I'm breathing shallow just thinking about cracked ribs. When I have had rib injuries that have forced me out of my regular exercise routines, I have turned to the leg extension machine for a safe, relatively pain-free way to keep some leg tone. If you do it without breaks (alternate legs), then it is a bit like riding a trainer. Yes, that means it is boring, but at least you can hear the music better on a leg machine than you can on a trainer.