After two years of Covid-19 and its disruptions to our exercise routines, many of us may feel like we have forgotten how to be fit. But an encouraging new study suggests that our muscles remember. The study involved mice, but it builds on similar experiments with weight training and people. It found that muscles developed a pervasive and lasting molecular “memory” of past resistance exercises that helped them bounce back quickly from long layoffs.
In the study, animals that completed a rodent form of resistance training developed changes in their muscles’ DNA that lingered long after they stopped exercising. The mice then packed on muscle mass much faster than other animals when they began training again. And as an encouraging side note to those who are taking up weight training for the first time, the findings also suggest that we should be able to build new muscle memories, regardless of our age.
Until recently, the term “muscle memory” usually described our ability to bike, ski, throw to first base or repeat other common physical tasks, even if we had not pedaled, schussed or beelined a baseball in years. Our bodies remember how. But this type of memory, while real, is not really a muscle memory. These memories exist within motor neurons in our brains.
But scientists knew that something happened within muscles themselves when they were worked hard, especially during weight training, and that these changes affected how muscles later responded to exercise. “Anecdotally, people say things like, ‘I used to be an athlete, then took time off, but my muscles came back as soon as I started’” lifting weights again, said Kevin Murach, a professor of health and human performance at the University of Arkansas, who oversaw the new study.
Those stories piqued his and other researchers’ interest. How, they wondered, do muscles “remember” past workouts? And in what ways do those memories help muscles rebound after time away from the gym?
Some preliminary studies with animals suggested that genes inside the nuclei of muscle cells worked differently after resistance exercises. Then, in 2018 and 2019, several much-discussed studies of people looked into the epigenetics of resistance training. Epigenetics refers to changes in the ways that genes operate, even though the gene itself does not change. It mostly involves a process called methylation, in which clusters of atoms, called methyl groups, attach themselves to the outside of genes like minuscule barnacles, making the genes more or less likely to turn on and produce particular proteins.
In the recent human experiments, resistance exercise changed methylation patterns on a number of genes in people’s muscles, and those changes remained evident weeks or months later, even after the volunteers stopped exercising and lost some of their muscle mass. When they began lifting again, they packed muscle back on much faster than when the studies started, the researchers found. In essence, their muscles remembered how to grow.
But those studies, while intriguing, lasted a few months at most. It was still unclear if exercise from much longer ago would linger as a genetic memory in our muscles, or just how many different cells and genes in muscles would be affected epigenetically by resistance training.
So for the new study, which was published recently in Function, a flagship journal of the American Physiological Society, Dr. Murach and his colleagues, including the lead author Yuan Wen, decided to recreate the human weight-training experiments as closely as possible in adult mice. Rodents’ life spans are far more condensed compared with ours, meaning that changes seen in the animals after several months might appear in people after several years.
But since mice cannot use barbells, the scientists had them run on weighted running wheels, which were designed to provide leg-muscle resistance training. The animals trained for eight weeks and then sat in their cages for 12 weeks — about 10 percent of their life spans, which would be years for us. The animals then trained again for a month, joined by mice of the same age that were new to the exercise and that served as controls. Throughout, the researchers biopsied and microscopically studied their muscles.
They noted plenty of differences in gene methylation in muscle cells after the mice trained; most of the changes remained months after they stopped exercising. In general, these epigenetic changes dialed up the operation of genes involved in muscle growth while quieting gene activity elsewhere, making the genetic process of building muscle “more refined,” Dr. Murach said. Even after months of inactivity, these changes helped the trained mice add more muscle more quickly during retraining, compared with the mice that had not previously trained.
Of course, this study involved mice, not people. It also looked only at resistance exercises and not at aerobic workouts.
But since many of the genes the researchers tracked are the same ones that researchers studied in the human experiments, the findings most likely have relevance for any of us who hope to build up our muscles in 2022. They suggest that:
It may never be too late to start laying down muscle memories, even if we have rarely or never lifted weights. The mice in the study were all adults when they began the weighted-wheel workouts, yet they all managed to build muscle memories that allowed them to bulk up faster after a period of inactivity. “It’s better to start sometime than not at all,” Dr. Murach said.