This popular story from the Daily Briefing's archives was republished on Aug. 2, 2023.

After a long break from exercising, it can feel "daunting" and "miserable" to try to get back into a workout routine. Writing for the New York Times, Knvul Sheikh speaks to experts about the best ways to rebuild your fitness after a break—and what you can do to prevent fitness loss in the first place.

What does it mean to lose fitness?

"When it comes to cardiovascular fitness and muscle strength, the adage is true: Use it or lose it," Sheikh writes. Regular exercise can improve your heart health and strength, but taking time off can reverse those benefits.

According to Edward Coyle, a professor of kinesiology and health education at the University of Texas at Austin, one of the first things to weaken when you stop exercising regularly is your cardiovascular endurance.

After a few days without exercise, the volume of blood plasma in the body decreases, which leads to several other cardiovascular changes. Research has shown that after 12 days without exercise, the total amount of blood the heart pumps every minute, as well as the amount of oxygenated blood available, declines. However, if you return to the gym at this point, you will only notice slight performance differences, Coyle said.

The three-week mark is often when people experience the biggest changes to their endurance when working out. At this point, your body will have to work harder to pump blood and oxygen, and "exercise will be more fatiguing," Coyle said.

When it comes to strength, it will take around eight weeks of inactivity for the size and strength of your muscles to begin decline. Over time, the maximum amount you can lift and the number of repetitions you can do will decrease, and you will likely experience more soreness after a workout.

"Your body adapts to the stimulus you provide," said Kevin Stone, an orthopedic surgeon. "Your muscles become used to the stress and the testosterone, the adrenaline and endorphins — all the wonderful things that circulate from exercise. When you take that away, the body initiates a muscle loss program."

In general, how much your fitness declines after inactivity can depend on your age, genetics, lifestyle, diet, and prior level of fitness. According to studies, older adults lose fitness twice as fast as those in their 20s and 30s. High-level athletes also "have more to lose in absolute terms" compared to people who exercise more recreationally, Coyle said.

How to regain your fitness

Although getting back into exercise after a long break can feel "daunting or miserable," it is possible to regain your fitness over time, Sheikh writes. In fact, people who regularly exercised may have an easier time regaining their fitness than those who were largely sedentary in the past.

Inactivity can shrink muscle fibers, but they don't disappear completely. These fibers also have a molecular "muscle memory" that can help them strengthen again even months later, allowing people to regain their strength and endurance more quickly.

"You can regain approximately one-half of your fitness in 10 to 14 days with moderately hard workouts," Coyle said.

After that, the time it takes to fully regain your fitness will depend on how fit you were to begin with. In one study, older adults needed less than eight weeks of retraining to regain their fitness after a 12-week break.

When rebuilding fitness, Coyle recommends people start with a goal of working out for a certain amount of time each day without worrying about strength and intensity. As you get more comfortable with a certain level of exercise, you can increase your pace or add heavier weights.

In general, personal trainers recommended increasing your exercise length and intensity by no more than 10% each week. People can also adjust their exercise routines based on how their bodies feels.

To prevent fitness loss in the future, Coyle recommends people do any kind of small exercise instead of completely taking time off. For example, you can do body-weight exercises instead of lifting with dumbbells or make time to do a few short high-intensity interval exercises throughout the day.

"If you spend just a few minutes a day doing interval training, that's sufficient to keep blood volume elevated and mitochondria relatively high," Coyle said.

For people who need to take a longer break, it may be beneficial to try cross training or switch to a different sport, such as swimming or skating. People can also work on their balance through aerobics or dance classes that help keep their muscles active in a different way.

"Overall fitness is a combination of many factors," Stone said. "It's not just muscle strength and cardiovascular fitness." (Sheikh, New York Times, 1/31)