Sixteen athletes wore casts on their non-dominant arms for four weeks. Half of them did three strength-training sessions per week, while the other half did none.
By the end of the study, published in the Journal of Applied Physiology, the people who stopped training lost 21.6 percent of their strength in the immobilized arm, while those who kept exercising the opposite arm lost only 2.4 percent. They were able to maintain muscle thanks to a neurophysiological phenomenon called cross-education, or mirroring.
For the most part, each of the two hemispheres in the brain sends information to one side of the body, explains study author Justin Andrushko, a Ph.D. student at the University of Saskatchewan in Canada.
But when people do single-arm exercises, information is sent between the two hemispheres via a metaphorical bridge called the corpus callosum, he says. This might explain why the left arm benefits when you train your right arm only.
Exercising muscles in your functioning arm can help you maintain strength in the opposite arm when it’s injured. Though research shows mirroring happens in the upper body, it probably won’t work in the lower limbs. “Leg muscles lose size and strength substantially faster than the upper limbs, and combating the natural declines would likely be a challenge,” Andrushko says.