When we start to lift weights, our muscles do not strengthen and change at first, but our nervous systems do, according to a fascinating new study in animals of the cellular effects of resistance training. The study, which involved monkeys performing the equivalent of multiple one-armed pull-ups, suggests that strength training is more physiologically intricate than most of us might have imagined and that our conception of what constitutes strength might be too narrow.
Those of us who join a gym — or, because of the current pandemic restrictions and concerns, take up body-weight training at home — may feel some initial disappointment when our muscles do not rapidly bulge with added bulk. In fact, certain people, including some women and most preadolescent children, add little obvious muscle mass, no matter how long they lift.
But almost everyone who starts weight training soon becomes able to generate more muscular force, meaning they can push, pull and raise more weight than before, even though their muscles may not look any larger and stronger.
Scientists have known for some time that these early increases in strength must involve changes in the connections between the brain and muscles. The process appears to involve particular bundles of neurons and nerve fibers that carry commands from the brain’s motor cortex, which controls muscular contractions, to the spinal cord and, from there, to the muscles. If those commands become swifter and more forceful, the muscles on the receiving end should respond with mightier contractions. Functionally, they would be stronger.
But the mechanics of these nervous system changes have been unclear. Understanding the mechanics better could also have clinical applications: If scientists and doctors were to better understand how the nervous system changes during resistance training, they might be better able to help people who lose strength or muscular control after a stroke, for example, or as a result of aging or for other reasons.
So, for the new study, which was published in June in the Journal of Neuroscience, researchers with the Institute of Neuroscience at Newcastle University in England decided to teach two female macaque monkeys to lift weights, and the researchers watched what happened to their nerves.