Ankle Injuries
An ankle sprain can take you out of commission in several sports and activities. You can keep fit by doing other activities, such as rowing, that rely on the upper body and do not stress the ankle.

"And there's no reason why you can't do upper leg exercise like the leg curl or leg extension," says John Jay Wooldridge, a Reebok Master Trainer. Just make sure there is no stress on your ankle.

Knee Injuries
Knee injuries can be very serious, and trying to keep going through a knee injury could make it much worse. Talk to your doctor or trainer to find out if cycling or riding the recumbent bike (which takes the weight off your legs) will exacerbate your injury. If not, these might be good replacement activities. If your knee injury is more serious than that, Cotton recommends swimming with a pull buoy to support your legs. "It's an excellent aerobic workout," he says.

Pulled Muscles
Trying to play through a pulled muscle or tendon is tempting, because once your muscles warm up, you may not feel the injury as much.

"In many cases, an athlete will feel a muscle injury more while at rest," Wooldridge says. Hamstring injuries are especially common, often caused by rapid deceleration or change of direction. If you pull a muscle, Wooldridge says, you need to let it rest and follow the RICE (rest, ice, compression, elevation) method.

One way to work up a sweat using only upper body muscles is to toss around a medicine ball with a partner for a while. Try it for 20 or 30 minutes just tossing it back and forth, not running around at all. "You'll get a great workout using your abs and shoulder muscles," Wooldridge says.

Achilles Tendinopathy
An aching Achilles tendon has been the downfall of many athletes. So, if you feel pain, do not ignore it. Take a break from all sports that exacerbate it, especially running or activities that involve running. Using a rowing machine is a good way to stay fit while resting your Achilles tendon. You may be able to use an elliptical trainer as well.

Tennis Elbow
Players of racquet sports are familiar with elbow pain. If a case of tennis elbow starts to interfere with your game, take time off to let the injury heal. You can keep running and doing interval training, including plenty of short bursts of speed and side-to-side movements to stay in top tennis shape.

Shoulder Injuries
Hurting your shoulder can interfere with your ability to participate in a host of popular recreational sports. You do not want to serve, swim, swing, or throw with a shoulder injury. With particularly painful shoulder injuries, even the arm-swinging inherent to running can be too much to take.

"Avoid overhead movements and any arm position that causes pain," Cotton says. And make sure you get professional guidance before picking up a racquet or swimming again. Do as much lower-body weight training as you can handle. Try running, cycling, stair climbing, and even rowing.

When You Can't Run

Running is the foundation of many fitness programs, because it produces such effective cardiovascular results. But it is also an activity that is very likely to be hampered by a sports injury. It's impossible to mimic the effects of running, but you can come close by using a stair climber or elliptical trainer.

"Recent research shows that women on a stair-climbing program stayed in as good shape as women on a running program," Cotton explains. Stair climbers and elliptical trainers eliminate the repetitive pounding of running.

Wooldridge recommends circuit training for runners who are taking time off the road. Have 10 or 12 weight machines that you want to use, and go through them in a regular pattern. Do one set on each machine with light to moderate weight (less weight than you would usually use).

"Move as fast as you can between each station, and go through the whole circuit four or five times." Wooldridge recommends. "It will take almost an hour, and you'll get a great workout."