If they arent new boots I doubt that's the problem. It's probably due to you just jumping around where as on real terrain you jump and land and then there is time where you have to cruise to the next jump or ride the lift much more time for your body to rest. It's like lifting a 20 lb dumbell once every 30 min it's easy and light. Or trying to lift it 500 times every thirty minutes your bicep would get real torn up and cramped. not that the motion is different or the weights heavier just volume.