Published October 6, 2026 10:40AM
How we reported this: The writer, Erica Sloan, is a health journalist. She interviewed four experts for additional context on how the body responds to detraining over weeks to months.
Improving your physical fitness—whether it’s to excel at a sport like running or lifting, or simply to stay active and healthy as you age—hinges on regular exercise. So when you have to stop working out for any reason, it’s natural to wonder how, and how soon, your body will notice.
“Our bodies are incredibly adaptable,” Jacque Crockford, director of product development at the American Council on Exercise (ACE), told Outside. “They adapt both to the exercise we do and to the exercise we stop doing.” If you’re an athlete, you likely know that brief breaks, including both rest days and periods of low-intensity movement, are essential for recovery, allowing you to avoid injury and ultimately optimize performance. But “if the pause is longer, some of the adaptations we’ve built through training can begin to diminish,” Crockford said.
The timeline for detraining (aka, the loss of fitness gains such as strength or stamina with stopping exercise), is a “sliding scale,” Kevin Murach, associate professor of Nutrition and Exercise Physiology and principal investigator of the Molecular Muscle Mass Regulation Laboratory at the University of Missouri, told Outside. Someone who cuts out all movement, for instance due to injury or illness, will have a quicker drop-off in fitness than a person who stops hitting the gym, for instance, while continuing to walk and move about their day.
But overall, the trajectory follows a uniform pattern in the body, with decreases in aerobic capacity and psychological shifts occurring first, then muscular changes hitting later. Keep reading for the details on how stopping exercise affects key body systems over time.
After One Week
Heart and Lungs
With a week off, “we can be assured that there’s really no change in fitness,” Greg Grosicki, staff research scientist at wearable health and fitness technology brand WHOOP, told Outside. If anything, there can be “very, very minor reductions” in plasma volume, or the amount of fluid in your blood, he said. (Research shows that, in endurance athletes, plasma volume can fall by five percent with a couple of days off of exercising.)
Without the strain of exercise, your heart no longer needs to shuttle as much blood to your muscles and to the surface of your skin to cool you down. This decrease in cardiac output can lead to a small drop in VO₂ max, or the amount of oxygen your body can use during exercise, Farah Hameed, medical director of Women’s Health Rehabilitation at NewYork-Presbyterian/Columbia University Irving Medical Center, told Outside.
As Outside previously reported, a study of runners and cyclists found that VO₂max can decline by seven percent 12 days after stopping training. In this window, you might find cardio exercises tougher or feel wiped out more quickly.
Muscles
Your muscles might appear a little smaller or less visible because of “shifts of glycogen and water stores,” Hameed explains, “but this can quickly revert once you start working out again.”
Overall, strength won’t decrease yet, Murach said. He added that many people who resistance-train only target a given muscle once a week and still gain strength.
Brain
Exercise can be such a mood-booster that you might feel a little down or less joyful within even a day of stopping, Grosicki said.
After One Month
Heart and Lungs
By this point, a loss of cardiorespiratory fitness—or how efficiently your heart, lungs, and blood vessels supply blood to your muscles—can become more noticeable. Plasma volume continues to decrease; therefore, the heart pumps less blood with each beat, Crockford explained, “and has to beat faster at a given exercise workload.” VO₂ max continues to decline as muscles become less capable of taking up oxygen from blood and using it to produce energy, Grosicki said.
If you were to restart exercising, you’d feel these changes as a loss of endurance. “Either you’re huffing or puffing [when trying to finish a workout],” he said, or your nine-minute mile has turned into a ten-minute mile.
Ventilatory efficiency can also start to drop at one month of stopping exercising, Hameed said, which means you have to breathe more air to clear out the same amount of carbon dioxide. This can contribute to gasping for air or feeling wiped during activity that used to be no problem.
Muscles
Reductions in muscle performance can crop up by the one-month mark, Grosicki said, mainly because of changes in the brain’s ability to coordinate muscle movements.
“Muscle is made up of all these fibers that need to work synchronously to produce force,” Grosicki explained. He uses the example of a bicep curl. Consider how all your bicep fibers have to flex at the same moment as all your tricep fibers relax for the most efficient curl. With a month of no exercise, neural coordination of muscles can decline, Grosicki said. So, you might struggle to lift the same weight as you used to because your muscles have become less efficient at generating power—even if their size hasn’t changed much.
Brain
A 2017 review of studies on exercise withdrawal found that the effects on anxiety and depressive symptoms only worsen beyond the two-week point, Crockford said.
“Evidence also suggests that some cognitive benefits associated with exercise can diminish during detraining, although much of that research has focused on older adults,” she said.
A 2024 review of studies on detraining effects among older adults found that breaks ranging from ten days to 16 weeks were linked to decreases in cognitive ability, particularly in executive function, which includes core skills like planning, multitasking, and emotional control.
After Three Months
Heart and Lungs
VO₂ max may continue to decrease through this point by as much as 20 percent. But you can also see changes in heart structure, including decreases in left chamber size and right ventricle size, which can continue to reduce cardiac output, Hameed said.
Muscles
Changes in enzymes in the muscles can make them less adept at turning fatty acids into fuel, Murach said. That means it may get tougher for you to sustain a longer workout.
Some changes in muscle size and strength may begin at this point, too. “Muscle adapts to what we ask it to do,” Crockford said. “When we stop asking our muscles to produce force against resistance, we remove some of the mechanical stimulus that supports maintaining and building muscle tissue.” Without that impetus for growth, she explained, muscle can start to shrink.
Brain
Though shifts in the brain can happen when exercise stops, “The brain doesn’t suddenly change at one specific point on a detraining timeline, but we know regular movement is an important contributor to cognitive health and mental well-being, particularly as we age,” Crockford said.
After Six Months
Heart and Lungs
Some good news: cardiorespiratory fitness largely plateaus after a few months off rather than continuing to drop, Hameed said. Studies have found no significant dips in VO₂ max beyond the three-month window of stopping exercise—meaning that if you were relatively fit before pausing, you’ll still retain a higher VO₂max than if you’d never worked out, Grosicki said.
Muscles and the Brain
At this point, a shift in the types of fibers in your muscles can occur, Murach said. Several months of no exercise can cause muscles to revert to an “untrained phenotype,” he added, with fibers returning to a less-optimized hybrid state. You might struggle to sustain the types of activity you were used to doing previously, whether that’s weightlifting or running, because the muscle is not as well set up for it anymore, he said.
And like the muscles, with a longer pause, the brain is further deprived of exercise’s benefits.
Can You Actually Lose All of Your Fitness Gains?
It’s unlikely that even a long exercise break will erase all of your gains—but the truth is, “it’s unethical for us to figure that out,” Grosicki said. The benefits of exercise are so well-known that it would be unfair to ask someone who regularly works out to stop for a prolonged period, he explained. It’s the reason many clinical trials have gone just eight to 12 weeks, Murach added.
Based on longer studies in mice, however, “there does appear to be some memory of adaptation [in the muscle],” Murach said. If we apply that to humans, certain molecular changes stick around, which may help you regain strength more easily when you restart than if you were a true beginner, he said.
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