Continuing to exercise during your recovery from injury is critical for your physical and mental health, however, it needs to be done in a fashion that facilitates, not impedes, your healing and return to full activity.
The guiding principles for cross-training around your injury during your recovery include: 1) avoid specific activities that caused you pain before your treatment until you are fully recovered, 2) as you progress, proceed gradually and test each new movement or exercise. For example, if your knee injury has recovered enough to walk for exercise, start by walking a quarter of a mile every other day. If after a two or three sessions your knee feels ok, increase to a half of a mile every other day. Once your are up to two miles a day of fast walking, if your goal is to return to running, start with a quarter of mile easy jog and gradually build distance and then speed, resting every other day. If at any time you feel the injury site having anything other than normal workout soreness, significantly reduce your distance and intensity. Remember, even after the best treatments or surgeries, significant injuries still take 3 - 4 months for initial healing to occur. Treated injuries continue to remodel and heal over the next 9 months to a year.