Amuscle cramp is a sudden, painful contraction of a muscle, during which movement is difficult. what is happening at the cellular level during a muscle cramp? a.   a lack of calcium causes the sarcomeres to lock together, preventing relaxation. b.   myosin and actin do not fully detach, so the muscle cannot relax. c.   the proteins in the myofibrils become denatured, preventing relaxation. d.   the motor neuron does not send a signal, so the muscle cannot relax.