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The Science Behind Falling Out of Shape

Story at-a-glance -

  • After about 10 days to two weeks without exercise, your VO2 max, a measure of cardiovascular endurance, will start to drop
  • Skipping workouts for just two weeks can significantly reduce your cardiovascular fitness, lean muscle mass and insulin sensitivity
  • After three to four weeks of inactivity, your muscles will start to atrophy and your body will start to revert to using carbs, rather than fat, for fuel

By Dr. Mercola

If you've worked hard to get in shape, the last thing you want is for your fitness gains to disappear. But if you skip your workouts for just two weeks, that's exactly what might happen, at least on some level.

While people who are in shape can generally regain their fit physique easier than those who are starting from scratch, it may sound somewhat surprising that your body can fall so quickly out of shape. The phrase "use it or lose it" definitely applies when it comes to your muscle strength, cardiovascular fitness and more.

The bright side is that beneficial changes happen quickly too when you exercise regularly. For instance, after just a few strength-training sessions neuromuscular adaptations occur, which means your brain has an easier time communicating with your muscles so you can use them more efficiently.

After exercising just over a week (assuming you've gotten your heart pounding), you'll enjoy increases to your plasma and blood volume and, as you continue, to your muscle mass and strength.

These gains can be fleeting, however, if you don't keep training, and once they're gone you'll have to work hard to get back in shape. According to sports physiologist Iñigo Mujika, it takes about twice as long to get back into shape as the time you've spent being inactive.1

Skipping Workouts for Two Weeks May Be Detrimental to Your Fitness Level

Let me preface this by saying that your body needs recovery time when you exercise, especially if you exercise at high intensity. That being said, skipping workouts for two weeks will catapult your body quickly into the "out of shape" category.

For instance, a study in the Journal of Applied Physiology suggested that skipping workouts for just two weeks can significantly reduce your cardiovascular fitness, lean muscle mass and insulin sensitivity.2

Mujika told Outside Online that after about 10 days to two weeks, your VO2 max, a measure of cardiovascular endurance, will start to drop and continue doing so at a rate of 0.5 percent a day.3

Indeed, another study found that after just 12 days without exercise, VO2 max dropped by 7 percent while blood enzymes associated with endurance performance dropped by 50 percent.4

Likewise, four weeks of inactivity among endurance cyclists resulted in a 20 percent decrease in VO2 max.5,6 Keep in mind this is among trained athletes — among those new to exercise, gains in VO2 max completely disappeared after four weeks of inactivity.7

Curiously, the opposite holds true for strength loss among newbies, with studies showing newly made gains in strength tend to hold on even after months of inactivity.

For instance, among previously untrained men who engaged in a 15-week strength-training program, taking a three-week break in the middle had no impact on strength levels at the end of the study.8

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The Older You Get, the Faster Your Muscles Atrophy

Mujika noted that after three to four weeks of inactivity, your muscles will start to atrophy and your body will start to revert to using carbs, rather than fat, for fuel, which sets the wheels of insulin resistance in motion.

Generally speaking, if you're very fit to begin with your body will remain in a fitter state longer than someone who's not fit, even as your workouts cease. If you're an Olympic athlete, for instance, taking two weeks off at the end of the season may be just what your body needs. As noted by Outside Online:9

"Mujika tells his athletes, including three-time Olympic triathlete Ainhoa Murúa, to take two weeks completely off from training at the end of their seasons, then spend two weeks doing physical activity that's not sport specific."

The older you get, however, the faster your muscles atrophy if you're not regularly engaging in appropriate exercise. In addition, it will take you longer to gain it back. When comparing 20- to 30-year-olds with 65- to 75-year-olds, the older group lost strength nearly twice as fast during six months of inactivity.10

If you need to cut back, incorporating some form of high-intensity exercise on a weekly basis seems to improve your chances of maintaining your conditioning, even if you can't resume your full fitness routine for several months.

In order to do this successfully, you need to exercise at about 70 percent of your VO2 max at least once per week, according to sports medicine expert Elizabeth Quinn.11

If You Skip Your Workouts, You'll Miss Out on These Phenomenal Benefits

Just meeting the minimum requirement of 30 minutes of moderate physical activity a day, five days a week, can reduce your risk of death from any cause by 19 percent.12 Those who engaged in moderate intensity activity — a full seven days a week — further reduced their risk of death, from 19 percent to 24 percent.

A separate study also found that, compared to those who exercised daily and often vigorously, sedentary people had a six times greater risk of dying from heart disease over the course of 15 years.13

The greatest gains are often seen among people who go from being sedentary to physically activity, although benefits also increase with exercise frequency and intensity (to a point, as overdoing it will backfire).

If You Fall Out of Shape, You'll Feel Older Faster

Part of what makes exercise so useful is that it not only extends your life but also adds quality of life to those extra years.

All of those movements that you might now take for granted — walking up and down stairs, carrying in groceries, climbing a ladder to change a light bulb — can start to become more difficult by the time you're in your 70s.

This is when sarcopenia (age-related muscle loss) tends to accelerate. You might start to feel weaker and find you can't do things, physically, that you used to do. But exercise can change that by helping you to maintain your muscle mass and strength (and even increase your muscle size).

At the same time, exercise lowers your risk of chronic diseases so much that researchers described it as "the best preventive drug" for many common ailments, from psychiatric disorders to heart disease, diabetes and cancer.14

It's worth noting, too, that walking 7,000 to 10,000 steps a day (which is just over 3 to 5 miles, or 6 to 9 kilometers) in addition to your exercise program is also important for keeping your body healthy.

Are There Times When You Should Skip a Workout?

One of the benefits of being fit is that you can take time off from exercise and use the "reserves" that you have built up during your time off. If done infrequently, skipping a workout is unlikely to negatively affect your overall fitness level, and in the cases that follow is probably more beneficial than not.

  • You're sick below your neck (i.e., you have coughing or chest congestion, fatigue, muscles aches, vomiting or stomach cramps)
  • You have a fever
  • You're injured (there are some cases when light exercise can help an injury; seek the opinion of a physical therapist or personal trainer if you're not sure)
  • You're exhausted

As mentioned, you should also skip workouts at part of your recovery process, although on your "off" days you might still do gentle exercise, like stretching or yoga, and you can still do your 10,000 steps. As your fitness increases, the intensity of your exercise goes up, and the frequency that your body can tolerate goes down. As a result, you need to continuously customize your program to your own fitness level and other lifestyle issues.