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.
How Quickly is Running Fitness Lost?
Cardiovascular Affects 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.
What Does This All Mean in Terms of Your Running Fitness?
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 Quickly Fitness is Lost & How to Rebuild
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.
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.
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How long have you taken off of running? How did that impact your fitness?