If you are injured or recovering from almost any type of injury, running in a pool is a wonderful cross-training activity. You can wear a specially designed belt that allows you to move your arms and legs just as if you were running on land. Or, if you don't have a belt, you can use a kickboard. Place the kickboard under your chest and wrap your arms under it. Your feet will hang down into the water. Use a normal running motion to propel yourself forward. The kickboard version of pool running is less aerobic than using a belt, but you will still work your leg muscles.