A muscle 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.
