I have a client whose right leg is injured. He is wearing an orthopedic boot and has been advised by a physical therapist to strength train the left leg only until the right leg recovers. The belief is, once the right leg has recovered from the injury, the right leg will catch up in strength quicker if he has been strength training the left leg all along. I am interested in finding out if any research has been done on this training approach.
A number of research studies have been conducted that conclude strength training one leg can have a crossover effect to the opposite leg. This is helpful to know when recommending exercise to a client with a single leg injury. The reason that training one leg affects the other is because the body’s nervous system can learn to engage stronger movements from one leg and send that same message to the other leg. Just how much of a crossover effect exists is not exactly known, but nonetheless, the injured leg will be able to “catch up” in strength more quickly if you continue to train the uninjured leg during the recovery period.
One interesting factor to be aware of is that strength training one’s dominant leg may have less of a crossover effect to the non-dominant leg as opposed to the crossover from the non-dominant to the dominant leg. It is theorized that this is because strength gains in one’s non-dominant leg would be greater and would thus have more of a crossover effect to the dominant leg.
An ankle injury rehabilitation trial was conducted by sportsinjurybulletin.com involving 20 individuals, half who trained the dominant side and half who trained the non-dominant side. They found that, on average, subjects who trained the dominant leg only increased peak torque values by 8.5 percent in the trained ankle and by 1.5 percent in the untrained ankle. Subjects who trained the non-dominant leg only increased peak torque values by 9.3 percent in the trained leg and 3.5 percent in the untrained leg. This supports the above theory of a greater crossover effect from non-dominant to dominant. This study did not require each subject to have the unused ankle immobilized, as would be the case for a person with a serious injury. The crossover effect may be more or less for people with injuries, depending on the body’s response to training.
Numerous other studies have been done showing a minimal crossover effect of increased strength in one leg by training the other one. The studies I’ve seen reported a 10 to 15 percent crossover effect. In conclusion, training the uninjured leg will help the injured leg regain its strength more quickly once it is healed. Of course, pool workouts and/or physical therapy exercises for the injured leg should also be incorporated as soon as possible, pending approval by the client’s doctor.