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This is What Happens to Your Body When You Stop Exercising


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.

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.

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.

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.

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.

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.

“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.”

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.

“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.

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.

“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.”


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