It took me more than five years of hard training to break 4:00 for 1500 metres for the first time. Not long after, I missed almost two years of training with a knee injury. When I was able to start running again, I wondered (and worried) whether it would take me another five years to regain my form.
In fact, I was able to break 4:00 again after only a few months of track workouts and ended up running 3:48 that summer. How could something that was so hard the first time be so easy (relatively) the next time?
The answer to that question has many components. Confidence is certainly part of it. Once you know you’re capable of something, it’s easier to do it again. There are also likely neuromuscular adaptations (i.e. in the signals that travel from the brain to your muscles) – that’s the common explanation for why a similar effect is often observed in strength training. But it turns out that your muscles themselves are also changed by training, in a way that makes it easier to regain fitness than it was to gain it in the first place.
Back in 2010, Norwegian researchers published a study on muscle memory in strength training in mice and rats that suggested a mechanism. Muscle cells are unusual: because they’re so big compared to other cells, they have multiple nuclei within a single cell. When you strength train, your muscle cells get even bigger, so they add more nuclei. When you stop training, the muscles cells get smaller – but the new results suggested that the number of nuclei remains elevated long after the training stops. Then, when you start training again, those nuclei are already there, ready to stimulate muscle protein synthesis.
What about endurance? At the ACSM conference last week, Hojun Lee and his colleagues from Temple University presented some data on the role of muscle cell nuclei in mitochondrial adaptations, which is one of the key cellular responses to endurance training. They put rats through one or both of two eight-week training protocols, with 20 weeks in between. As expected, the rats who did the first training session still had more muscle cell nuclei and were able to get fitter and stronger when it came time to do the second training session.
Most interestingly, the “retrained” rats also showed greater mitochondrial activity and greater expression of proteins related to mitochondrial growth. The key? Even though the mitochondria themselves might disappear when you get unfit, the genes that regulate mitochondrial growth are located in the nucleus – so all those extra nuclei that stick around are ready to respond quickly to training when you start up again.
That’s kind of nice to know, and should make people more willing to take a few weeks off every year to recharge – because no matter how much fitness you lose, you’re not starting from scratch.
One aside: These muscle memory mechanisms shed a different light on the long-term benefits of performance-enhancing drugs. Even after serving a ban of a few years, it’s possible that your muscles are still primed to respond to training at a higher level than they would have been without drugs. It’s hard to know how big that effect would be, but it’s interesting to consider.