Published July 14, 2026 11:05AM<\/p>\n<\/div>\n
There are occasions when we all get sick, injured, or too busy to run. That\u2019s life. But for runners, it\u2019s a hard part of life to accept. When we take time off running, we tend to panic, worried that we are losing fitness.<\/p>\n
It\u2019s 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\u2019s Moon-bound astronauts, but that study and a 1996 follow-up study<\/a> discovered that the short-term effects of three weeks of total inactivity were worse than the effects of 30 years of aging. When they emerged from their inactivity in 1966, the volunteers\u2014two 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<\/a> during their first treadmill tests.<\/p>\n Keep in mind that it\u2019s unlikely that anyone but a research subject or ill hospital patient would 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\u2014walking, not running\u2014six weeks after a heart transplant.) But we all, eventually, take time off running.<\/p>\n There is no perfect roadmap to what happens to your body during a layoff. \u201cThere\u2019s a bell curve,\u201d Thomas Schwartz<\/a>, a kinesiology professor and cross-country coach at Dickinson State University, North Dakota, says. \u201cYou have people who have a small amount of change, people [with] a bit more change, and [people who] have a lot of change.\u201d<\/p>\n 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.<\/p>\n 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.<\/p>\n Here\u2019s a breakdown of the timeline.<\/p>\n If you take even a few days off, you may feel sluggish when you first 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. \u201cMy heart rate was five beats higher, [and] my breathing was a little more rapid,\u201d he says.<\/p>\n To make sure this wasn\u2019t 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.<\/p>\n \u201cEvery time I ran a race where I took a day off, I ran worse,\u201d he says. Other runners refer to this as \u201cgoing flat\u201d 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\u2019s just a 20-minute jog and a few 100-meter strides.<\/p>\n Schwartz suggests that part of what\u2019s going on is that taking a day off before the race doesn\u2019t mean it\u2019s just a 24-hour layoff. It\u2019s 48 hours. \u201cPeople don\u2019t get that,\u201d he says.<\/p>\n What exactly is going on physiologically isn\u2019t clear\u2014the conventional wisdom, in fact, is that nothing physiological has changed.<\/p>\n A study<\/a> 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 (a 12 percent reduction in Coyle\u2019s study).<\/p>\n A subsequent study<\/a>, by researchers I\u00f1igo Mujika and Sabino Padilla, described the effect as \u201crapid,\u201d 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\u2019s ability to fill to capacity with blood, forcing it to beat faster to achieve the same output.<\/p>\n After the first week of no running, there is more research. The leading study is a 1984 paper<\/a> 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\u2014typically a mere 500 meters of slow walking per day or about 750 steps.<\/p>\n 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 percent 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 percent at 12 days, then also staying stable until day 21.<\/p>\n Most likely, these changes are due to the rapid reduction in blood plasma observed by Mujika and Padilla. Coyle\u2019s team tested this by detraining a different cohort of athletes for 2 to 4 weeks, then infusing them with enough saline solution to restore their blood volume to baseline. That instantly eliminated a major portion of the reduction in their stroke volumes and VO2max. A 2023 study on male runners<\/a> 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.<\/p>\n Somewhere between 21 and 56 days, Coyle\u2019s data show other major downturns. The loss in stroke volume increases by 30 to 40 percent. The loss in VO2max doubles.<\/p>\n 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\u2019s aerobic capacity and function. \u201cIf you take somewhere around a month off,\u201d Schwartz says, \u201cthat declines.\u201d Coyle\u2019s research shows that the effect is really dramatic, starting early, but kicking in at an accelerating rate somewhere between 12 and 56 days.<\/p>\n Due to the changes in mitochondrial enzymes, you are also seeing a shift from fat-burning metabolism to glucose-burning metabolism. \u201cThat is a profound effect,\u201d Schwartz says. \u201cOne of the things you want to improve [in training] is to burn more fat at any given pace. You\u2019re going to be more efficient.\u201d<\/p>\n When layoffs extend beyond a month, the rate of physiological change generally slows. Coyle\u2019s study, for example, didn\u2019t 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.<\/p>\n 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, \u201cgetting closer to the sedentary state, or what that would be.\u201d In other words, there\u2019s a limit to how much fitness you can lose.<\/p>\n But you\u2019re not losing everything. The capillaries built by your prior training remain, at least until the end of Coyle\u2019s 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.<\/p>\n And in a 2025 case report in Frontiers in Physiology<\/em><\/a>, 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.<\/p>\n During the layoff, he experienced a 10 reduction reduction in VO2max and associated other losses of conditioning, much like Coyle\u2019s athletes. By the end of the 12-week retraining, however, he found that \u201calmost all variables,\u201d including VO2max, had returned to baseline or even slightly improved\u2014the big exception being lean body mass and running economy, neither of which had fully recovered by the end of his study. Three months after that, however, he was racing as well as ever.<\/p>\n The lesson from Lepers is that a break is not the disaster that runners believe it to be. \u201cAthletes should not hesitate to take a training break when they feel physically and\/or psychologically tired,\u201d he told journalist and former marathoner Amby Burfoot when his report was released. \u201cYou will lose some fitness, but you will be highly motivated to retrain.\u201d<\/p>\n Want more\u00a0<\/b>Outside<\/i><\/b>\u00a0health stories?\u00a0<\/b>Sign up for the Bodywork newsletter<\/b><\/a>.<\/b><\/p>\nOf Course, All Bodies Are Different\u00a0<\/b><\/h2>\n
A Timeline of What Happens to Your Body When You Stop Running<\/b><\/h2>\n
One Day to a Few Days of No Running<\/h3>\n
One to Three Weeks of No Running\u00a0<\/b><\/h3>\n
One Month of No Running\u00a0<\/b><\/h3>\n
One Month or More of No Running<\/b><\/h3>\n