He's simply reacting to an unpleasant or even painful sensation in the only way he knows how. (nehumanesociety.org)
Don't forget to pay attention to your body while practicing yoga — if you feel painful sensations, don't continue without consulting your doctor or another trusted professional. (lovingmomentsbras.com)
All of a sudden I felt a pain, not a contraction, but a pulling and moderately painful sensation. (chabad.org)