When you were born, it was already predetermined whether you would be a sprinter or an endurance athlete. Muscle fibers, either slow-twitch or fast-twitch, are a result of the DNA you got from your parents. No training can change a fast-twitch fiber into a slow-twitch fiber. Yet this doesn’t mean you should design a workout that targets only one muscle type, because the exact ratios vary from muscle to muscle.
Glancing at the different running disciplines can help you understand fast- and slow-twitch muscle fibers. Sprinters are usually built thick, with bulging muscles and solid mass. Marathon runners are long and lean, without an extra ounce of body fat. This is because sprinters have more fast-twitch, or Type II, muscle fibers, while endurance athletes have more slow-twitch, or Type I, muscle fibers.
Fast Twitch Versus Slow Twitch
The smaller slow-twitch fibers contract slowly but are resistant to fatigue. These muscles tend to stay the same size, regardless of exercise. Your larger fast-twitch fibers contract quickly but wear out quickly. Targeting these muscles with high-intensity exercises will cause them to grow in mass. Everyone has both fast-twitch and slow-twitch muscles throughout their body. Muscle groups such as your back or abdomen are composed of mainly slow-twitch fibers because they are used constantly to maintain posture. This is fortunate because you wouldn't want to lose that strength midworkout.
While you may have a natural inclination toward endurance or speed, that doesn’t mean you shouldn’t train both types of muscles. When you work out, you use entire muscle groups, not just individual fibers. Each group of muscles features both types of fibers; the percentage of each is individual for each person. To uniformly strengthen your muscles, try a combined workout.
Endurance aerobic workouts, such as jogging or cycling at a moderate intensity, focus on slow-twitch fibers. Lifting weights, sprinting and circuit training stimulate your fast-twitch fibers. To design a workout that engages both muscle fibers, you need to combine the two. High-intensity intervals can be designed to target both muscle fibers. Mix up 15 to 30 seconds of high-intensity sprints followed by three to five minutes of lighter fitness activity. Another option is merging your strength training with your cardio. Combine a high-weight, low-repetition lifting session, where you are struggling to finish the last rep, with some cardio. In between lifting exercises, jump on the treadmill or elliptical for 10 minutes and cruise at a moderate intensity.
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