Effects of training, detraining, and retraining on strength, hypertrophy, and myonuclear number in human skeletal muscle

Abstract

Previously trained mouse muscles acquire strength and volume faster than naïve muscles; it has been suggested that this is related to increased myonuclear density. The present study aimed to determine whether a previously strength-trained leg (mem-leg) would respond better to a period of strength training than a previously untrained leg (con-leg). Nine men and 10 women performed unilateral strength training (T1) for 10 wk, followed by 20 wk of detraining (DT) and a 5-wk bilateral retraining period (T2). Muscle biopsies were taken before and after each training period and analyzed for myonuclear number, fiber volume, and cross-sectional area (CSA). Ultrasound and one repetition of maximum leg extension were performed to determine muscle thickness (MT) and strength. CSA (~17%), MT (~10%), and strength (~20%) increased during T1 in the mem-leg. However, the myonuclear number and fiber volume did not change. MT and CSA returned to baseline values during DT, but strength remained elevated (~60%), supporting previous findings of a long-lasting motor learning effect. MT and strength increased similarly in the mem-leg and con-leg during T2, whereas CSA, fiber volume, and myonuclear number remained unaffected. In conclusion, training response during T2 did not differ between the mem-leg and con-leg. However, this does not discount the existence of human muscle memory, since no increase in the number of myonuclei was detected during T1 and no clear detraining effect was observed for cell size during DT; thus, the present data did not allow for a rigorous test of the muscle memory hypothesis. NEW & NOTEWORTHY If a long-lasting intramuscular memory exists in humans, this will affect strength-training advice for both athletes and the public. Based on animal experiments, we hypothesized that such a memory exists and that it is related to the myonuclear number. However, a period of unilateral strength training, followed by detraining, did not increase the myonuclear number. The training response, during a subsequent bilateral retraining period, was not enhanced in the previously trained leg.

The concept of skeletal muscle memory: Evidence from animal and human studies

“…However, muscle fibre size remained unchanged during detraining as well as 5 weeks of bilateral retraining. Furthermore, no changes in myonuclear content could be detected in response to the initial training, detraining, or retraining period 11 . The degree of muscle fibre growth observed during the initial 10 weeks of exercise training may have been too small to elicit a group mean increase in myonuclear content which is required to firmly establish whether muscle memory by myonuclear retention exists in humans.…”

“…Currently there is only one study that attempted to directly test the hypothesis of muscle memory by myonuclear permanence in humans. In the study by Psilander et al, 11 healthy young men performed 10 weeks of unilateral resistance exercise training, followed by 20 weeks of detraining and a subsequent 5 weeks of bilateral retraining. Skeletal muscle thickness and strength increased significantly in response to the initial exercise training period, which was mostly lost during the successive 20 weeks of detraining.…”

“…The degree of muscle fibre growth observed during the initial 10 weeks of exercise training may have been too small to elicit a group mean increase in myonuclear content which is required to firmly establish whether muscle memory by myonuclear retention exists in humans. As Psilander et al 11 published their raw study data, a secondary analysis on these data was recently performed by Murach and colleagues. Here, Murach et al 12 retrospectively divided the participants in two groups, based on the immunohistochemical analyses of both muscle cross sections and single muscle fibres: one group in which a significant increase in myonuclear content was observed (n = 11) following the initial training program, and one group in which no change was observed (n = 8).…”

“…Further experimentation is clearly warranted to more firmly establish whether muscle memory by myonuclear retention exists and, more importantly, whether it plays a relevant role in muscle reconditioning in humans. The most effective study designs should be comparable to that of the recently published study by Psilander and colleagues, 11 who applied a unilateral approach. Moreover it is critical that exercise training intensity, volume and/or duration progresses enough to induce considerable (and measurable) muscle fibre hypertrophy as well as myonuclear accretion.…”

Transcriptional memory in skeletal muscle. Don't forget (to) exercise

“…It should, however, be mentioned that, in experimental models, hypertrophic responses in skeletal muscle have reportedly been observed without myonuclear accretion (Blaauw et al, 2009;McCarthy et al, 2011). Similarly, a recent human study found no increase in myonuclear number that would accompany gains in muscle force and mass in the course of 10 weeks of leg strength training (Psilander et al, 2019). Further complication arises from a recent murine study that reported traininginduced increases in myonuclear density but found no evidence for a retention of these newly recruited myonuclei that would last over a subsequent period of detraining (Dungan et al, 2019).…”