Fast-twitch muscle fibers are one of two muscle fiber types in your body. The other type is slow-twitch fibers. Your strength, speed and power depend on fast-twitch muscle fibers, which perform your anaerobic movements such as sprinting, jumping and heavy weightlifting. Slow-twitch fibers perform your aerobic movements such as walking, swimming and cycling. The number of fast-twitch fibers you have is dependent on genetics, so you can't alter how many you have. You can, however, increase the size of the fibers by gearing your workout toward fast-twitch fibers. The result is an increase in muscle size and strength.
You build fast-twitch muscle fibers when you overload the fibers during strength training with weight, number of repetitions and speed. In contrast, you build slow-twitch fibers when you increase the duration of your aerobic workouts. Overload occurs in one of three ways. The first way to overload muscles is to subject your muscles to a dramatic increase in load or amount of resistance. For example, instead of curling 20 pounds for 10 repetitions, curl 40 pounds for five or six reps. The second overload principle is to perform more repetitions of an exercise. For example, push your muscles beyond their current limits; if you struggle to perform eight repetitions, make yourself complete 10 even if you require assistance from a partner. The third way to overload muscles is to increase the pace at which you perform the movement. For example, instead of lifting a weight to a count of two and lowering the weight to a count of four, use one second to lift and one second to lower the weight. You can arrange your workouts to combine two overload principles or use all three during one exercise.
One way to build your fast-twitch muscle fibers is to lift heavy weights with few repetitions. Warm up with five minutes of movement such as walking or light rowing. Organize your strength workouts with exercises that use large movements, such as chest presses, squats and deadlifts. These exercises use a higher number of fast-twitch muscle fibers. Select a weight that causes muscular fatigue within six repetitions. The heavy amount of weight uses the first progressive overload principle. Combine it with the third principle and use a fast acceleration as you squat, press or lift to further build your fast-twitch fibers. Perform one set and add a second when the first is easy. Increase to three sets when you are able to complete two sets of six, fast-paced repetitions with ease. This type of workout should be used only once or twice a week with at least two days of rest in between sessions.
Plyometric exercises are jump-training drills that rely on your fast-twitch muscle fibers. Your muscles enter a pre-stretched state immediately prior to performing the jump. This occurs, for example, when you squat before you box jump. In the pre-stretched state, the muscles quickly contract to provide maximal power upon release of the stretch during the jump. This overloads the fast-twitch muscles and increases their size. Examples of plyometrics include jump squats, box jumps, depth jumps, vertical jumps and power cleans. Add plyometrics gradually to your workout routine. After a five to 10-minute, full-body warm-up, such as walking or cycling on a light resistance, select one or two plyometric exercises. Perform five or six repetitions of each. When five or six repetitions is easy, add a second set. Complete your two exercises once a week, then gradually increase the number of repetitions. Add a third exercise once you can easily perform three sets of 12 repetitions of your initial two exercises. Use plyos once or twice a week with at least two days of rest in between sessions.
Rapid acceleration relies on your fast-twitch muscle fibers. When you consistently include speed workouts in your routine, you cause your fast-twitch fibers to adapt, change their structure and increase in size. Sprints are an example of speed training. Warm up with five minutes of walking or a light jog before you sprint. Begin with sprint intervals and lightly jog for 30 seconds, then sprint for 30 seconds. Repeat the cycle for 20 minutes. Other examples include agility drills, such as zig-zag running through cone markers, and ladder drills, in which you quickly move your feet in and out of the ladder markings. Keep in mind that it is the short-bursts of anaerobic movement that build fast-twitch fibers rather than the long, steady runs or miles of cycling that improve your slow-twitch fibers.