It’s like getting paid for someone else’s work. In a phenomenon called the crossover effect, the muscles of one ankle get stronger even when exercises are done only by the opposite ankle.
Researchers measured crossover improvements by comparing two groups of people. Half went through eight weeks of a supervised strengthening program for one ankle using specialized equipment. The other group did normal activities and avoided exercise for the legs. Afterwards, the people in the training group had as much as a 19% improvement in muscles of the untrained ankle.
The authors present a variety of theories on how this could happen. Of the many possibilities, they find it most likely that the nerves going to the muscles on one side of the body cause an effect on the same exact muscles on the other side of the body. The researchers suggest that this crossover effect is most likely controlled by the central nervous system. The authors are optimistic that people who are unable to use their ankle due to injury or surgery could benefit from the crossover effect by using the efforts of one ankle to keep the other ankle strong.