Biker joke

A professor at University of Minnesota was giving a lecture on "Involuntary Muscular Contractions" to his first year medical students.

Realizing that this was not the most riveting subject, the professor
decided to lighten the mood slightly.

He pointed to a young woman in the front row and said,
"Do you know what your ass hole is doing while you're having an orgasm?"

She replied, "Probably at a biker bar hanging out with his buddies."

The professor laughed so hard he could not continue with the class.