How Quickly Is Running Fitness Lost?

Inevitably, all runners take time off of running. You may be taking a necessary post-race recovery break, a few days off for illness, or a longer time off due to injury, loss of motivation, or pregnancy/postpartum. Naturally, the first thought that enters the mind of a runner is: how quickly is running fitness lost?

Now, let’s add the first caveat: when we talk about detraining, we are talking about it in the context of zero to minimal (erratic) running. If you go through a busy few weeks of life where you are only able to run two times per week instead of five, you will still maintain a certain degree of baseline fitness with minimum running

Running fitness is a combination of cardiovascular, aerobic, neuromuscular, and other aspects of fitness. Detraining affects each of those individually; the individual changes then compound to cause an overall loss of running fitness. Initially, the detraining effects are minor; after 3-4 weeks of no training, however, loss of fitness accelerates. 

Cardiovascular Effects of Detraining

Plasma volume can decrease with as little of two days of detraining, although it takes approximately twelve days for exercise stroke volume to significantly decrease. After twelve days off, maximum cardiac output also declines. These changes are small, though, until a longer period of time off of running. 

A longitudinal study in the Journal of Applied Physiology examined participants of the 2016 Boston Marathon during an eight-week detraining period. This detraining period did not include a complete cessation from exercise (probably because they would not get enough subjects if they did!); runners were permitted approximately 1/10th of their previous training load. 

By four weeks of detraining, blood volume and plasma volume significantly decreased. After eight weeks of detraining, exercise-induced changes to the heart began to regress. Due to these cardiac changes, peak treadmill exercise time declined. 

How Detraining Affects Aerobic Capacity

Now, VO2max is not everything when it comes to fitness. Running economy, lactate clearance, force output, and other factors all affect running performance. In fact, multiple studies show that detraining can occur without changes to VO2max. 

In the above-mentioned Boston Marathon study, none of the participants experienced a change to their VO2max. However, research beyond this study repeatedly points to the fact that changes in performance are not correlated to changes in VO2max. All participants did experience performance loss, as indicated by reductions in time to exhaustion at both 4 weeks and 8 weeks. 

Other research (including this 2016 study published in The Journal of Sports Medicine and Physical Fitness) shows that after four weeks of detraining, time trial performances suffered. While you certainly will not lose all off your fitness, it is safe to conclude that that detraining of four or more weeks temporarily reduces peak performance. 

Musculoskeletal Changes from Detraining

A 2020 article in Frontier of Physiology delved into the issue of detraining during Covid-19 quarantine and what it meant for athletic performance. Within a few days of training cessation, muscle oxidative capacity and mitochondrial enzymes decrease. After approximately six weeks of detraining, your musculoskeletal system will lose many of its adaptations to the specific demands of running. 

What does that mean? Your muscles, bones, and tendons lose their ability to handle the mechanical load of running. Your body is no longer adapted to the high impact of running. In short, you will need to gradually resume training so that you do not get injured. 

After 8-12 weeks of detraining, force-generation capacity will decrease due to neuromuscular changes. After twelve weeks, muscle fiber (both type I and type II) will decrease in size and muscle atrophy will occur. 

This is why I as a coach program runners with a 6-8 week or greater hiatus to return with run-walk intervals. The last thing you want after one stress fracture is a strain, sprain, or worse. 

Can You Lose Running Fitness in One Week?

I am not going to share a chart to project x amount of fitness lost after y time off. Your fitness level prior, any cross-training you did, your age, and even your individual genetics will all influence the rate of fitness deterioration. There is no simple way to project what your VO2max, running economy, or lactate threshold will be after detraining. 

What you can most practically learn from this is that you should not despair short amounts of time off. A few unplanned rest days to calm a niggle will not cost you an entire training cycle. You will not lose all of your fitness by taking a week off when sick

It is essential to remember a key principle of physiology: it is always easier to rebuild fitness than it was to improve it the first time. You may have spent years building up to your pre-injury fitness; even with 6-8 weeks off, you will return to that level much more quickly than when you first achieved it. 

Certain reasons for time off will naturally require longer to rebuild fitness. For example, returning from a bone stress injury requires caution and a slower rebuild. The physiological changes of childbirth plus the demands of infant care will often require a gradual return to previous fitness. 

How To Rebuild after Losing Running Fitness

Dr. Jack Daniels provides a reasonable approach to rebuilding fitness: for eight weeks or less, spend equal amount of time rebuilding to your previous fitness level. For more than eight weeks, you will have lost more running fitness (for the reasons above) and require longer to rebuild. (This article explains in-depth how to start running again after time off.)

I will add the caveat that Jack Daniels’ formula is not infallible; it is simply a sound theory supported by scientific evidence.  

  • After five days off: No change to training load
  • One to four weeks off: equal time to reload as spent off, with first half at 50% of previous load, second half at 75% 
  • Four to eight weeks off: equal time to reload as spent off: first third at 33% previous load, second third at 50%, and final third at 75%
  • More than 8 weeks: Three weeks at 33%, three weeks at 50%, three weeks at 70%, three weeks at 80%

For example, six weeks off of running would mean three weeks at 50% load (or less) and three weeks at 75% load (or less). Think of these as maximums, not absolutes. Many runners returning from injury will resume training at lower than 50% of their original load.

Most importantly…remember the individual factor

How quickly fitness is lost will vary individually, to a certain degree. An elite like Desi Linden can regularly take a month off with seemingly no effect – but she is also a true genetic outlier. You do not want to interpolate conclusions for the general running population based on outliers. 

Some runners will lose fitness more quickly than others. Genetics, current fitness level, and age play a role, as does any cross-training done when not running. For example, if you are out for six weeks with an injury, you will lose less fitness if you are able to swim or cycle than if you are unable to exercise at all.

How long have you taken off of running? How did that impact your fitness?

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10 Responses

  1. Sound advice!
    Especially this point is very reassuring for injured runners: it is always easier to rebuild fitness than it was to improve it the first time.
    The come-back will be a lot easier than the first build-up.

  2. This is so interesting to me–throughout a lifetime of running, I’ve taken prolonged time off for pregnancy (I didn’t run during either one), surgery, and injuries. What you said is true–that it is easier to come back after prolonged time off than to start off new. The body remembers!

  3. I need to run consistently. I run 10 miles every weekend. I took two weekends off. And I felt it when I ran 10 again.

    But everyone is different.

  4. I have been forced to take of weeks of running due to injury in the past. Because I was allowed to bike and strength train, I kept up my cardio pretty well. I came back cautiously and was able to come back even stronger. Our bodies do have muscle memory

  5. This is so interesting, and really gives me insight into how long is too long to take a break before losing fitness! Right now I am running pretty casually (about 3 times per week), but when I start training more seriously again, I will keep this in mind!

  6. I’ve had two major “sabbaticals,” both were 2-3 months in length. I kept active while rehabbing, so I think my endurance and cardio maintained, and I think that is why everything else came back so easily when I was allowed to resume running. I had to ease back, with walking, then run/walk intervals, then gradual increased mileage, but the time-off seemed to actually enhance my running when the rally-backs were done. It’s amazing what the body can do it we respect it đŸ˜‰

  7. Very interesting! My longest break from running was when I had hip surgery and I stopped for 4 months before surgery and was cleared to start running on the Alter G 12 weeks post surgery. I kept up with cardio and had a slow return back. I think coming back after my c-section was harder even though I only took off about 8-9 weeks. During that time I wasn’t doing much besides walking.

  8. In my brain, I feel like I lose so quickly, but that’s in my head. I’ve felt great after a week. And even coming back after my injury wasn’t as bad as I imagined. This is a great overview of how it all works!

  9. I remember thinking, about 10 years ago when I had my hysterectomy and had to take off about six weeks, great! This will be great for my body, I’ll get very rested, and I’ll feel awesome when I start back. Um, no. It was almost like starting over! It took a long time to get my base back (though I’m sure major surgery had something to do with it too).

  10. Interesting, Laura. I know that in January I was sick (not COVID), but just a lingering low grade temp & fatigue & I chose to take 3 weeks off running. I made sure to return very slowly, since I wasn’t training for anything. I definitely noticed that I’d lost some fitness, but with a slow & steady return it came back.

    I also occasionally will take a week off running here & there, and I definitely don’t lose much fitness there and usually will pick up where I left off.

    It definitely is individual though!

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