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It’s not exactly Arnold Schwarzenegger and Danny DeVito, but the “identical” twin brothers who submitted to a battery of tests at California State University Fullerton’s Center for Sport Performance were pretty different.

One was a 52-year-old endurance junkie who worked as a high-school track coach, competed regularly in marathons and triathlons, and logged 63,458 kilometres of running between 1993 and 2015. The other was a truck driver who didn’t exercise at all. When scientists at CSU Fullerton heard about the pair from a friend of one of the brothers, recalls researcher Katherine Bathgate, “the wheels started turning.”

Bathgate is the co-lead author of a new study in the European Journal of Applied Physiology that explores the remarkable physical differences that accrued between the brothers as their lifestyles diverged for more than 30 years. The results, along with a series of other recent twin studies, offer a crucial reminder of how malleable our bodies really are. Your genes matter, but what you do with them may matter even more.

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The external differences were the most obvious. The untrained twin weighed 10 kilograms more than the trained twin, with a body mass index classified as obese. The extra weight was entirely fat, since an enhanced X-ray scan showed that they had essentially the same amount of muscle. The trained twin also scored better on a long list of health measures: his resting heart rate was 30 per cent lower, and he had lower blood pressure, cholesterol and blood sugar.

But the biggest surprise lurked inside their muscles. We all have a mix of fast-twitch muscle fibres, which are good at generating explosive force but tire quickly, and slow-twitch fibres, which are less powerful but more fatigue-resistant.

“Historically, it was believed that fibre type was very highly genetic,” Bathgate explains. “You were born with what you were born with and that was that.”

But when the researchers sliced a small sample of muscle from the thighs of each twin, they found completely different profiles. The running twin had 94 per cent slow-twitch fibres, ideal for an endurance athlete; the non-runner twin had just 40 per cent slow-twitch fibres. While scientists have known for several decades that your muscle fibres can adapt to the demands you put on them, this difference was far greater than any previously reported in identical twins.

It wasn’t all bad news for the untrained twin: he outperformed his brother in tests of leg strength. All the endurance training the trained twin did may have interfered with his ability to put on muscle and gain strength, Bathgate says; it may also be that the extra body weight the untrained twin carried around strengthened his legs.

Either way, the big message is that your health and athletic destiny aren’t written immutably at birth – and the question, then, is what determines which path you follow. Both twins played a variety of sports in high school, but a relatively minor ankle injury effectively ended the untrained twin’s recreational sports career, and he never got back in the habit of exercising.

From such minor twists of fate large differences can accrue. A study of 41 pairs of Finnish identical twins published earlier this year found that pairs with significantly different weights also got significantly different amount of exercise. When they wore accelerometers for a week, the lighter twins spent 30 per cent more time being moderately or vigorously active, and took 21 per cent more steps.

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(Interestingly, when they filled out questionnaires about their habits, the heavier and lighter twins self-reported getting identical amounts of physical activity. What may have seemed like a quirk of metabolism was actually a difference in perception.)

What makes Bathgate’s twin pair so unique is not just the huge difference in exercise levels, but also the duration of their natural experiment. Most training studies last for a few weeks, or a few months at most. Stretch it out to a few decades, the results suggest, and your very fibres will change so fundamentally that it will appear that you were born to do whatever it is you’ve been doing all along.

Alex Hutchinson is the author of Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance. Follow him on Twitter @sweatscience.

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