Many people worry that taking a break from exercise will cause them to lose all their progress.
But a new study gives hope that your muscles may remember past workouts, and restarting exercise could be even more effective the second time around.
Researchers from the University of Illinois Urbana-Champaign studied mice to understand how muscles respond after stopping and restarting exercise. The mice were given a running wheel to use freely.
They exercised for four weeks, rested for four weeks, and then exercised again for another four weeks. Surprisingly, the mice showed bigger muscle gains after the second round of running—even though they didn’t run as much as they did the first time.
The study was led by professor Diego Hernandez-Saavedra and Ph.D. student Clay Weidenhamer. They wanted to know if our bodies “remember” exercise and become stronger or healthier because of it. This concept is often called “muscle memory.”
Muscle memory is believed to involve special muscle cells called satellite cells. These cells help muscles grow by adding new nuclei to muscle fibers when we exercise. However, scientists have not been sure whether these new nuclei stay in the muscle after we stop working out.
To explore this, the researchers looked at changes in the muscles’ gene activity. They even added a one-week “washout” period where the mice didn’t exercise, to make sure they weren’t just seeing temporary effects.
What they found was interesting: after the first exercise period, most of the muscle changes disappeared during the rest period. But once the mice started exercising again, the muscles responded strongly and grew even more.
The mice gained up to 30% more muscle in their legs after the second round of exercise, compared to mice that only exercised once. This happened even though the second round of running was less intense.
In the first round, the mice ran about 10 kilometers per day, but only about 6 kilometers per day in the second round. This shows that previous exercise somehow prepared the muscles to grow more later on.
So, what made the muscles grow more during the second round? The researchers believe the answer lies in the mitochondria, which are the parts of our cells that produce energy. Their study showed that after the second round of exercise, the genes related to mitochondrial function became more active. This didn’t happen after the first round.
The researchers also looked at how diet affected muscle memory. Some mice were fed a regular diet, while others were given a high-fat diet to make them obese. Even with the unhealthy diet, the mice still saw big muscle gains after the second round of exercise. This suggests that exercise memory may help overcome some effects of poor eating habits.
The study also supports the idea that aerobic exercise, like running, can build muscle—not just strength training. The gains weren’t as large as what you’d get from lifting weights, but they were still important.
The researchers hope that in the future, this type of knowledge could help older adults maintain muscle strength or help people with health problems avoid frailty and muscle loss. They plan to do more studies over longer time periods to learn even more.
So if you’ve taken a break from working out, don’t worry. Your muscles may still remember what they’ve learned—and be ready to bounce back even stronger when you return to exercise.
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The study is published in American Journal of Physiology-Cell Physiology.
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