Injuries upset competitive athletes because they know their competitors are still training. They can maintain fitness by using a training technique called cross transference, and so can you. Exercising one leg or arm helps to maintain strength, endurance and power in the other limb. The muscles in the injured limb are not strengthened directly because they are not being used. Cross transference strengthens nerves in both limbs, even though only one is being exercised (Journal of Applied Physiology, November 2005). Each muscle is made of millions of fibers, and each fiber is stimulated by a single nerve.

When you exercise, your brain sends messages along these nerves, telling only about five percent of the nerves to contract at the same time. With training, your brain learns to contract a greater percentage of muscle fibers simultaneously. The more you practice a specific exercise, the greater percentage of your muscle fibers you can contract at the same time. When you stop exercising, your brain quickly loses its ability to contract as many fibers at the same time and you lose strength, endurance and coordination. However, if you continue to exercise one arm, your brain retains its ability to contract the fibers in the opposite arm. This concept applies only to opposite limbs; you can maintain strength in an injured arm by continuing to exercise the uninjured one, but exercising your legs will not strengthen your arms and vice versa. So if you are a runner who injures a leg muscle, you can work the uninjured leg on resistance machines to keep up the strength of both legs. If you are a baseball pitcher, you can help to maintain strength in an injured arm by using your other arm to throw and do resistance exercises.