"I have a bad back and/or knees."
Don't let an injury totally sideline you: Experts say you need to build up the area around your problem spot. "If it's your back, you
work your abs as well as the muscles that laterally bend and rotate the spine," says celebrity trainer Gunnar Peterson. Check with your doctor or physical therapist first, but for a low-impact cardio workout try indoor cycling or swimming. Or work your core with yoga or Pilates.