Covid-19 has disrupted most people's workout routines over the past two years, leading many people to feel like they've "forgotten how to be fit"—but new research suggests that our muscles have a lasting "memory" that helps us bounce back, even after long periods of inactivity, Gretchen Reynolds writes for the New York Times.

Background

In 2018 and 2019, several studies on humans analyzed the epigenetics of resistance training. According to Reynolds, epigenetics refers to any changes in the way a gene functions, even though the gene itself does not change. This largely occurs through a process known as methylation, during which clusters of atoms, called methyl groups, attach to the outside of genes, which makes the genes more or less likely to activate and produce certain proteins.

In those studies, researchers discovered that resistance training altered methylation patterns on multiple genes in people's muscles that resulted in long-term changes that could be seen weeks or months later—even after volunteers were no longer exercising and had lost some of their muscle mass.

Once the participants started lifting weights again, they increased their muscle mass at a much faster rate than they had at the beginning of the studies. "In essence, their muscles remembered how to grow," Reynolds writes.

Unfortunately, those studies only lasted a few months, making it unclear whether exercise that occurred much earlier in a person's life would result in lingering "muscle memory." The length of the studies also prohibited researchers from determining how many different cells and genes within a person's muscles would be affected epigenetically through resistance training, Reynolds writes.

Study details

For the new study, published in Function, a flagship journal of the American Physiological Society, researchers mirrored human weight-training experiments as closely as they could in adult mice. Notably, however, since rodents have significantly shorter life spans, changes seen in the animals after several months may not appear in people for several years, Reynolds writes.

Specifically, to mimic weight training in humans in the new study, scientists had the rodents run on a weighted running wheel, which was designed for leg-muscle resistance training. Each of the rodents was engaged in training for eight weeks, followed by a 12-week period of inactivity—roughly 10% of their life spans, which would equate to multiple years of inactivity for a human.

After sitting in their cages for 12 weeks, the animals trained again for one month, along with a control group of mice that were the same age but were new to the exercise. Throughout the study, researchers biopsied and microscopically studied the mice's muscles.

How long does 'muscle memory' last?

The researchers observed several differences in gene methylation in the cells of the mice's muscles after they trained—most of which remained even months after their training ended.

Specifically, these epigenetic changes increased the operation of genes involved in muscle growth while decreasing gene activity elsewhere, essentially making the genetic muscle-building process "more refined," said Kevin Murach, a professor of health and human performance at the University of Arkansas, who oversaw the new study.

And these improvements lasted, the researchers found: Even after the mice had been inactive for months, these changes helped the trained mice increase their muscle mass more quickly during retraining than the mice in the control group that had not trained previously.

And according to Reynolds, although the study involved mice instead of people, and only incorporated one type of exercise, many of the genes analyzed in this study were the same as the ones researchers tracked in the human experiments—meaning the findings most likely have relevance for anyone hoping to build up their muscles.

Ultimately, researchers said the results suggest that no matter how long it has been since a person's last workout, their muscles should respond to the exercises we do when we begin a workout regimen.

In addition, they found that "muscle memory" can be built at any age—even if an individual has little to no experience lifting weights. Notably, the study used adult mice—and they were all able to build muscle memories that allowed them to gain muscle faster after a period of inactivity.

According to Murach, "It's better to start sometime than not at all." (Reynolds, New York Times, 1/5)