content: Combination of that and hyperextending his legs during lockout. A correctly locked out leg can usually support sufficient surplus over the muscle's yield strength such that health and safety of the knee joint should be a distant concern compared to ripping or tearing the quads. If, however, you push the weight to lockout in an uncontrolled manner and accidentally hyperextend the knee (like this guy)... well, the video says it all. I hope for his sake that amputation wasn't necessary after that bad break.