non of these answer makes sense, and there a few minor errors in them. i know that action potential may keep on firing continuously down the axon. one reason may be because acetylcholinesterase is not coming to the rescue and cleaning up all the acetylcholine in the synaptic cleft. but that only happen usually if there are drugs involve. so in term of natural muscle cramps, it really does not make any sense what so ever.
a really good reason is mainly because of lack of potassium. the function of potassium inside the muscle cell is to repolarize the membrane. however if there are not enough potassium, the time it takes for it to repolarize is pretty damn slow. on the other hand, sodium is used to depolarize the membrane. As a result, since there are an unequal distribution of sodium to potassium, less potassium is flowing out the membrane while more sodium will flow in the membrane causing it to depolarize faster than it suppose to repolarize. since the inside of the cell is becoming more positive with the help of sodium and the outside of the cell is becoming negative with the help of potassium, the inside of the cell is pulling back the potassium. but again, there is an unequal distribution of potassium compare to sodium so it will never achieve that electrochemical equilubrium (resting membrane potential). since electrochemical equilibrium can not be achieve, sodium is flowing inside the membrane causing it depolarize across the membrane at a rapid rate forming an unfused tentanus at a rapid rate.
the rate in which this is happening is so fast that it wont let the myosin head detach from actin for a long period of time, so it continuously making the sacromere shorter and shorter causing a pain receptor to travel toward the CNS. these is my hypothesis.