Muscle memory in action: a second exercise round boosts muscle growth
A new study in mice suggests that a second bout of voluntary wheel running, after a four-week pause, can provoke greater muscle growth than the initial training. The research, published in the American Journal of Physiology: Cell Physiology, found that the second round led to larger increases in muscle fiber size, even though the mice ran less distance per day than during the first period. The findings offer intriguing clues about how the body may “remember” prior exercise and respond more strongly when activity resumes.
The experiment: training, washout, and retraining
In the study, mice voluntarily ran on an exercise wheel for four weeks, stopped for four weeks (the washout period), and then ran again for another four weeks. The team, led by Diego Hernandez-Saavedra, Ph.D. candidate Clay Weidenhamer, and colleagues at the University of Illinois Urbana-Champaign, monitored muscle mass and fiber size throughout. By the end of the second period, leg muscles showed up to a 30% gain in mass compared with mice that completed only a single training bout. Importantly, the second phase used less daily running distance (roughly 6 km per day versus 10 km in the first phase), suggesting that prior training primes muscles to respond more vigorously to retraining, even when the effort is reduced.
Is the effect due to lasting structural changes?
Muscle memory has long been linked to satellite cells—cells that fuse with muscle fibers to add nuclei, enabling growth. Weidenhamer and colleagues took a broader look, examining gene expression during different stages of training. They found that most adaptations faded during the washout period, but a second exercise bout reactivated a robust program of gene activity, particularly genes tied to energy production in mitochondria—the cell’s powerhouses. This pattern pointed to mitochondria playing a central role in the enhanced response seen after retraining.
Mitochondria and the memory of exercise
The team reports a striking finding: while the first training period did not markedly boost mitochondrial function markers, the second period did. The data imply that the muscle priming effect may involve improved mitochondrial efficiency or capacity, allowing muscles to generate energy more effectively during subsequent exercise. In other words, the second bout appears to “unlock” a metabolic readiness that the first bout did not fully establish.
Diet, obesity, and the resilience of muscle memory
To test the robustness of this memory, researchers included mice on a control diet and others on a high-fat diet designed to induce obesity. Remarkably, the second training still produced comparable gains in muscle growth regardless of diet. The authors concluded that exercise memory can, at least in this model, override dietary factors to some extent, highlighting the potential value of continued aerobic activity for preserving muscle health amid dietary challenges.
What this means for aging and health beyond the lab
Beyond the lab, the findings reinforce the idea that aerobic or endurance exercise contributes to muscle health, not just endurance. The observed improvements in muscle mass from retraining, coupled with mitochondria-linked gene expression, suggest strategies to preserve muscle function during aging or to counteract metabolic disturbances from a poor diet. While translating findings from mice to humans requires caution, the concept of a lasting, retraining-related boost to muscle growth is increasingly plausible and could inform exercise guidelines for different life stages.
Future directions
The researchers plan to study the durability of this exercise memory over longer time scales and to explore the molecular signals that transmit the memory from prior training to the next bout. If these pathways can be harnessed safely in people, they could help design interventions to counter frailty or metabolic disease and optimize aging-related muscle maintenance.
