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. the proteins in the myofibrils become denatured, preventing relaxation. b. a lack of calcium causes the sarcomeres to lock together, preventing relaxation. c. the motor neuron does not send a signal, so the muscle cannot relax. d. myosin and actin do not fully detach, so the muscle cannot relax.