Dunno who wrote this but it is a good little article Fast and Slow Twitch Muscle Fibers. How they affect your performance: It is generally accepted that there are two basic types of muscle fibers. Slow twitch (Type I) muscle and fast twitch (Type II) muscle fiber. Research is currently looking at the specific makeup of these fibers and the 'fast' and 'slow' categories are much simplified. It appears that the fast twitch fibers can be further categorized into Type IIa and Type IIb fibers (1)(2). Until further evidence is available however, these distinctions will help us discuss and understand how muscle fibers affect performance. Human muscles contain a genetically determined mixture of both slow and fast fiber type. On average, we have about 50% slow and 50% fast fibers in most of the muscles used for movement. The slow muscles contain more mitochondria and myoglobin which make them more efficient at using oxygen to generate ATP without lactate acid build up. In this way, the slow twitch fibers can fuel repeated and extended muscle contractions such as those required for endurance events like a marathon. The two fiber types generally produce the same amount of force per contraction, but fast twitch fibers produce that force at a higher rate (they fire more rapidly). So a lot of fast twitch fibers can be an asset to a sprinter when there is a limited amount of time to generate maximal force. The slow twitch fibers, on the other hand, fire less rapidly, but can go for a long time before they fatigue. Fiber Type and Performance The genetic component of muscle fiber type can not be overlooked when it comes to performance. Olympic athletes tend to be genetically blessed with large variations in fast and slow twitch fibers that perfectly suit their sport. Olympic sprinters have been shown to possess about 80% fast twitch fibers while those who excel in the marathon may have 80% slow twitch fibers. Can you change your muscle fiber type by training? This is a hard question to answer because good studies are just now being conducted (3). Currently, there is limited evidence to show that human skeletal muscle switches fiber types from "fast" to "slow" due to training (4). Researchers have demonstrated a fast-to-slow fiber transformation in animal skeletal muscle, and the human studies are showing similar outcomes. There is decent evidence that pure fast (Type IIb) fibers can transition to "hybrid" (Type IIa) fibers with chronic endurance training. What can I do to improve my performance? Keep in mind that genetic differences may be dramatic at the elite levels of athletic competition, but for the typical athlete, following the principles of conditioning will dramatically improve personal performance. Following the principle of overload is the cornerstone of training. With consistent endurance training muscle fibers can develop more mitochondria and surrounding capillaries. In this way training improves your muscle's ability to cope with and adapt to the stress of exercise. Fiber type alone is a poor predictor of performance, even among elite endurance athletes. There are many other factors that go into determining athletic success, including mental preparedness, proper nutrition and hydration, getting enough rest, and having appropriate equipment and conditioning. References: (1) McArdle, W.D., Katch, F.I. & Katch,V.L. (1996). Exercise physiology : Energy, nutrition and human performance (4th ed.). Philadelphia : Lea & Febiger (2) Lieber, R.L. (1992). Skeletal muscle structure and function : Implications for rehabilitation and sports medicine. Baltimore : Williams & Wilkins. (2) Andersen, JL; Schjerling, P; Saltin, B. Muscle, Genes and Athletic Performance. Scientific American. 9/2000 (3) Thayer R, Collins J, Noble EG, Taylor AW. A decade of aerobic endurance training: histological evidence for fibre type transformation. J Sports Med Phys Fitness. 2000 Dec;40(4).