What Causes Numbness In Leg While Sperm Comes Out?

when having sex at the time of climax the legs go numb lock up an straighten out ,thesperm comes out but its not felt cause all rthe numbness an stiff ness ,,the legs involuntarily straighten out an wont bend untilitits over ,,at night while aleepin the legs dothe same but thwy curl in instead of extending out involuntarily ,,do you know what the dignosis is for this or what symptoms are these interms of diseae