When we start to lift weights, our muscles don’t change at first, but our nervous systems do, and that makes us stronger.

A fascinating new study 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.

People who take up weight training often feel some initial disappointment when their 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 might not look any larger and stronger.

Scientists have known for some time that these early increases in strength 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.

There were theories that as those commands become swifter and more forceful, the muscles on the receiving end would respond with mightier contractions. In other words, they would be stronger. But the mechanics of these changes have been unclear.

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 surgically implanted tiny transmitters and electrodes in two female macaque monkeys (complying with regulations for humane treatment of animals during research) so the researchers could track how nerve responses changed.

Macaque monkeys, like humans and other primates, have two major bundles of nerves that transmit messages from the motor cortex. One, called the reticulospinal tract, has connections throughout the brain and into the brainstem. These nerves generally direct broad motor skills, like posture. A separate bundle of nerves, called the corticospinal tract, is more refined, controlling fine motor skills like grasping objects.

The researchers used treats to get the monkeys to pull a weighted lever with their right arms, while the scientists measured which nerves became most activated — and by how much — before, during and after the workouts.

For almost three months, the monkeys trained five times a week, with the researchers increasing the amount of resistance on the lever until the monkeys could complete a workout equivalent “to a human doing 50 one-armed pullups,” said Isabel Glover, a neuroscientist and author of the new study with Stuart Baker, the director of the Movement Laboratory at Newcastle University.

This impressive gain in strength was driven, the electrode data showed, by changes in the reticulospinal tract.

This finding underscores that “strength isn’t just about muscle mass,” Glover said. “You get stronger because the neural input to your muscles increases.”

Of course, this study was conducted with macaques, which are not people, although they “have a very similar nervous system to humans,” Baker said. It also was small, involving only two monkeys.

Even with those caveats, though, the results intimate that we most likely are hard-wired to respond quickly and well to weight training and should not be deterred if our muscles do not at first bulge.

“Initial gains are all about strengthening the reticulospinal tract,” Baker said. “Only later do the muscles actually start to grow.”