"The Impact of Training on the Loss of Cardiorespiratory Fitness in Aging Masters Endurance Athletes, " published in the International Journal of Environmental Research and Public Health.

I remember years ago while cresting the top of a neighborhood hill that I had descriptively named "Mt. Tendinitis," I had the thought that the majority of what we associate with old age is the slow accretion of the results of decades of sedentary self-indulgences. At the time, I had just returned to exercise and was feeling the years peel off my body. Now, twenty years later, I have undoubtedly aged, but certainly not at the rate of my sedentary peers. I came across "The Impact of Training on the Loss of Cardiorespiratory Fitness in Aging Masters Endurance Athletes, " published in the International Journal of Environmental Research and Public Health.

I have often written about VO2max or maximal aerobic capacity, defined by UC Davis Health as "the body’s ability to deliver oxygen to your muscles, which allows them to do work or engage in activity…VO2max allows one to produce more energy, thereby performing more work. With this in mind, VO2max is the 'gold standard' measure of overall fitness." The American Heart Association considers cardiorespiratory fitness of such fundamental importance to overall health that, at one time, it considered making VO2max a clinical vital sign. It is also strongly inversely associated with all-cause mortality: each metabolic equivalent increase in cardiorespiratory fitness is associated with an 11% reduction in all-cause mortality.

Sedentary individuals exhibit a broad range of VO2max values determined by genetics and age. VO2max can be elevated with training in most individuals but declines linearly with age, independently of training status. Similarly, VO2max drops when training is stopped and rebounds upon starting training again. Aerobic capacity declines linearly in young and old athletes and non-athletes by as much as -20 % after 12 weeks of becoming sedentary. Bed rest results in even more significant declines in VO2max , with studies showing declines of as much as -16.5% in three days. Additionally, two chemical markers of mitochondrial structure and energy production in the mitochondria, citrate synthase and succinate dehydrogenase, show declines of up to -40% over a 12-week sedentary period. This decline is eliminated in both young and old athletes, with VO2max returning to pre-cessation levels once training is resumed. The rate of re-capillarization is also similar between age groups.

Longitudinal studies of athletes between the ages of 6 and 34, with follow-up sampling ranging from 46.5 to 82.8 years, have shown that VO2max declines between -5% and -46% per decade. In healthy sedentary adults, the decline average is -10 % per decade, increasing slightly after ages 60 to 70. The decline in athletes is closely related to changes in training volume. Regression analysis shows that between 39% and 54% of this decline is due to a reduction in training, not aging. Moderately trained individuals or those who reduced volume between 11% and 20% had a VO2max decline of -8 % to 26% per decade. Those with a reduction greater than 20% or who became sedentary had declines of between −15% and −46% per decade. 

This shows that training can slow, but not stop, the aging process. 54% percent of the variance in the observed VO2max decline is due to training volume decreases, which rises to 70% when the age of the athletes is considered. Amongst serious or competitive athletes, the VO2max loss is between  −5% and −6.5% per decade. This is roughly half the loss in healthy, sedentary adults.