There are two types of runners: those who have superior speed—whose performance gets better as the race gets shorter—and those who have superior endurance—whose performance gets better as the race gets longer. Most runners, unless they are individually coached, follow some generic training program, but those plans don't acknowledge differences in runners' muscle fiber types and their associated metabolic profiles. The types of fibers that make up individual muscles greatly influence your performance.
Humans have three different types of muscle fibers, with gradations between them. Slow-twitch (ST) fibers are recruited for all of your aerobic runs, while fast-twitch B (FT-B) fibers are only recruited for short anaerobic, high-force production activities, such as sprinting, hurdling and jumping. Fast-twitch A (FT-A) fibers, which represent a transition between the two extremes of ST and FT-B fibers, are recruited for prolonged anaerobic activities with a relatively high-force output, such as racing 400 meters.
It's a given that you have more ST fibers than FT fibers, otherwise you would be a sprinter rather than a distance runner. However, even within a group of distance runners, there is still a disparity in the amount of ST fibers. Some runners may have 90 percent ST and 10 percent FT fibers (marathoners), while others may have 60 percent ST and 40 percent FT fibers (milers).
In lieu of a muscle biopsy to determine your exact muscle fiber type composition, ask yourself the following questions:
1. When you race:
a) Are you able to hang with your competitors during the middle stages, but get out-kicked in the last quarter to half-mile?
b) Do you have a hard time maintaining the pace during the middle stages, but can finish fast and out-kick others?
If you answered a, you probably have more ST fibers. If you answered b, you have more FT fibers.
2. Which type of workouts feel easier and more natural:
a) Long intervals (800-meter to mile repeats), long runs and tempo runs
b) Short, fast intervals (200s and 400s)
If you answered a, you have more ST fibers. If you answered b, you have more FT fibers.
3. Which workouts do you look forward to more:
a) Long intervals and tempo runs
b) Short, fast intervals
If you answered a, you have more ST fibers. If you answered b, you have more FT fibers. (People tend to get excited about tasks at which they excel, while being more anxious about tasks that are difficult.)
Understanding your fiber type can help you train smarter. While most runners do the same workouts to focus on a specific race, your training and racing should reflect your physiology. For example, if you have 90 percent ST and 10 percent FT fibers, your best race will likely be the marathon and your training should focus on mileage and tempo runs.
More: 4 Tempo Run Workouts
If you have 60 percent ST and 40 percent FT fibers, your best race will likely be the 800 meters or mile, and your training should focus less on mileage and more on interval training.
If both of the runners mentioned above want to race a 5K or 10K, the former runner should initially do longer intervals, trying to get faster with training—for example, complete 1,200-meter repeats at 5K race pace, increasing speed to 3K race pace or decreasing the recovery as training progresses.
The latter runner should do shorter intervals, trying to hold the pace for longer with training, such as 800-meter repeats at 3K race pace, increasing distance to 1,200 meters or increasing the number of repeats as training progresses. Thus, there can be two paths to meet at the same point.race.