Of course, we are all different, when it comes to our bodies and our respective levels of fitness. Certainly, we also differ in the way we train, in what kind of sports we engage, and we all have unique physical strengths and weaknesses. Consequently, the effects of inactivity, or “detraining,” will manifest differently from person to person. There are, however, some conclusions that are important to know:

Within two weeks: Your endurance will drop

Typically, VO2 Max, which is the maximum amount of oxygen your body can use, is the first fitness measure to be affected by inactivity. It can drop by about 10% after two weeks. After four weeks, your VO2 max can decrease by about 15%, and after three months, it can fall about 20%.

Speed and high intensity training are highly dependent on VO2 max, if this is in decline, the inability to transport sufficient oxygen will impact your endurance and makes it harder to recover. When the amount of oxygen is limited, the muscles produce lactate.

The ability to train at higher intensities becomes also more difficult because lactate accumulates quicker and at a lower intensity.

Within four weeks: Your strength will start to diminish

An individual undergoing detraining will also reduce the amount of muscle tissue. Our strength seems to diminish at a slower rate than our endurance, but the effect of detraining on the structural system, will manifest itself between 10 to 28 days. This will become noticeable in diminished muscle strength and a loss in power, including speed and agility.

Within eight weeks: You might gain fat

Regular exercise increases muscle strength, power, coordination, stability, and flexibility, while improving endocrine measures, such as sugar and fat levels.

In terms of metabolism and muscle function, detraining also has a significant impact on the trained body. The body’s respiratory exchange ratio goes up, meaning fat metabolism becomes impaired, resulting in increased fat storage.

Therefore, after about six to eight weeks of detraining, people usually will start to notice a physical change related to an increase in body weight and waist circumference.

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In conclusion, this is what happens to your body when you stop training:

There are plenty of benefits to exercise, but they are not permanent. In fact, many of those hard-earned gains will start to disappear in as little as two weeks.

The effects of detraining are profound, but are closely related to the initial fitness level. The fitter you are, the profounder are the negative effects on the body.

For this reason, it is important to stay physically active, even if you’re not training as you used to.

• Reduce the amount of training volume.

• By maintaining some measure of intensity and decreasing frequency only moderately, many of the negative effects of detraining can be avoided.

• Cross train in sports that are similar and even dissimilar. Using the elliptical trainer when injured, can help retain running benefits. Even completely different sports can still have crossover aerobic benefits.

Just don't quit moving altogether—your body, brain, and waistline will thank you.

More from MD Thordis Berger expert medical tips and advice.

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Ready, A.E., Quinney, H.A. Alterations in anaerobic threshold as the result of endurance training and detraining. Medicine and Science in Sports and Exercise. 1982, 14(4).

Ormsbee MJ, Arciero PJ. Detraining increases body fat and weight and decreases VO2peak and metabolic rate.J Strength Cond Res. 2012 Aug;26(8):2087-95. doi: 10.1519/JSC.0b013e31823b874c.

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