Okay, yeah, it looks a little silly. But unlike the leg curl machine, a hamstring curl on a physio ball takes a lot of stress off your knees, and forces you to coordinate your upper and lower body as you hammer your hamstrings—without putting any major joints at risk. It’s a great finishing move for hamstrings, and the physio ball variant involves the glutes enough that makes it a great assistance move for bigger lifts like the deadlift.