Dear Doctor:

My son, who plays several sports, recently broke his right arm. Now he’s worried that the muscles will waste away while it heals. I read that exercising his left arm could help his broken arm. When I told him this, he scoffed. It does seem crazy, but is it true?

Dear Reader: Improbable as it sounds, a new study found that when a limb on one side of the body is immobilized, as in the case of your son’s broken arm, it’s possible to preserve muscle strength and size by exercising the corresponding limb on the other side of the body.

This is a phenomenon known as “cross education,” which has been written about since at least 1894. Since then, a number of studies have backed up the existence of cross education. Whether through voluntary muscle contractions through exercise, imagined contractions via targeted visualization or contractions that were electrically stimulated, activating the muscles of the opposite limb preserved both strength and muscle mass in the immobilized body part.

In this newest study, researchers in Canada studied the forearms of 16 male and female college students for four weeks. All were right-handed, and none had recent experience with resistance training. Using CT scans, ultrasound and a weight machine, the researchers measured the dimensions and strength of the muscles in the participants’ forearms. The students were then fitted with casts on their left forearms to immobilize their wrists, hands, thumbs and fingers up to the knuckles. Each was randomly assigned to either a training group or to a control group. The training group took exercise classes three times a week that focused on certain muscle groups in the wrist, hand and forearm of the free limb. The control group did not take part in those classes and was asked to refrain from any other exercise as well.

A month later, after the casts were removed, measurements showed that participants in the control group had lost about 20 percent of the strength in their left forearms and had lost 3 percent of their muscle mass. For the participants who had exercised those specific muscle groups in their right forearms, however, both strength and mass measurements in their immobilized forearms were virtually unchanged. Even more interesting was the fact that the muscle groups in the right hand that were deliberately not exercised had measurably atrophied in the left hand. Only the same muscles that were exercised in the free limb maintained their strength and mass in the limb that was in a cast.

How and why cross education takes place is not yet known. Although sensors placed inside the casts of the resistance training group showed that there was indeed some muscular contraction taking place while the students exercised their free hands, these were too weak to have a physical effect.

So what’s going on? Most popular among the many theories that have been floated is the idea that something is happening in the neural circuits of our brains. In the meantime, let your son know that, thanks to cross education, he has a way to emerge from his cast stronger than he expected.