you must be kidding dude....in no way does your lower back come in danger with this unless your stomach is as weak as an infants....the hammer bounces off the tire so theres no extreme low back stress besides that it's a 16 pound sledge at most what the fuck...i mean if thats too much for you you could go to a halloween store and get yourself a plastic sledge hammer and then stand in front of a mirror and pound your head with it for 5 sets of 10 reps each side
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"To tell you the truth I can beat anyone in the world" -Bruce Lee
Been doing sledgework for a year.One of the best power/speed workout with conditioning there is.If you do it hard enough you can get your back and legs into it, which is better.