One of the papers I had bookmarked on muscle memory recently got cited by another paper covering disuse-associated muscle atrophy . So, I finally got around to reading the muscle memory paper. Some interesting stuff!
We’ve previously talked about muscle memory a number of times, most recently in our November 2024 Research Review,.
Briefly, the term muscle memory describes an improved efficiency of gaining strength and muscle size in an individual who returns to training after a period of detraining lasting weeks to months, or perhaps longer.
There are a few suggested mechanisms for this phenomenon, but the main one over the years has been the myonuclear permanence hypothesis.
Myonuclei are the muscle cell nuclei, and each muscle fiber has many. One of their main functions is to produce protein for the muscle to maintain/alter its size and function by producing muscle protein. They can only produce so much protein though, so growing more muscle typically requires additional myonuclei to be added to the muscle.
During detraining, the myonuclei produce less protein and the muscle gets smaller. The myonuclear permanence hypothesis suggests these myonuclei persist and when the individual gets back to training, they regrow muscle and gain strength more rapidly than they did initially to get back to where they were.
The evidence on this is incomplete and, as mentioned, other mechanisms have been suggested.
For example, persistent neural adaptations such as coordination, movement proficiency, and skill may allow a rapid restoration of strength, use of heavier weights, and more effective training compared to being completely untrained. This wouldn’t really explain an increase in muscle size however, as most data does not support greater hypertrophy from using higher weights. Perhaps the “real” mechanism has to do with more substantial muscle recruitment leading to greater stimulus.
Another mechanism put forth is that some individuals may become “resensitized” to exercise after a break, where a number of molecular mechanisms involved in muscle hypertrophy become blunted after continuous training and are more responsive to training stimuli) after a break. To me, this implicates either a sort of ceiling to muscle size that the body protects against going over, or alternatively, the need for greater training stress as people become more trained in order to “make up” for the blunted signal.
Now, this paper I finally got around to reading has introduced a new concept into my understanding of muscle memory, epigenetic muscle memory.
Epigenetics refers to the impact of the environment, behavior, and other non-genetics factors on gene expression. In this context, earlier exercise-driven muscle growth modified gene expression at the level of the muscle, which are kept during periods of detraining and leveraged when people return to the gym. There’s also some thought this can work negatively too, e.g. people with a history of long-term immobilization or muscle-wasting condition may be more susceptible to losing muscle during detraining, and/or having a harder time building muscle when hitting the gym.
Pretty interesting stuff. Here’s the paper:
https://journals.physiology.org/doi/full/10.1152/ajpcell.00099.2023