How Quickly Do I Lose My Run Fitness?

Published July 10, 2023 06:00AM

There can be a myriad of reasons why you might lose run fitness: injury, loss of motivation, a planned break in training, a hectic work or family schedule that impedes your regular routine. Unfortunately, your physiology cares very little what the reason(s) might be, and the impact on your fitness is the same. That is, you’ll lose it. But it’s not all bad news—and it won’t all vanish overnight.

How quickly do I lose my run fitness?

Run fitness will typically drop by 2-3% each week that you’re not logging miles, but the process is more complicated than that. The process of losing fitness, sometimes referred to as “detraining,” can be divided into two phases: short-term and long-term, each having different impacts on your physiology and performance.

Certain adaptations, like blood volume increases, are fleeting, changing constantly throughout your training. These are the first to go and, in the case of blood volume, their disappearance can significantly increase your heart rate for the same level of effort (which is usually what most athletes report noticing on resuming run training after a break). Other adaptations affecting heart rate, such as the enlargement of your heart chambers, are structural changes that take longer to develop and also decline at a slower rate.

Rob Pickels, a Boulder-based physiologist and coach, said: “In the initial four weeks after a break from run training, we primarily see a decline in VO2max and an increase in heart rate due to reduced blood volume. There is also a reduction in the capillary network that supplies our muscles, an increase in carbohydrate utilization for the work we’re doing, and a decrease in our lactate threshold.”

Should you take longer than four weeks off from running, you’ll start to see further decreases in VO2max and increases in heart rate, as your heart chambers start to shrink and breathing efficiency decreases. Carbohydrate storage begins reverting to pre-training levels, carbohydrate usage rises even further, and the ratio of slow-twitch (oxidative) fibers to fast-twitch (glycolytic) fibers decreases. As a result, the best performance predictor—your pace at threshold—continues to decline. It’s here that there is some good news, though. As these changes are structural, they occur much slower than in those first four weeks, with the rate of decline being around 1 to 2% per week.

Factors affecting how quickly you’ll lose run fitness

Age, current fitness level, and accumulated years of training can significantly impact performance decline during periods of detraining. As you accumulate training years, you often develop more structural changes (e.g., larger heart chambers, improved fat and carbohydrate oxidation) that result in a more gradual decrease in fitness. However, once you hit your 40s, dwindling hormone levels make it harder to retain fitness gains.

Interestingly, of all these factors, it is your current fitness level that holds the most influence on your rate of detraining. Although highly-trained athletes have further to fall before they revert to their sedentary baseline, their high-level training load is vital for maintaining their fitness. Consequently, they often detrain rapidly at first, but then subsequent losses are less severe. Pickels said: “This fact sparks a debate about how long to pause or decrease training at the end of a season. For some athletes, a longer hiatus of more than four weeks can put them in a position where regaining their peak fitness for the next season becomes challenging.”

Of course, there are ways to mitigate the loss of your run fitness, one of which can be keeping active with other sports or activities.

Pickels said: “Any amount of training will decelerate the rate of decline, even if it’s just one training session [in another sport] per week. Remarkably, an 80% reduction in training volume (for instance, cutting down from 10 hours per week to two hours) can halve your fitness losses.”

Keeping some higher-intensity work in your routine while taking a break from running can also help your cause: “During a reduced training period, the intensity of your workouts can play a crucial role,” Pickels said. “High-intensity training appears to be a stimulus that aids in preserving your fitness. As a personal tip, even when I’m traveling and unable to maintain my usual exercise routine, I always try to spare 30 minutes for some quick intervals before carrying on with my day. That seems to help prevent my run fitness from falling off a cliff entirely.”

How long does it take to get run fitness back?

Ah, the million dollar question. Making that journey back to full fitness is a hotly debated topic with some researchers suggesting that you can regain your fitness in half the time it took to achieve it, whereas others argue it takes twice as long. Pickels said that when it comes to guiding his athletes back into training he typically budgets one week to regain fitness for every week that was taken off.

He said: “This is a good formula for the average amateur athlete. Of course, some athletes might regain fitness slower or faster than this, but that’s where it’s important to follow a flexible training program as you make your return.”

Particularly when it comes to the classic Type-A personalities (who typically gravitate towards marathon running and/or triathlon), it is not uncommon for them to want to be back to their previous levels of fitness as quickly as possible. Yet as Pickels warned: “This is the fastest route to failure! It’s important to readjust training zones to your current fitness level, not where you used to be. If you have to rely on one metric, heart rate is going to be the most reliable, but don’t be discouraged when your pace feels slow and your effort feels high!”

Remember, too, that if you’ve taken a break from running that was longer than four weeks, you should be cautious with total training volume as you return. A good rule of thumb is to return to 50-75% of your previous training volume, gradually increasing this every one to two weeks until you’re back to full volume.