I think it must be a scaling problem ,I don't have scaled exact enough,but when in a run the calf bone is not staying exactly with the thigh (knee end) properly it moves a bit so I was thinking maybe it needed to be constrained to the nee end and instead of just thigh,that maybe there was a way to use the constraint as a aim at two one to the thigh nee end and the other to the foot,allthough the bvh call the calf the shin.But on looking at it I think it could just be a scaling problem