How ‘muscle memory’ may help us get in shape

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 pedalled, 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.

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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, 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% 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.

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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 dialled up
the operation of genes involved in muscle growth while quieting gene activity
elsewhere, making the genetic process of building muscle “more refined,” 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:

— No matter how long it has been since we’ve been to
the gym or joined an online body-weight workout, our muscles should remain
primed to respond to the exercises when we start working out again.

— 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,” Murach

© 2022 The New York Times Company

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