Can training make you a premier long distance runner — or are your abilities determined at birth?

Credit: Staff Sgt. Natasha Stannard/U.S. Air Force
Credit: Staff Sgt. Natasha Stannard/U.S. Air Force

All of our skeletal muscles are made up of a combination of two types of fibre: slow-twitch muscle fibres and fast-twitch muscle fibres. 

“Muscles have fibres of both types, but the percentages of each may differ from muscle to muscle and person to person,” says Courtenay Dunn-Lewis, a physiologist at the University of Pittsburgh.

“About 80% of an elite athlete’s muscle fibres are either fast-twitch, if they are a power athlete, or slow-twitch, if they are endurance athletes,” says Dunn-Lewis. “Consider the long, slender physique of a marathon runner, whose predominantly slow-twitch muscle fibres may be small but are resistant to fatigue and provide lasting energy kilometre after kilometre. This person is also burning less energy in a given unit of time.”


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“By comparison, an American football player or hockey player has predominantly large fast-twitch muscle fibres, moves with power and speed, but fatigues quite quickly. Athletes with 80% of one fibre type are simply born lucky. For the rest of us, the percentages are closer to 50% fast-twitch and 50% slow twitch, and that percentage is determined at birth. Fibre type is determined strictly by the nervous system, and for that reason cannot be changed with exercise.”

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