First, do no harm. Talk with your doctor about which activities are safe. You want to get the benefits of aerobic exercise without aggravating your injury or creating a new one. Swimming and pool running, for instance, are typically safe with IT band syndrome, Achilles' tendon strains and stress fractures. Stationary bikes are often safe with shin splints, plantar fasciitis and IT band syndrome, as long as you remain seated.
Let pain be your guide. If any cross-training activity hurts, stop right away. If you feel pain afterward, talk to your doctor before doing it again. Popping over-the-counter painkillers is not wise, as they may allow you to aggravate your injury.
Find probable cause. Often injuries recur because rehab plans focus on vanquishing symptoms, not correcting the cause, said Adam Tenforde, a sports medicine doctor at Harvard. "Steroid injections, heat, ice and ultrasound might feel good at the time," he said, "but they're not going to translate to a longer-term change." Work with a physical therapist to identify strength, flexibility, biomechanical or training issues that led to the injury, and devise a plan to correct them. See a sports dietitian to determine whether a nutrient deficiency played a role.
Mimic your regular workouts. If you get your doctor's green light, try to replicate the workouts you'd do in your regular routine. You can do intervals of vigorous running in the pool or on the elliptical, which can also help you stay focused and break up the monotony.
Eat well. Many athletes restrict calories in an effort to avoid weight gain during their layoff. That's not wise, said Dr. Stephen Pribut, a Washington podiatrist, especially if a nutrient deficiency contributed to the injury. Although you may have to cut calories a little if you drastically cut your activity, if you're maintaining a workout routine with cross-training, you shouldn't have to worry about weight gain. Pribut recommends that those with stress fractures get extra calcium and vitamin D, plus plenty of omega-3 fatty acids, which have been linked to lower inflammation.