There have been hints, though, that exercising one limb can affect the other. In past studies, when someone pedals a bike with one leg or lifts weights with one arm, muscles in the other limb often contract, a development known as mirroring.

But in most of those experiments, the unused limb was not completely immobilized with a cast and scientists did not focus on specific muscles, making it difficult to know whether exercising certain muscles in one limb affects all muscles in the other or only some.

So for the new study, which was published in April in the Journal of Applied Physiology, researchers from the University of Saskatchewan in Canada gathered 16 male and female college students and closely examined their wrists.

Using ultrasound and CT scans, the scientists determined the precise dimensions of two separate sets of muscles in that joint: the extensors, which move the wrist back and away from the body; and the flexors, which pull it in, toward the forearm.

The researchers also tested each volunteers’ wrist strength using a weight machine for the hands.

Then they covered each student’s left forearm and wrist with a hard cast to freeze the wrist in place. (All of the students were right-handed.)