How quickly do I lose fitness if I stop exercising for a while?

Your aerobic fitness declines the fastest. After 8 weeks of complete rest, maximum oxygen uptake dropped by around 22% in one study. In well-trained endurance athletes, heart mass shrank by roughly 20% after just 3 weeks, and after two months the heart pumped noticeably less blood per beat during exercise. Those are substantial drops in a short period of time.

Muscle strength holds up for longer. Those same participants lost barely anything in muscular endurance or functional performance after 8 weeks, even though some muscle mass did disappear. Aerobic fitness therefore detrains faster than strength.

Six weeks of rest presents a mixed picture. Improvements in the autonomic regulation of the heart (the variation in your heart rate between beats) disappeared completely after 6 weeks. Yet endurance fitness itself was largely preserved after the same period in a separate measurement. Both findings come from small studies in breast cancer patients, so it is difficult to translate these results directly to healthy athletes.

An additional point of attention for older athletes: in a small study of athletes with an average age of 59, 3 out of 10 people developed new abnormalities on an exercise ECG after 3 months of stopping. The clinical significance of this is unclear, but it does indicate that prolonged inactivity in later life deserves extra attention. In larger groups of older adults, studies spanning multiple years show a clear association between high levels of sitting and a decline in both physical and mental fitness. Moderate to vigorous exercise protected against this; gentle walking made no measurable difference.