Losing Running Fitness: A Scientific Look at How Much You’ll Slow Down When Not Able to Run

How much will taking a few days off from running hurt my fitness?

It’s one of the most common questions I get from runners struggling with an injury, fighting the flu, or hesitant to take a much needed rest from training. As runners, we are all paranoid about taking a few days off, generally thinking it will ruin our months of meticulous training.

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As a coach, I am not immune to being frightened by this irrational fear.

While I was training for the NCAA championships while in college, I had a swimming accident that left my left shoulder separated and required a visit to the hospital to get it back in place. The doctors told me I needed to take a few days off to let the shoulder heal.

Not wanting lose any precious training time, I strapped my left arm tight across my body using a combination of saran wrap and duct tape and went on a 12-mile run the next morning. Luckily, I didn’t suffer any lingering affects from the imbalances I created by running with one arm.

However, I wanted to share this story with you to demonstrate that I write this article with the deepest understanding of how hard it can be to listen to science and understand that a day off isn’t going to end your hopes of running as fast as you’ve dreamed.

When we look at the effects of taking time off from running, we have to analyze the detraining from two perspectives: (1) your metabolic systems such as aerobic fitness, threshold and VO2 max; and (2) your structural systems such as your muscles and neuromuscular coordination (how fast and efficiently your brain can tell your body to perform and execute a specific movement).

Effect of detraining on the aerobic system

Because VO2 max is one of the best measurements of a runners physical fitness, I will use it as the baseline to compare the effects of detraining on your aerobic system. To be brief, VO2 max is an individual’s maximum ability to transport and use oxygen during exercise (here’s more information if you need).

Recent studies show that there is little reduction in VO2max for the first 10 days following inactivity in well-trained athletes. It is prudent here to mention that all of these guidelines assume you are a decently trained runner, having trained consistently for a 4-6 month period. Beginner runners will lose fitness at a slightly faster rate since they have a smaller base of fitness.

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After two weeks of not running, studies show that VO2 max decreases by 6%. After 9 weeks VO2 max drops by 19% (sorry, I couldn’t find any data on 3-8 weeks post inactivity). After 11 weeks of no running, Studies demonstrate that VO2 max falls by 25.7% from peak physical fitness.

So, as you can see, from an aerobic standpoint, you have very little to worry about if you have to take a break from running for two weeks or less.

This is very important for those runners that need to take a hiatus because of a small injury or are nervous about taking downtime after a long training segment. A 6% decline in VO2 max can be made up with one or two weeks of solid training.

While percentages are fantastic, what do those numbers really mean for runners?

Let’s use an example of a 20 minute 5k runner. A 20 minute 5k runner has a VO2max of roughly 49.81 ml/kg/min (estimated using a formula). After 2 weeks of no running, the 5k runner would lose 6% of his VO2 max, which would be 46.83 and would now be in 21:05 shape, according to most estimates.

After 9 weeks of no running, the same 20-minute 5k runner would now be in 24:00 minute 5k shape. After 11 weeks of no running, our poor running friend would be in 25:30 shape.

Effect of detraining on the structural system

While the reduction in aerobic fitness has been tolerably studied in an applicable manner, the effect of detraining on specific running muscles has been harder to find.

However, the little research that does exist about detraining in general proposes that the most dramatic reduction in fitness occurs within a 10-28 day window. Before and after this window, detraining from a structural perspective isn’t severe.

Also, there is some research on how quickly use benefits from strength training (see our full write-up here). In short, most of the research shows muscle power declines significantly slower than metabolic factors.

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What does this mean? After 7-10 days of not running, you will lose some muscle power and coordination, but not enough to totally derail your goals.

With a few specific workouts such as hill sprints, you’ll be back to your pre-detraining levels before you know it. If your break from training is longer than two weeks, than you’ll have a little bit to make up before you can get back to personal best shape.

What does it all mean?

Research shows you shouldn’t be too worried about losing significant fitness if your break from running is less than two weeks.

You’ll lose some conditioning in your aerobic system and muscles, but pre-inactivity fitness will return quickly. Again, this assumes that you have built a healthy and consistent base of training of 4-6 months prior to taking time off. It’s not the end of your career if you haven’t been training for this long; it simply means that the reduction in fitness will be slightly more pronounced.

After two weeks of not training, significant reductions in fitness begin to occur and you’ll have about 2-8 weeks of training (depending on the length of inactivity) ahead of you to get back to your previous level of fitness.

Basically, here is an easy to follow form chart:

Days of not running Reduction in fitness What this means for a 20 minute 5k runner 1-7 days Negligible reduction in VO2 max and muscle power Now running 20:10 10-14 days 6% reduction in VO2 max and minimal reductions in muscle power Now in 21:05 shape 14-30 days Estimated 12% reduction in VO2 max and decrease in muscle power Now in 23:00 shape 30-63 days 19% reduction in VO2 max and significant decrease in muscle power Now in 24:00 shape 63 days or more 25.7% reduction in V02 max and significant decrease in muscle power Now in 25:30 shape

By no means am I suggesting that taking time off from running is an enjoyable experience. However, sometimes it’s inevitable or for the best in the long-term. I hope this article answers all your questions in a practical yet scientifically supported way. Please feel free to comment and share with your running friends.

Now that you know that a few days off, how do you adjust your training when you need to take time off? Here’s an exact outline on how to return to running after injury, sickness or missing training.

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