More mileage during training results in faster finish times, right? Recent research suggests that might be true for some runnersbut not for everyone.

In a study published in the Journal of Applied Physiology, researchers recruited 24 highly trained, middle-distance runners and had them complete three weeks of their normal (which was prescribed by their individual coaches), followed by three weeks at an increased training volumea 10-, 20-, and 30- percent increase each successive week. Then, they did a one-week taper at a 55 percent reduction in training volume from their highest level.

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Before and immediately after each training period, running performance was assessed, as well as physiological responses, like resting metabolic rate (the total number of calories burned when your body is completely at rest) and muscle fiber composition.

Researchers found that half the runners decreased their overall running time because they gained speed, but the rest did noteven though they reported increased fatigue levels.

We challenged the idea that all runners adapt to increased mileage positively, and found that an increase in weekly mileage resulted in very individual responses, Philip Bellinger, Ph.D., the studys lead author and a lecturer in exercise science at Griffith University in Australia, told Runners World. Some runners increased their performance directly after the increase in training volume, while others had a substantial decrease.

Some of the runners had improved performance after the taper period, he added, while others only returned to their baseline level.

These findings show that not all runners respond the same way to a given training program, and what works for one runner may not work as effectively for another, he said.

Since there were no major differences in the runners resting metabolic rate or blood biomarkers (such as blood pressure or heart rate), the researchers believe their responses could come down to muscle fiber types. The runners who had performance increases tended to have a higher proportion of type I fibers (also called slow-twitch fibers), which are the kind used most for endurance, rather than fast-twitch fibers that switch on with short energy bursts. Basically, you use type I for a longer-distance run and type II for sprinting.

Bellinger said having more type I fibers made the runners better able to tolerate an increase in training volume (as opposed to having more type II fibers), leading to better performance adaptation.

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If you lack robust type I fibers, does that mean increasing your mileage is a wash? Not necessarily, according to Bellinger. He admits that few runners would be eager to get muscle biopsies, but theres a simpler way to determine if your mileage is working: Track your results.

Runners should communicate with their coach and monitor their own training very closely, he said. Look at training volume, duration, and intensity, and take note of responses to training, such as heart rate and perceived exertion.

Most of all, dont be hard on yourself if people in your running group are seeing major results from increased mileage and youre not. As the study suggests, you may need a different training program, not a different mindset.

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The Link Between Muscle Fiber Types and Your Bodys Response to Training - runnersworld.com