If you’ve ever fallen out of your gym routine and worried that all your hard work was wasted, new research offers some encouraging news.
A study from the University of Illinois Urbana-Champaign suggests that when you return to exercise after a break, your muscles may actually grow more than they did the first time around—even if you’re not working out as hard.
The study, published in the American Journal of Physiology-Cell Physiology, focused on mice that ran voluntarily on exercise wheels.
For the first four weeks, the mice ran daily, covering about 10 kilometers per day. Then they rested for four weeks with no exercise before resuming wheel running for another four weeks, this time averaging only 6 kilometers per day.
Surprisingly, the second round of exercise produced larger gains in muscle growth than the first.
The mice that retrained after a break had up to 30% more muscle mass in their legs compared with mice that only completed one round of running. In other words, the break did not erase their progress—it may have set them up for even bigger gains.
This led the researchers to explore the idea of “muscle memory,” the concept that muscles retain some form of benefit from earlier exercise, allowing them to respond more strongly the next time.
“Our lab is very interested in understanding whether our body remembers that we’ve exercised before,” said kinesiology professor Diego Hernandez-Saavedra, who led the study with Ph.D. candidate Clay Weidenhamer.
Scientists have long suspected that muscle memory involves satellite cells—specialized cells that add extra nuclei to muscle fibers during exercise.
These nuclei were thought to stick around and give muscles the ability to grow faster during retraining. But earlier research has produced conflicting results about whether these nuclei truly stay after exercise stops.
In this new study, the Illinois team looked deeper at what was happening inside the muscle cells, focusing on changes in gene activity.
They introduced a one-week “washout period” between the exercise phases to avoid measuring only short-term effects. What they found was striking: during the second bout of exercise, genes linked to mitochondrial function switched on much more strongly.
Mitochondria are the “powerhouses” of cells, producing the energy muscles need to keep moving.
The robust boost in mitochondrial-related genes after retraining suggests that the muscles had been primed by the first exercise phase. While most adaptations from the first round seemed to disappear during the break, something about the mitochondria carried over, making the muscles more responsive the second time.
Even diet didn’t seem to block these benefits. Some mice were fed a standard diet while others were given a high-fat diet to induce obesity. Both groups experienced similar improvements in muscle growth after retraining, pointing to a strong underlying effect of exercise memory.
The study also highlights that aerobic exercise—like running—does more for muscles than many people think. While not as powerful for building bulk as weightlifting, consistent aerobic exercise can still cause meaningful muscle growth, especially when done over time.
Hernandez-Saavedra said the findings may eventually help researchers design strategies to maintain muscle health in aging adults or people with poor diets. “These insights may help identify strategies to preserve muscle health during aging or counteract the effects of a poor diet, opening the door for new interventions against frailty or metabolic disease,” he explained.
For anyone who has skipped the gym for a while, this research is a reminder that it’s never too late to start again. Your muscles may remember more than you think—and reward you with even bigger gains when you come back.
