Q: How quickly does your fitness level decline when you stop exercising?
A: It varies. The “detraining” effect depends as much on how fit you are and how long you have been exercising as it does on how long you have gone without exercising and what type of exercise you used to do.
With a complete cessation of aerobic training, significant losses in conditioning can occur within two weeks; all gains dissipate within two to eight months, even if you do routine low-intensity activity.
In one study, runners, cyclists and swimmers who had trained for years abstained from all exercise. After three months they lost more than half their prior gains in aerobic conditioning. But in another study, when sedentary people undertook an eight-week bicycling regimen and then stopped for eight weeks, they lost all their aerobic gains.
Keep in mind that when people merely cut back on aerobic exercise, they are often able to minimize the effects of detraining.
Muscle strength is reduced at a slower rate than aerobic conditioning, particularly during the first few months after people stop training, assuming they remain moderately active.