It would depend on the injury. I would do an assessment or an evaluation on where the knee hurts. If the knee hurts where the petela tendon is I would not do any high impact. If the client is able to walk slow down the belt on the treadmill and increase grade to keep the intensity all relevant. If the outside of the knee is hurt, maybe have them do a gait analysis to see if they are over pronating. If they are maybe they have too high of an arch in their shoes. If you have any questions just ask.