Bike, Jump rope, Swim. There's also the "arm bike" which you could find in any physical therapy shop...doesn't do much for the entire body but will keep your upper body cardio intact Anything of a metabolic nature. You don't necessarily have to be on your feet to maintain endurance although not being on your feet presents a new challenge. I would imagine he's doing a lot of trampoline and band work (elastic) for the knee and then primarily core, and hip work so that he's not starting from ZERO when he is cleared to run and cut. Obviously you don't need a knee to do flys, bench, dumbell press etc etc etc so he should be ok as far as upper body strength is concerned. His biggest challenge, a second OBVIOUSLY will be the analysis of the game. Someone mentioned ODB but ODB was used to lining up everywhere so that helped his comprehension. Not saying Perriman can't have a similar story, but I would have to think that that had something to do with his success