Hi LeAnn! Perhaps it is due to striving for balance. Many times you don't realize how hard you tense your feet when working on maintaining your balance during lower body exercises. The feet will literally cramp and or tense up in attempt to maintain balance. Also, if you feel this could be the case, try not to curl your toes so hard while fighting for balance. Just being aware of this on your next workout should help tremendously. Good Luck!