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

When you stop exercising, your cardiorespiratory fitness (heart and lung fitness, measured as maximal oxygen uptake, or VO2peak) declines the fastest. In one study of recovered breast cancer patients, VO2peak dropped after 8 weeks of stopping from an average of 29 to 22.7 ml/kg/min, a decline of more than 20%. That is a substantial setback in a relatively short period of time. Because this was measured in a specific patient population, it is not certain whether this figure applies one-to-one to healthy recreational athletes, but the direction of the effect is clear.

Muscle strength and practical muscle function (such as rising from a chair) largely held up during those same 8 weeks, even though total muscle mass had decreased slightly. Fitness is therefore lost more quickly than strength during a short break. At the cellular level it is now well understood why: when muscles are placed under less load, they switch to a breakdown mode. Breakdown signals become more active, build-up signals less so. Even a short period of inactivity can therefore already lead to measurable loss of muscle mass.

The effects on autonomic cardiac regulation (how well the nervous system controls the heart, measured as heart rate variability) disappeared completely after 6 weeks of stopping. The training gains from 12 weeks of endurance training were entirely gone. Notably, in that same study the overall cardiorespiratory fitness after 6 weeks of stopping still sat just above the pre-training level. This seems to contradict the finding from the 8-week study, and shows that the rate of fitness loss can vary from person to person, depending on baseline fitness and the measure used.

Over the longer term: structural inactivity and prolonged sitting are associated in older adults with decline in muscle mass, muscle function, and overall physical and mental health across multiple years. Inactivity produces effects that closely resemble those of ageing itself, and it is not yet fully clear scientifically how much of the muscle loss seen in later life is due to ageing and how much is simply due to too little movement. More vigorous physical activity (brisk walking, cycling at pace) was associated with maintaining or improving capacity in older adults; leisurely strolling did not have that effect.

In practical terms: your cardiorespiratory fitness begins to decline noticeably within a few weeks of stopping completely, while muscle strength holds on somewhat longer. How quickly this happens exactly depends on your baseline level, your age, and whether you keep moving at all. Even staying lightly active (brisk walking at at least 70% of your maximum heart rate) helps to preserve training effects for longer than being completely inactive.