From my research it's only a handful of people over the 200 centers and 10+ years. I would like to know how many though - it's an important stat. And it's horrible for those it happened to. And people certainly fracture during other forms of weight training and even yoga and pilates. But no exercise, nor anything in life, is 100% risk-free. It seems OsteoStrong does have a system for making it incredibly safe to put incredible amounts of force (up to several multiples of bodyweight) on bones.