Humans possess two fundamental classifications of muscle fibers: slow twitch and fast twitch. Slow twitch muscle fibers are used constantly during almost all activities of daily living. Fast twitch muscle fibers are reserved for activities that require greater power and speed. Fast twitch muscle fibers are also used when our slow twitch fibers aren’t capable of completing a task. Approximately 90-95% of people possess roughly a 50/50 split of slow and fast twitch muscle fibers. As we age, our fast twitch muscle fibers atrophy (decrease in size) and number. By our early 30’s, we begin to witness a reduction in fast twitch muscle fibers. This is why the career of an NFL football player declines rapidly after the age of about 30, especially among running backs.
Long distance runners are generally assumed to have high percentages of slow twitch muscle fibers (this fiber type allows them to endure and resist fatigue). However, long distance runners racing distances from the 1500-meter to the marathon will call upon fast twitch muscle fibers in a variety of scenarios: during surges, climbing hills, and during the end of the race “kick.” Runners over the age of 30 can maintain (and improve) their fast twitch ability by strength training, but only if the strength training is done properly. The key to recruiting fast twitch muscle fibers when strength training is “intent.” When we strength train, we should do so in an intentionally slow and controlled manner, especially at the beginning of the set. This intentionally slow movement minimizes momentum and keeps tension on the targeted muscle. As the set continues and our muscles fatigue, we begin recruiting our fast twitch fibers. When we are really struggling through the last few reps of the set, the weight is, in fact, moving very slowly; the key is that at this point, we have the “intent” to lift the weight as fast as we possibly can. Of course, fast movement will be utterly impossible at this point. An important note: If you actually lift the weight fast (if the weight is actually moving fast); you aren’t recruiting fast twitch muscle fibers as the muscle itself is actually unloaded and momentum is essentially carrying the weight. This has important implications for training. If your goal is to increase speed, avoid exercises where the weight moves fast (“cleans,” “snatches,” and “swings”). This approach is valid for the 15 year-old runner and the 25 year-old runner, but is especially important for the 40+ runner who has undoubtedly lost fast twitch muscle over the years.
And of course, this isn’t important for runners alone; as we age, we should fight to maintain fast twitch muscle fibers to retain our function and performance in a variety of tasks.