Use it or lose it is a popular saying when it comes to fitness. While it is true that you will lose fitness when you stop exercising, just how quickly you lose it depends on several factors, including how fit you are, how long you have been exercising and how long you stop.

Losing fitness when you stop working out, which is known as detraining or deconditioning, is one of the key principles of conditioning, according to sportsmedicine.com. The principle of use/disuse simply means that when we stop exercising, we generally begin to decondition, with a subsequent lose of both strength and aerobic fitness.

No matter how dedicated you are, there are times where you have to stop exercising for any number of reasons. Illness, injury, holidays, work, travel and social commitments often interfere with training routines. When this happens, you will often see a decline in your level of conditioning.

Detraining in Fit Athletes: Deconditioning in fit athletes doesn’t appear to happen as quickly or drastically as in beginning exercisers. One study looked at well-conditioned athletes who had been training regularly for a year. They then stopped exercise entirely. After three months, researchers found that the athletes lost about half of their aerobic conditioning.

Detraining in Beginning Athletes: The outcome is much different for new exercisers. Another study followed new exercisers as they began a training program and then stopped exercise. Researchers had sedentary individuals start a bicycle fitness program for two months. During those eight weeks, the exercisers made dramatic cardiovascular improvements and boosted their aerobic capacity substantially.

At eight weeks, they quit exercising for the next two months. They were tested again and were found to have lost all of their aerobic gains and returned to their original fitness levels.

Detraining and Exercise Frequency and Intensity: Other research is looking at the effects of decreasing training level, rather than completely stopping all exercise. The results are more encouraging for athletes who need to reduce training due to time constraints, illness or injury. One study followed sedentary men through three months of strength training, three times a week. They then cut back to one session per week. They found that these men maintained nearly all the strength gains they developed in the first three months.

There are many individual differences in detraining rates so it’s impossible to apply all of these study results to all athletes. But it appears that if you maintain some higher intensity exercise on a weekly basis, you can maintain your fitness levels fairly well.

Studies have shown that you can maintain your fitness level even if you need to change or cut back on you exercise for several months. In order to do so, you need to exercise at about 70 percent of your VO2 max at least once per week.

If you stop exercise completely for several months it’s difficult to predict exactly how long it will take you to return to your former fitness level. After a three-month break it’s unlikely that any athlete will return to peak condition in a week. In some athletes it may even take as long as three months to regain all their conditioning. The time it takes to regain fitness appears to depend on your original level of fitness and how long you’ve stopped exercise.