) must be present in sufficient quantities to allow both contraction and relaxation. After contraction, ATP must attach to the myosin heads for them to disassociate from the actin and allow relaxation; the absence of ATP in sufficient quantities means that the myosin heads remains attached to actin.