Involuntary Muscle Contraction

Professor Higgins at the University of Sydney was giving a lecture on 'Involuntary Muscle Contraction' to the first year medical students.
This was not an exciting subject and the professor decided to lighten up the mood.
He pointed to a young woman in the front row and asked, 'Do you know what your arsehole is doing while you're having an orgasm?'
She replied, 'Probably out on his bike with his mates.'
It took 45 minutes to restore order in the classroom!