Endurance training may leave a mitochondrial "muscle memory" that enhances training adaptations after a period of inactivity.
The extent to which exercise leaves a lasting mark on future muscle responses after a period of inactivity remains only partly understood. To address this gap, researchers at the University of Illinois examined whether endurance training could create a metabolic memory that influences later training adaptations.
The team used eight-week-old male mice that were divided into two diet cohorts, one receiving a standard control diet and the other a high-fat diet. In a retraining experiment, mice completed four weeks with access to a running wheel, followed by four weeks without wheel access, then a second four-week period with access to a running wheel. Another group of control-diet mice remained sedentary for eight weeks before getting access to a running wheel for four weeks. In two other experiments, mice on both diets completed either only a four-week training period or a four-week training period followed by a sedentary phase. All of these groups were compared to mice on the same diet that stayed sedentary for the same length of time.
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Across both diets, prior endurance exercise changed how body weight, muscle size, and mitochondrial programs responded to later running:
- On the control diet, retrained mice maintained body weight while gaining lean mass and losing fat, whereas sedentary mice gained fat and naive group mainly lost fat without adding lean tissue.
- On the high-fat diet, retrained mice ended the study with roughly half the body fat percentage of sedentary mice.
- Retraining produced larger hindlimb muscles than first time training over the same four weeks, with some 12 to 30% heavier than in mice of the naive training experiment.
- Retrained mice showed a higher proportion of muscle fibers that contain more mitochondria and rely more on oxygen based metabolism, which is a typical effect of endurance activity, and these fibers also became larger. Changes that did not appear in the mice that trained for the first time.
- Retraining increased the activity of genes that help mitochondria generate ATP (the cell's main energy source) and genes that build the protein-making machinery inside mitochondria.
- After retraining, muscles had higher levels and activity of proteins involved in energy production and fat metabolism compared with sedentary controls.
Muscles that had been trained before responded more strongly to exercise, as if they "remembered" the earlier training. Mitochondria appear to be a central driver of this muscle memory and raise the possibility that early endurance training leaves lasting benefits for muscle growth and metabolism.
More studies in animals and humans will be needed to further test and confirm these findings. In this clip, Dr. Martin Gibala outlines the relationship between exercise types, mitochondrial growth, and their combined effect on fat metabolism.