Aging changes the body, but it does not write every line of the story in advance. Aerobic capacity usually falls after early adulthood, often summarized as a loss of about 5% to 10% per decade. Yet people who continue vigorous endurance training can remain far above their sedentary peers, and in one influential longitudinal study their rate of decline was roughly half as large.
That does not mean exercise freezes the cardiovascular system at 30. Even highly trained athletes lose capacity, especially in later life. It does mean the slope commonly attributed to age contains at least two intertwined processes: biological aging itself and the shrinking amount or intensity of activity that so often accompanies it.
The distinction is hopeful, but it also demands precision. “Aerobic capacity” has a particular meaning, percentages can be calculated in different ways, and the cleanest half-rate comparison came from only 29 men. The evidence is stronger as a demonstration that decline is modifiable than as a promise of one exact rate for everyone.
What aerobic capacity actually measures
Researchers usually quantify aerobic capacity with maximal oxygen uptake, written VO2max. During a progressively harder treadmill or cycle test, VO2max captures the highest rate at which the body can take in oxygen, move it through the heart and circulation, and use it in working muscle. It is an integrated measure, not simply a score of lung power.
The number can be expressed in litres per minute, an absolute measure, or in millilitres per kilogram of body weight per minute, a relative measure. The second form is common because moving a larger body requires more work, but it also means gaining or losing weight can change the value even if the body’s absolute oxygen-processing ability changes less.
A large modern analysis from the FRIEND cardiorespiratory-fitness registry observed an average VO2max decline of about 9% per decade. That sits neatly inside the familiar 5% to 10% shorthand. It should still be read as an average across people, not a metronome ticking at the same pace in every body.
The loss does not follow one straight line
Age-related decline often accelerates. An analysis of the Aerobics Center Longitudinal Study described in a longitudinal study of lifestyle and fitness found that cardiorespiratory fitness fell nonlinearly after age 45. Earlier Baltimore Longitudinal Study of Aging data estimated losses of roughly 3% to 6% per decade in the third and fourth decades, rising above 20% after age 70.
Women’s data tell a similarly complicated story. A recent review of aging and endurance training in women noted cross-sectional estimates of about 4% to 10% per decade, while longitudinal measurements accelerated from 6.4% in young adulthood to 21.1% at age 70 and older in one cohort.
Cross-sectional and longitudinal studies answer different questions. A cross-sectional study compares younger and older people at one moment, so generational differences can masquerade as aging. A longitudinal study retests the same people, but participants who return over many years may be unusually healthy. Neither design gives a perfectly pure rate.
This is why “typically 5% to 10%” is useful orientation rather than a biological constant. Sex, starting fitness, body composition, disease, medication and the age range studied all move the estimate.
The famous half-rate comparison was small but revealing
The most direct source for the “roughly half” claim is a 1990 study in the Journal of Applied Physiology. Researchers retested 15 well-trained male master endurance athletes and 14 sedentary men after an average of about eight years.
Both groups were around 61 to 62 years old at the first assessment. The sedentary men’s relative VO2max fell from 33.9 to 30.6 millilitres per kilogram per minute, equivalent to 12% per decade. The athletes fell from 54.0 to 51.8, or 5.5% per decade. Their decline was approximately half as fast while they maintained vigorous endurance training.
The physiological details were interesting too. Maximum heart rate fell by eight beats per minute in the sedentary group but was unchanged in the athletes. Both groups experienced a modest reduction in maximal oxygen pulse, a rough indicator of oxygen used per heartbeat. Continued training appeared to preserve one part of the system without preventing every loss.
But this was not a lifelong experiment. The athletes reported training for an average of 10.2 years before the first assessment, and the sample contained only men. The study provides a strong longitudinal clue, not a universal coefficient that can be applied to women, casual exercisers or every decade of life.
Lifelong athletes show reserve, not immunity
Separate research supplies the genuinely lifelong comparison. In one study, nine male lifelong endurance athletes in their eighties, most of them former competitive cross-country skiers, averaged a relative VO2max of 38. Their healthy untrained peers averaged 21. The trained men’s aerobic capacity resembled values seen in untrained men about 40 years younger.
The athletes also had higher activity of skeletal-muscle enzymes involved in oxidative metabolism. Their bodies retained a much larger reserve above the level needed for ordinary independent living.
That is striking, but it is cross-sectional. It cannot tell us how quickly each person declined, and selection matters. People capable of decades of endurance competition may begin with favourable genetics, avoid certain illnesses or have resources that make sustained training easier. Exercise is central to the difference, but the precise share attributable to training cannot be isolated from that comparison alone.
A 2022 review of longitudinal studies in master athletes helps bridge the designs. Men who maintained training volume and intensity commonly lost about 5% to 6.5% per decade. Declines became much larger when athletes reduced training. Across the reviewed data, changing training volume explained 54% of the variation in VO2max decline among men and 39% among women.
Inactivity can change capacity surprisingly quickly
Aerobic fitness is partly a record of recent use. Blood volume, cardiac filling, stroke volume, capillary support and muscle mitochondria all respond to training and detraining. Some changes happen on a timescale far shorter than aging.
The unusually vivid example is the Dallas Bed Rest and Training Study. Five healthy men underwent three weeks of strict bed rest at age 20, then were studied again across four decades. The 26% loss in VO2max caused by those three inactive weeks was comparable to their net 27% decline from age 20 to 60. Training after bed rest restored capacity, while later-life retraining also produced meaningful improvement.
Five men cannot define a population, and bed rest is more extreme than an ordinary sedentary week. Still, the experiment makes an important point visible: a large decline in aerobic function can arise from disuse without requiring decades of cellular aging.
Biology remains. Maximum heart rate generally falls with age, and reductions in maximal cardiac output, stroke volume and the muscles’ ability to extract oxygen can all contribute. Athletes who maintain training usually decline more slowly, not indefinitely. Illness, injury and recovery also become more consequential in later decades.
What “modifiable” means in ordinary life
These studies do not require everyone to become a master marathoner. They mostly reveal the upper edge of what sustained training can preserve. The distance between that edge and sedentary aging suggests that some loss commonly blamed on the calendar is tied to doing less.
How much is not settled. The half-rate result came from a small male cohort; later reviews show wide variation; and observational athlete studies cannot fully separate training from selection. Even the word “lifelong” covers very different histories, from uninterrupted competition to decades of recreational exercise.
Public-health advice is broader than the athlete literature. The US Centers for Disease Control and Prevention recommends at least 150 minutes of moderate aerobic activity or 75 minutes of vigorous activity each week, plus muscle strengthening on two days. More activity can bring additional benefits, but a safe starting point depends on health, mobility and current conditioning.
For context, earlier ScienceBlog coverage of running and longevity found that even slow, short runs were associated with lower mortality. Preserving elite VO2max is not the only worthwhile outcome, and competitive mileage is not the price of meaningful benefit.
Aerobic capacity does decline with age, including in people who train. The more useful truth is that the rate is not wholly fixed. Lifelong endurance athletes do not defeat aging, but they show how much functional room regular movement can keep open.
