Where is my strength gone?

Cause: Inefficient muscle-fiber recruitment
Occurrence: Sustained medium efforts like 10K to half marathon
Antidote: Train your fast-twitch fibers to be more efficient.

The early miles of a half marathon often feel pretty easy. You’re not sprinting fast enough to accumulate high levels of lactate and other metabolites; and unlike in a marathon, you’re not running far enough to empty fuel stores. So why does it eventually—and inevitably—get hard?

The answer, according to studies from the University of Copenhagen, once again depends on oxygen kinetics. Over the course of a sustained run at half marathon pace or faster, the amount of energy (and thus oxygen) needed to maintain that pace gradually inches upward. Over the course of 10 or 20 minutes, your oxygen consumption can drift upward by as much as 25 percent, making it progressively harder to hold your pace.

This drift is the result of a shift to less-efficient muscle fibers. When you start running, you automatically recruit mostly slow-twitch muscle fibers, which are suited for long-distance running because they’re efficient and take a long time to fatigue. As time goes on, though, individual fibers begin to fatigue and run low on fuel. To replace them, your brain must recruit fast-twitch fibers, which demand more energy—and oxygen—to deliver the same output.

One way to tackle this problem is to train your fast-twitch fibers, which are usually deployed for explosive movements, to be more efficient. “This might be one reason long runs are so important for marathoners,” Jones explains. A 2.5-hour run, even at a slow pace, will eventually deplete slow-twitch fibers and force fast-twitch ones to practice delivering slow-and-steady power. In response, they’ll build endurance by ramping up mitochondrial content and adding capillaries to supply more blood.

Build intervals in your running schedule (split run) or combine with our SlimAndFitClub.com program to improve your overall strength, speed and endurance.

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