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It's the chicken-and-egg question of aging: Do we become less active as we get older because our bodies start to break down, or do our bodies start to break down because we allow ourselves to become less active?

For years, it was widely accepted that humans would start getting slower, weaker and more fragile starting in their 30s. But new studies on topics ranging from the cellular mechanisms of aging to the time-defying performances of masters athletes are forcing researchers to question this orthodoxy. It seems increasingly likely that the first signs of decline are more a function of lifestyle than DNA: If you keep using it, you'll be well into middle age before you start losing it.

One of the classic approaches to studying age-related decline is to look at how age-group records change in sports such as running and weightlifting. The fastest and strongest 45-year-olds are considerably slower and weaker than their 25-year-old counterparts, which supports the traditional view of inevitable decline.

But there are several problems with this analysis. One is that even the best older athletes train less as they age, notes York University kinesiology professor Joe Baker – though the reasons for this aren't entirely clear.

"I've always wondered whether the change in training habits that we see as athletes get older reflects a change in their life situation – 'I'm not making it to the Olympics, so goodbye speed work' – or whether this results from injury or pain-related issues – 'Speed work hurts a lot more than it used to,' " he says.

Another problem with looking at age-group records is that this extreme level of performance requires a punishing training regimen that can seldom be sustained for longer than 10 years. That means those setting world records in their 20s, when the rewards are biggest and the talent pool deepest, are seldom still competing two or more decades later.

A better approach, according to scientists at the German Sport University Cologne, is to look at the times of mid-pack recreational athletes of different ages, since their training is more likely to be consistent over several decades.

In a study published last year in the journal Deutsches Ärzteblatt International, Dieter Leyk and colleagues crunched finish-time data for more than 900,000 marathon and half-marathon finishers in German races, with ages ranging from 20 to 79.

The results were startling: No significant age-related decline in performance appeared before the age of 55. And even beyond that age, the decline was surprisingly gentle. In the 65-to-69 group, a full 25 per cent of the runners had times that would have ranked as above average among 20- to 54-year-olds.

Crucially, the runners in the study trained a similar amount (typically three to four times a week, for about an hour at a time) no matter what their age.

The conclusion is clear, the study authors write: "Performance losses in middle age are mainly due to a sedentary lifestyle rather than biological aging."

Of course, few people manage to sustain even that fairly moderate level of exercise into their 40s and beyond. There are numerous possible explanations for the drop-off, including the time pressure of raising families and increased responsibilities at work. But a more subtle contribution may come from a gradual decline in the "intrinsic drive" to exercise.

When mice are left to their own devices in a cage with an exercise wheel, the amount they choose to run declines with age, long before any changes in their physical capacity are evident. While it's not clear what determines this "intrinsic drive," research by Mark Davis and colleagues at the University of South Carolina has suggested that it's linked to levels of mitochondria in the brain.

Mitochondria are the "power plants" that provide energy to cells and muscle fibres. It has long been known that aerobic exercise boosts levels of mitochondria in your muscles, but a study just published by Dr. Davis's group shows for the first time that aerobic exercise also stimulates the growth of new mitochondria in the brain.

This research remains in its preliminary stages, but the implication is that the brain responds to training like a muscle – and if it gets out of shape, it too is susceptible to fatigue and more likely to opt to stay on the couch. Regular exercise, on the other hand, doesn't just preserve your ability to exercise as you get older; it also preserves your desire to exercise.

Alex Hutchinson blogs about research on exercise at His new book – Which Comes First, Cardio or Weights? – is now available.