Detraining, or reversibility, is at the forefront of endurance athletes’ and their coaches’ minds. Nobody wants to put in weeks and months of hard work, early mornings, and personal sacrifices to simply lose their adaptations and fitness improvements in a matter of days and weeks. Often we see athletes pushing through sickness and injury, or not scheduling time off during the year to recover, because they do not want to revert back to their pre-training “unfit” state. The following will discuss the science behind changes which occur when we cease exercise, how it can be combated, and how long we should take off training.
The Principle of Reversibility
The principle of reversibility states that when training stimulus is removed/reduced, the ability to maintain performance at a particular level is reduced and eventually gains will diminish to pre-training levels. Basically, you use it or lose it. But it’s not quite as bad as it seems…
Research from Mujika & Padilla (2001) found that highly trained endurance athletes reduced their time to exhaustion tests by 9-25% after 2 weeks of detraining, 8-21% in 4 weeks of detraining, and 23% in 5 weeks of detraining. A different study from Kenney, Wilmore and Costill (2012) shows that endurance trained cyclists with 10 ± 3 years of intense training experience reduced their VO₂ max values by an average of 7% in 21 days and 16% after 56 days of no training.
What Happens to Our Bodies During Detraining?
Several processes occur which reduce our endurance performance.
After 21 days (3 weeks) of no training:
Stroke volume is reduced.
Blood plasma volume is reduced.
Total peripheral resistance (blood pressure) is increased.
Left ventricle heart volume and wall thickness decreases.
Citrate Synthase (CS) activity decreases 20% (1-2.5% per day). This enzyme plays a key role in aerobic energy production in mitochondria.
After 21-56 days (3-8 weeks) of no training:
A-VO₂ difference decreases about 5%.
CS drops 40% at 56 days, but is stable thereafter (however this value is still 50%> than untrained individuals).
After 56-84 days (8-12 weeks) of no training:
There is NO CHANGE in muscle capillarisation.
CS has stabilised.
Additionally, Powers and Howley (2012) show that in a study which involved highly trained individuals training for 5 weeks followed by completing no exercise for the next 5 weeks that:
About 50% of the increase in mitochondrial content was lost after one week of detraining.
All of the adaptations were lost after 5 weeks of detraining.
It took 4 weeks of retraining to regain the adaptations lost in the first week of detraining.
I Thought You Said it's Not as Bad as it Seems?
Athletes are forever completing more and more volume in a quest to create bigger, better and more mitochondria to fuel aerobic performance, right? This is why we complete months of base training. Detraining causes a loss in mitochondrial content, not mitochondrial structure. Mitochondrial content is made up of proteins, enzymes, lipids etc which all help the mitochondria to create aerobic energy. Mitochondrial structure, however, is the actual organelle we require to house these enzymes which helps us use oxygen. So what does this mean? Think of mitochondria as a bath tub, and the water filling it as mitochondrial content. If we fill the tub to 40% full (the equivalent of CS activity lost after 8 weeks of detraining) and pull the plug, we lose that content quite quickly. On the flip side, once the tub is empty and, after putting the plug back in, we turn the tap on to fill it back up, it takes a lot longer to fill up to 40% than it did to drain it out. Unfortunately, that’s just the way it is. The good this is, however, that the bath tub did not change its structure. The same is true for mitochondria. We complete season after season of base training to increase the number, size and surface area of mitochondria. The structure of these do not change as easily as the content. All we have to do is turn the tap on (begin training again) and fill up the bath, we don’t have to stimulate the growth of new mitochondria to get back to the fitness we were once at. This is why you see the ultra-distance endurance athletes peak at 30+ years of age: they still have all the mitochondria they’ve created over years of training, so they simply have to regain the content.
Capillarisation does not change after 12 weeks of doing absolutely no training. Capillarisation is an adaptation which helps supply our muscles with oxygenated blood. The more capillaries we have surrounding our muscles, the greater opportunity to diffuse oxygen into the working muscles and carbon dioxide out of it. Detraining does not affect this adaptation (at least not after 12 weeks).
Therefore, the only endurance adaptation loss that we’re really concerned about is atrophy of the left ventricle and a reduction in blood plasma volume. These are not major concerns, however, as their capacities are regained quite quickly. The heart is, after all, just another muscle. If you do not train it, it will shrink. The good thing is, however, that it isn’t affected as much as other variables and won’t happen overnight or in a week without training. Likewise, the majority of our blood volume loss is from plasma fluid and not our oxygen-carrying red blood cells. This will still cause a significant change in our stroke volume and heart rate variables, but plasma volume is regained pretty rapidly once we begin exercising again.
So basically, you have four options which come to mind...
Maintain the majority of your fitness while reducing your training load. Completing as little as two high intensity 20 minute sessions twice per week has been shown to maintain the majority of our aerobic adaptations in terms of heart size, mitochondrial content, VO₂ max etc. An example would be a 5km time trial once per week and 5x1km efforts on another day. This method will keep you fundamentally fit while only having to train for 40 minutes per week.
Take 3-6 weeks off structured training during the end of the season. This is a good opportunity to remain somewhat active but have the mental refresher from training structure and physical refresher from high volume and intensity. Cross training is great here, as is going for family walks, trips to the snow, bike rides with the kids etc. Something is better than nothing. Exercise when you feel like it, not when you’re told to.
If you’re physically and mentally exhausted, this is understandable! It’s not going to hurt you to take 3-6 weeks completely off training. Your adaptation structures will still be in place, so you’ll just have to fill up the bath tub again and re-strengthen that heart a bit. This can be a particularly good option if you have a solid 5-6 months before you’re required to be peaked again for a race.
Take no time off and train all year around. This, in my opinion, is the least sensible option as you will be prone to overtraining, injury, illness, and all types of mental fatigue and demotivation. Everybody needs a break at some stage.
The Take Home Message
Structuring in some time off into your training program is crucial not only physically, but mentally as well. The amount of time is up to you, but generally 3-6 weeks is enough to clear out any lingering soreness, have a mental refresher, and limit the amount of detraining effects. Rest assured that your fundamental adaptations are still in place, and we only need to regain some mitochondrial content, blood plasma and heart strength to be back fighting fit. It will take some time, but will pay dividends in the long run. Possibly the most important thing to remember is that we don’t see significant losses in aerobic capacity until 2-3 weeks of no exercise whatsoever. This means that you shouldn’t stress if you have to take up to a week-10 days to get over lingering soreness or the flu. You’re better off resting and getting the body right and getting back to proper training in 7 days than pushing through and being sick for 3 weeks. Likewise, if you’re busy with work or traveling and can’t stick to your regular training schedule, remember that even just two 20 minute high intensity sessions is sufficient to maintain the majority of your aerobic adaptations.