Today I was doing natural glute/ham raises with my knees on a couple of pillows and my brother holding my feet (first time trying this- did a variation i saw in an article somewhere on T-Nation where you push yourself up enough to do the rep) during the third rep both of my calves cramped up extremely hard. The pain was very intense and i had to quit the set. My calves stayed cramped for about 5 seconds then unclenched but were and still are rather sore. Anyone else ever experience or know why it happened- flexibility problem, form issue, etc. thanks a lot.