A juvenile climbing exercise establishes a muscle memory boosting the effects of exercise in adult rats

Abstract One of the ideas stemming from the discovery of a cellular memory in muscle cells has been that an early exercise period could induce a long-term muscle memory, boosting the effects of exercise later in life. In general muscles are more plastic in younger animals, so we devised a 5-week climbing exercise scheme with food reward administered to juvenile rats (post-natal week 4-9). The juvenile exercise increased fiber cross-sectional area (fCSA), and boosted nuclear accretion. Subsequently the animals were subjected to 10 weeks of detraining (week 9-19, standard caging). During this period fCSA became similar in the animals that had been climbing compared to Naive controls, but the elevated number of myonuclei induced by the climbing were maintained. When the Naive rats were subjected to two weeks of adult exercise (week 19-21) there was little effect on fCSA, while the previously trained rats displayed an increase of 19%. Similarly, when the rats were subjected to unilateral surgical overload in lieu of the adult climbing exercise, the increase in fCSA was 20% (juvenile climbing group) and 11% (Naive rats) compared to the contralateral leg. This demonstrated that juvenile exercise can establish a muscle memory. The juvenile climbing exercise with food reward led to leaner animals with lower body weight. These differences were to some extent maintained throughout the adult detraining period in spite of all animals being fed ad libitum, indicating a form of body weight memory.