It takes a lot of hard work to stay in shape, which is why it’s important to exercise on a regular basis. But it’s not always possible to remain active, and sometimes a few days off can turn into a more... extended hiatus. Here’s what happens to your body when you suddenly stop exercising.

Measuring Fitness

To understand what we stand to lose when we stop exercising, it’s helpful know about how we quantify fitness in the first place.

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When it comes to our bodies and our respective levels of fitness, we’re all different. We train differently, we engage in different sports, we vary in age, and we all have unique physical strengths and weaknesses. Consequently, the effects of inactivity, or “detraining,” as it’s often called, will manifest differently from person to person. There are, however, some standard measures that can be used to assess a person’s level of fitness.

Among the more important measures is oxygen uptake, or VO2, which is a measure of the difference in oxygen concentration between the blood leaving the heart and the blood that returns to the heart.

An athlete having his VO2Max measured (Credit: Catedra/CC-SA 3.0)

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Another tool used by exercise physiologists is the measure of lactate a person produces at a particular exercise intensity; lactate is produced by the muscles when the amount of oxygen is limited, so it provides an indirect measurement of the body’s efficiency at delivering oxygen to the tissues.

Other measurements quantify things like muscle endurance, muscle strength, and flexibility.

Relatedly, there are “gym-type measurements,” which include push-ups, pull-ups, abdominal plank, toe-touching, and so on. But according to Andreas Bergdahl, an assistant professor in cardiovascular physiology at Montreal’s Concordia University, these tests are more subjective than, say, VO2 and lactate measurements.

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Lastly, there’s the science of endocrinology, which measures the effects of exercise on hormonal levels, metabolism, growth and development, tissue function, and a host of other things.

Our Bodies on Exercise

In order to understand the effects of sudden physical inactivity, it’s important to consider the effects of regular exercise on the body. Regular exercise, of course, encompasses many different things, from lifting weights (i.e. resistance training) to endurance work, such as rowing or running. But as Bergdahl explained to io9, endurance training benefits a person’s overall capacity, while resistance training has limited effects on the aforementioned measures of fitness.

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Bergdahl says that regular endurance exercise leads to four major consequences:

  1. Increased ability of the heart to eject blood
  2. increased ability of the blood vessels to send blood to where blood is needed
  3. Increased number of capillaries (the vessels that deliver oxygen and ‘food’ to the muscles)
  4. increased size and the number of mitochondria (the “power plants” of the cells).

All these changes lead to the more efficient use of oxygen, as well as nutrients.

“Instead of sending lots of blood to your gut, kidneys and skin, all with limited ability to enhance someone’s performance, your body has trained its capacity to use the resources for maximizing performance,” says Bergdahl.

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Harry Pino, a senior exercise physiologist at the Sports Performance Center at NYU Langone Medical Center, agrees that endurance training has a profound effect on the body.

“Research shows that thirty consecutive minutes of cardiovascular activity, when done regularly, will not only improve your cardiovascular system, but will also improve some core mobility,” Pino told io9.


“The fitter you are, the harder you fall.”

- Harry Pino, a senior exercise physiologist at the Sports Performance Center at NYU Langone Medical Center


What’s more, he says that regular exercise increases muscle strength, power, coordination, stability, and flexibility, while improving endocrine measures, such as sugar and fat levels.

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“There are so many different types of benefits,” says Pino. “But from a cardiovascular and cardiorespiratory standpoint, those are the key ones that we can visually observe, and also observe via changes to biomarkers.”

Apparently Healthy but Deconditioned Individuals

So what happens to the body when physical activity comes to a grinding halt?

People, be they elite athletes or regular gym-goers, get injured, take extended fitness breaks, or simply lose interest. Exercise physiologists refer to such people as “apparently healthy but deconditioned individuals,” and the effects of detraining are profound, to say the least—but it largely depends on the person’s initial fitness level. As Pino explained to me: “The fitter you are, the harder you fall.”

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It’s a sentiment mirrored by Bergdahl, who says an athlete’s fitness drops faster the fitter they are.

“This means that what applies to an elite athlete after one week might take a sedentary individual one or two months to experience,” says Bergdahl. The elite athlete will still be in better shape than the couch potato, but will have lost a greater percentage of her fitness. An analogy is useful here: Two pots of water —one just off the boil, the other hot but not scalding—will both lose heat as they cool and approach room temperature, but the hotter pot will lose more heat faster; after five minutes, the former pot will still be hotter than the latter, but it will have lost a greater proportion of its overall heat.

Disheartening Declines

Typically, VO2 Max is the first fitness measure to be affected, followed by declines in muscle structure, power, strength, stamina, and coordination. Athletes in detraining mode can also expect to experience a rise in sugar levels and blood pressure. And indeed, some changes happen very quickly. As Bergdahl explained, the aerobic enzymes which provide food for the mitochondria decrease within days, while the adaptation in blood supply decreases relatively slowly. Basically, the local (within muscle) metabolic system declines greatly when compared to the capacity to circulate and deliver oxygen.

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“There are studies indicating a decline of 7 to 10% of VO2 after 12 days of sudden inactivity, 14 to 15% after 50 days, and 16 to 18% after 80 days,” says Bergdahl. “Maximal values for cardiac output, stroke volume [the amount of blood pumped out of the heart during each contraction] and ability of mitochondria to extract oxygen each decline along the same lines while the heart rate increases.”

Like Bergdahl, Pino says that endurance is among the first things to go. In otherwise healthy people, detraining begins to produce its deleterious effects within the first week, though in very low percentages (less than 5%) in both VO2Max and muscle power. Pino says that runners demonstrate some of the most dramatic declines in these areas.

MAA 5k Fun Run (Credit: MAA/Public domain)

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For example, Pino considers a person who can run 5 kilometers in 20 minutes. After just one week of inactivity, that person will typically worsen their time by approximately 10 seconds. But after 10 to 14 days, that’s when the percentages really start to drop. Runners begin to experience a reduction in muscle power, and start adding around one minute and five seconds to their time. Between 14 to 30 days, athletes can expect to experience around a 12% reduction in VO2 and a noticeable decrease of muscle power.

“Notice that during the first two weeks I described it as being a reduction of muscle power,” adds Pino, “but now we’re describing it as a decrease in muscle power.”

What was once a 20-minute 5K has now turned into a 23-minute 5K. By the 1- to 2-month mark, athletes typically lose about 19-20% of the VO2, and a significant decrease in muscle power. At this stage, they’re pushing 24-minutes or so. After two months, athletes typically exhibit a 26% reduction in VO2.

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“It’s shocking to see what happens to the body,” says Pino. “We start to see lots of changes to muscle, strength, and fat levels—it really deteriorates your structural well-being.”

Not surprisingly, these dramatic declines can be disheartening for elite athletes, particularly after all the hard work they put in.

“This can really discourage people from starting exercises for the fear that they’re going to lose it,” he says. “Well, they will— and the subsequent recovery will take longer, so it’s important that they keep this in mind.”

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Interestingly, Pino says that some people blame these declines, particularly endurance, on age, but the reality is that a person’s VO2 does not change dramatically with age.

“Certainly, it will eventually change with age, but what typically changes are the habits of practicing at high level intensity, and that’s how you slow down,” he says. “It’s not so much the frequency of exercise, but the quality of exercise.”

Essentially, the fit individual gradually turns sedentary with detraining.

Muscles and Fat

Cardio may be the first to go, but losses in muscle mass eventually follows. Bergdahl says the rate of loss is largely dependent on age, as the older we are the faster we lose our muscles. But while it’s possible to produce substantial improvements in physiological function among the elderly, regular exercise can only slow, and not completely halt, muscle atrophy.

English weight lifter Zoe Smith at the London Youth Games (London Youth Games/CC BY-3.0)

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“Basically, the aging effect is the same as what will happen during detraining,” says Bergdahl. “The cross sectional area of the muscle fibers—in particular what we call the slow twitch fibers—will decrease, quickly at first and then more slowly. This means that an individual undergoing detraining will reduce the amount of muscle tissue they have. The changes will start within the first two weeks. Furthermore, the oxidative capacity (i.e. the ability of the mitochondria to produce energy) will decline in the explosive muscle type we call fast twitch muscle fibers.”

According to Pino, higher trained athletes tend to show a drop of aerobic endurance within three weeks of detraining, but even after 12 weeks they will still retain a significant amount of bulk strength and endurance.


“Basically, the aging effect is the same as what will happen during detraining.”

- Andreas Bergdahl, assistant professor in cardiovascular physiology at Montreal’s Concordia University


“That’s because they were at a high level of fitness,” he says. “But if we’re talking about deconditioning individuals, now they’re really going to start hitting those numbers already mentioned.”

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Pino says that regardless of whether a person is a pro athlete or just someone who works out regularly, the effect of detraining on the structural system, typically between 10 to 28 days, will manifest as noticeable diminished muscle strength and a loss in power, including speed and agility, mobility, moving from side-to-side, the ability to stop on a dime, and a loss of coordination.

And then there’s fat to consider— though Pino says there’s a big misconception about muscles converting into fat.

“What really happens is that the muscle cells —which are completely different than fat cells—become smaller, because now you don’t have a demand of power and strength—they’re not growing,” he told io9. “That’s atrophy. Then you have fat cells that are starting to get larger and bigger, which will lead to changes in your appearance. Instead of looking lean and trim, you start feeling bloated and round.”

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When it comes to weight loss via exercise, Pino says there are three important components to consider: frequency, duration, and intensity, with the first two being the most important.

Inactivity Badness

In addition to weight gain (and the potential for obesity), a completely sedentary lifestyle can lead to a host of related complications, including health issues such as diabetes, cardiovascular disease, and problems with joints and ligaments.

Another important aspect is that a lack of exercise will hasten the decline in functional capacity which happens with aging.

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“Physiologic and performance measures improve rapidly during childhood and achieve a maximum between late adolescence and approximately age 30,” says Bergdahl. “The decline starts shortly after—sometimes after 40 depending on the body system—and the changes are similar to detraining.”

Regular exercise, says Bergdahl, will slow down this deterioration and will keep your body “younger” for a longer period of time.

Lastly, there are the psychological effects of physical inactivity to consider: A lack of exercise has been shown produce certain psychological effects, including depression and lower self-esteem.

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“Part of that has to do with blood flow (again oxygen) to the brain,” explains Bergdahl. “More oxygen equals better brain function. Furthermore, exercise is actually a form of stress—but a good, controlled one—which is used to train your body to handle not only an increased oxygen demand but also increased levels of certain hormones such as norepinephrine. By exercising you desensitize the receptors which means that it will take higher concentrations of these hormones, as well as longer exposure for your body to feel stressed.”

Thus, a lack of exercise will produce the opposite effect.

Active Recovery

Taken together, it’s clear that the sudden adoption of a sedentary lifestyle, particularly after a period of regular training or exercise, has a profoundly negative effect on the body.

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“That’s the importance of staying physically active, even if you’re not training as you used to. Even if you take a break you still have to be training.” says Pino. “This is what we call ‘active recovery.”’

Athletes can still work out while taking it easy (Credit: US Navy/Public domain)

To that end, Pino recommends that athletes, even after running a marathon, or when nursing an injury, can still find ways to remain active. He recommends such things as the stationary bicycle, elliptical, or rower. Many of his athletes cross-train during their detraining phases.

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“This gives the muscles that work hard a break,” he says, “but they won’t lose that much aerobic fitness.”

Other sources: Ready, E. A.; Quinney, H. A., Alterations in anaerobic threshold as the result of endurance training and detraining. Medicine & Science in Sports & Exercise 1982, 14 (4), 292-296 | Coyle, E. F.; Martin, W. H.; Sinacore, D. R.; Joyner, M. J.; Hagberg, J. M.; Holloszy, J. O., Time course of loss of adaptations after stopping prolonged intense endurance training. Journal of Applied Physiology 1984, 57 (6), 1857-1864 | Madsen, K.; Pedersen, P. K.; Djurhuus, M. S.; Klitgaard, N. A., Effects of detraining on endurance capacity and metabolic changes during prolonged exhaustive exercise. Journal of Applied Physiology 1993, (75), 1444-1451.


Contact the author at george@io9.com and @dvorsky. Top image by Tara Jacoby