This brand new study suggests that, as we’ve previously written about, muscle memory is real and may have to do with myonuclei domain theory.
Muscle memory – the concept that you can regain lost muscle much faster than how long it took to first gain it – has been documented both experimentally and anecdotally. A famous example is the Casey Viator experiment, where Casey allegedly (re-)gained 60lbs of muscle in only four weeks.
It’s been speculated that myonuclear domain theory underlies the concept of muscle memory. The nuclei of your muscle cells, also referred to as myonuclei, can only oversee so much muscle mass at once. To keep growing muscle, in the long run, you would need to eventually add new myonuclei to allow for further muscle growth. When training is ceased, muscle cells do begin shrinking relatively quickly; however, some research, primarily in rodents, suggests that the myonuclei that were acquired stick around for much longer, potentially years or decades. As a result, when you resume training, you don’t need to acquire new myonuclei from scratch, facilitating and accelerating the muscle re-gaining process.
While myonuclear permanence – the idea that myonuclei stick around for a long time and that this plays a role in muscle memory – has been studied before, the results have historically been much more consistent in animal models than in human research (1, 2).
However, this study provides strong support in favor of myonuclear permanence, but not necessarily muscle regaining.
Let me explain.
A new study
On August 19th, a group of researchers from Norway published a new study in the Journal of Physiology. In their study, Cumming and colleagues had participants engage in a 38-week study. Following initial tests, 12 participants trained only one of their biceps for 10 weeks. Then, participants stopped training for 16 weeks. Finally, after this 16-week de-training period, participants resumed training, but training both their previously trained arm and their previously untrained arm. Between each phase, measurements of bicep thickness and biopsies were taken to assess myonuclear number.
Results
The trained arm experienced around a 30% increase in cross-sectional area over the training period. Likewise, during the first training period, myonuclei number per muscle fibre increased by 13 and 34% for slower- and faster-twitch muscle fibres, respectively.
Number of myonuclei in the trained muscle
As you’d expect, the control, non-training arm, didn’t increase in size during the first training period. Following the 16-week training cessation period, both arms were of a similar size, again – the trained arm had lost all size gained during the first training period.
Muscle thickness of biceps brachii and brachialis measured before and after first training period (trained arm only; n = 12; week 0/baseline-10), de-training (n = 11; week 10–26) and re-training period (n = 11; week 26–36)
The myonuclei gained during the initial training period in the training arm remained until the start of the second training period, providing support for the idea of myonuclear permanence. Furthermore, both the control and training arm observed an increase in myonuclei during training period 2. However, both arms grew similarly during the second training period, despite the difference in number of myonuclei, though the hypertrophy was nominally greater in the trained arm compared to the control arm. This lack of difference contrasts with previous human studies looking at muscle memory (1, 2).
Interpretation
So, what gives? Why wasn’t muscle growth greater in the trained arm, despite the presence of previously gained myonuclei? In truth, it appears to be a result of variance. Specifically, one participant appeared to lose muscle in the trained arm in the second training period, while gaining ~50% biceps functional cross-sectional area in the control arm. When excluding this potential outlier from analysis, hypertrophy was statistically significantly greater in the trained arm that started the training period with a greater number of myonuclei.
Notably, the average myonuclear domain – or “real estate” the average myonuclei was already overseeing – did correlate negatively with hypertrophy. This makes sense, since a lower average myonuclear domain would mean more “room to grow” before the myonuclei’s abilities were maxed out and new myonuclei needed to be added to keep growing.
Correlations between delta change in fCSA over the re-training period and myonuclear domain at pre re-training for the type 1 fibres and type 2 fibres
Takeaways
Overall, research is starting to suggest that myonuclei are at least semi-permanent in humans. They stick around a lot longer than muscle will. Importantly, just like lifting weights, anabolic steroids can enhance myonuclear accretion, conferring steroid users a potentially lifelong advantage. It also seems likely that myonuclei play a role in the concept known as muscle memory, wherein lost muscle and strength are regained much more rapidly than during the initial, grueling gaining phase.
