Published December 4, 2025 07:39AM
There are occasions when we all get sick, injured, or too busy to run. That’s life. But for runners, it’s a hard part of life to accept. When we take time off running, we tend to panic, worried that we are losing fitness.
It’s not an unreasonable fear. A landmark study called the 1966 Dallas Bed Rest and Training Study put five healthy college students in hospital beds for 20 days, with only one brief respite for a shower. The goal was to simulate the effect of weightlessness for NASA’s Moon-bound astronauts, but it and a 1996 follow-up study discovered that the short-term effects of three weeks of total inactivity were worse than the effects of 30years of aging. When they emerged from their inactivity in 1966, the volunteers—two of whom were competitive athletes (one had a 4:45 mile) were so feeble that they had to be wheeled on gurneys to the sports lab for testing. Two fainted during their first treadmill tests.
Not that anyone but a research subject or desperately ill hospital patient is likely to be that inactive. (In fact, one of the consequences of the Dallas Bed Rest and Training Study was the realization that surgical patients need to be prodded out of bed and into activity as soon as possible. I have a friend who started doing Parkrun 5Ks—walking, not running)—six weeks after a heart transplant.) But we all, eventually, take time off running.
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Of Course, All Bodies Are Different
There is no perfect roadmap to what happens to your body during a layoff. “There’s a bell curve,” Thomas Schwartz, a kinesiology professor and cross-country coach at Dickinson State University, North Dakota, says. “You have people who have a small amount of change, people [with] a bit more change, and [people who] have a lot of change.”
What you do during your time off also matters. When I lived in Minnesota, I often quit or dramatically reduced my running in the coldest part of winter, shifting to cross-country ski racing. When I switched back to running, I rebounded so quickly that my lifetime 5K PR was run in early May.
That said, it is still possible to draw some degree of a timeline to the changes that occur when you take anywhere from a few days to a few months off.
Even with just a few missed days of running, you might feel flat when you return. Your pep comes back soon, though. (Photo: Getty Images)
One Week Off Running
The first thing is that if you take even a few days off, you may feel sluggish the first time you return to running. For some people, this can happen with as little as one day off. When he was in high school, Schwartz got to wondering about this and tested it on himself by measuring his heart rate during a couple of mile repeats at a not-all-that-difficult pace. He then took a day off and did it again. “My heart rate was five beats higher, [and] my breathing was a little more rapid,” he says.
To make sure this wasn’t a one-off, he repeated the experiment several times, getting the same result. He also did some road races, with or without taking the day before off.
“Every time I ran a race where I took a day off, I ran worse,” he says. Other runners refer to this as “going flat” before the race, and it is one of the reasons why, even when tapering for a marathon, most want to do something the day before, even if it’s just a 20-minute jog and a few 100-meter strides.
Schwartz suggests that part of what’s going on is that taking a day off before the race doesn’t mean it’s just a 24-hour layoff. It’s 48 hours. “People don’t get that,” he says.
What exactly is going on physiologically isn’t clear—the conventional wisdom, in fact, is that nothing physiological has changed.
A study by Edward F. Coyle, now director of the Human Performance Laboratory at the University of Texas at Austin, does show, however, that one of the first effects of detraining is a substantial reduction in blood plasma (12% in Coyle’s study).
A subsequent study, by Spanish researchers Iñigo Mujika and Sabino Padilla described the effect as “rapid,” though there appears to be no research on the degree to which it starts in the first week. Schwartz, however, thinks it might set in during the first two to seven days. If so, it could produce the cardiovascular responses he experienced by impeding the heart’s ability to fill to capacity with blood, forcing it to beat faster to achieve the same output.
Testing VO2max is a cornerstone of decades of research about time away from running. (Photo: Getty Images)
Two to Three Weeks Off Running
After the first week, there is more research. The leading study is a 1984 paper by Coyle and colleagues, who recruited seven well-trained athletes (a mix of runners and cyclists) and asked them to stop training and reduce their exercise level to the minimum required for their sedentary jobs—typically a mere 500 meters of slow walking per day. Not the Dallas Bed Rest and Training Study, but a really serious layoff.
Prior to the time off, they were subjected to a battery of tests, then retested at 12, 21, 56, and 84 days. What they discovered was that a lot of changes occurred, but the rate at which they occurred varied. Stroke volume (the amount of blood pumped by the heart per beat) fell 10% by day 12, then remained stable until at least day 21. VO2max (the maximum rate at which your body can process oxygen) followed a similar pattern, dropping 7% at 12 days, then also staying stable until day 21.
Most likely, these changes are due to the rapid reduction in blood plasma observed by Mujika and Padilla. In a subsequent study, Coyle’s team tested this by detraining a different cadre of athletes for 2-4 weeks, then infusing them with enough saline solution to bring their blood volumes back up to baseline. That instantly eliminated a major portion of the reduction in their stroke volumes and VO2max. A 2023 study on male runners confirmed the drop off in VO2max after two weeks, finding that muscle strength also drops, but the body maintains muscle endurance in that time frame.
In the folds of a mitochondrion, chemical reactions to produce energy in the form of adenosine triphosphate (ATP) take place. (Photo: Getty Images)
One Month Off Running
Somewhere between 21 and 56 days, Coyle’s data show other major downturns. The loss in stroke volume increases by 30-40%. The loss in VO2max doubles.
A big, new factor that might be contributing to this is changes in citrate synthase, which is a key mitochondrial enzyme that indicates a person’s aerobic capacity and function. “If you take somewhere around a month off,” Schwartz says, “that declines.” Coyle’s research shows that the effect is really dramatic, starting early, but kicking in at an accelerating rate somewhere between 12 and 56 days.
Due to the changes in mitochondrial enzymes, you are also seeing a shift from fat-burning metabolism to glucose-burning metabolism. “That is a profound effect,” Schwartz says. “One of the things you want to improve [in training] is to burn more fat at any given pace. You’re going to be more efficient.”
When you make your return to running, your body remembers your prior fitness—making a strong comeback possible. Just be patient with yourself. (Photo: Canva)
Beyond a Month
When layoffs extend beyond a month, the rate of physiological change generally slows. Coyle’s study, for example, didn’t find huge additional changes in VO2max between 56 and 84 days. Nor did it find big changes in mitochondrial enzymes like citrate synthase and another enzyme, succinate dehydrogenase, whose detraining decay path strongly follows that of citrate synthase.
One explanation for the tail-off of the decline is that as a layoff extends from one month to two, then three, you are now, as Schwartz puts it, “getting closer to the sedentary state, or what that would be.” I.e., there’s a limit to how much fitness you can lose.
But you’re not losing everything. The capillaries built by your prior training remain, at least until the end of Coyle’s study at day 84. And there are indications that the body remembers its previous fitness and might be quicker to rebound off a layoff than it is to train completely from scratch. There are plenty of stories of women coming back stronger than ever after pregnancies.
And in a 2025 case report in Frontiers in Physiology, French triathlete and physiologist Romuald Lepers took 12 weeks (84 days) off from training, limiting himself to two slow 30-minute walks and two 15-minute core-strength sessions a week. He then returned to training, building up progressively over the course of an additional 12 weeks.
During the layoff, he experienced a 10% reduction in VO2max and associated other losses of conditioning, much like Coyle’s athletes. By the end of the 12-week retraining, however, he found that “almost all variables,” including VO2max, had returned to baseline or even slightly improved—the big exception being lean body mass and running economy, neither of which had fully recovered as of the end of his study. Three months after that, however, he was racing as well as ever.
The lesson from Lepers is that a break is not the disaster that runners believe it to be. “Athletes should not hesitate to take a training break when they feel physically and/or psychologically tired,” he told Amby Burfoot when his report was released. “You will lose some fitness, but you will be highly motivated to retrain.”