The AP are caused by the constant stimuli they are receiving (i.e. the sexual stimulation).
During an Action
Potential, there is an influx of
Calcium ions pushed into the muscle fibers which allows them to contract. In order for them to relax back to normal, the free Calcium must be sequestered or taken back up. However, due to the repetitive stimuli, the Ca does not have enough time to be taken back up. In this case, temporal summation occurs.