Fast-twitch, or Type II, muscles assist with power and speed. Unlike slow-twitch, or Type I, muscles, fast-twitch muscles fatigue easily and work for shorter distances. This is mostly because Type II muscles use anaerobic respiration, depending on stored glycogen rather than inhaled oxygen as an energy source. Sprinting, weight training and interval training will help build your fast-twitch muscles.
Sprinting
Sprinting pushes the body to its maximum speed in most forms of exercise, including swimming, running, biking and climbing. Every rapid muscular contraction exercises the fast-twitch muscles, causing them to contract once every 50 milliseconds, or 0.05 seconds. The harder and faster you sprint, the more you work the Type II muscles. However, because these muscles work faster than your body can consume oxygen, even functioning without the presence of oxygen in the body (i.e., anaerobically), lactic acid (the byproduct of anaerobic respiration) can quickly accumulate in the muscles, causing muscular fatigue. Keep sprinting to a minimum to reduce the risk of injury and soreness from lactic-acid build-up.
Weight Training
There are two main types of fast-twitch muscles in the human body: Type IIa and Type IIx. Sedentary people have more Type IIx muscle, which functions mostly during anaerobic respiration. Sprinters, weight trainers and other fit athletes have more Type IIa muscle, which can last longer, as it mostly depends on aerobic respiration. To build healthier fast-twitch muscles, engage in heavier weight training. According to professor Charles Mallery of the University of Miami, three months of intense resistance training can reduce Type IIx from 9 percent to 2 percent while increasing the amount of Type IIa. This form of training includes squats, leg presses, biceps curls, chest presses and most gym exercises that entail resisting the gravity, or force, of heavy weights. For the best development, increase your number of repetitions and sets, concentrating on fatiguing your muscles without straining them.
Interval Training
Interval training alternates high-intensity and low-intensity exercises. Research at SparkPeople.com suggests that interval training improves athletes' overall power, lactic acid resistance and speed. However, it should not be done unless you already have a lot of experience with the exercises. If so, alternate between a minute of sprinting and a 3-minute recovery period of slower exercise. For instance, you can bike at 10 mph for 3 minutes, followed by 30 mph for a minute and then back to 10 mph, repeating for as many intervals as you can handle.
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