Any time you exercise, your putting alot of stress on your body. Its a work out. It has been shown that running can make you short though. Due to running on a had surface like pavement, it really does a thing on your knees. As for dangerous exercises...try curling a rabid monkey. Their is always danger in doing things. There is always a chance of pulling a muscle or bracking a bone. Long term problems are out weighed by the good. Trust me. -J