Flexion of a limb or part beyond its normal range.
hyperextension
A volleyball player's wrist snaps forward after setting a ball.
The prefix would most likely be 'hyper-' and could include: hyperextension or hyperflexion.
Well if hyperextension is a type of extension then the opposite of extension is flexion and I'm not sure you can get hyperflexion, so it's probably just flexion.