A slow twitch muscle fiber is best suited for endurance activities such as running, cycling and swimming. This is because physiologically these muscle fibers have more mitochondria, more capillaries and perform better under aerobic conditions. A fast twitch muscle fiber is best suited to strength and power oriented activities such as sprinting, weightlifting, football (American) and wrestling. This is because these muscle fibers are 10 times faster than the slow twitch, can develop more tension and perform better under anaerobic conditions. Fast twitch fibers will also fatigue very quickly, and decline with aging.

Physiologist have assigned these muscle fiber types with numbers; slow twitch being Type I and fast twitch being Type II. Within the fast twitch category there also exists Type IIa and Type IIb. Type IIa muscle fibers have some fatigue resistance capabilities similar to Type I. Type IIb muscle fibers fatigue very quickly but have the strongest contractile capabilities.

The question every coach and serious athlete has concerning muscle fibers is; can training convert muscle fiber types? In other words through training can an athlete convert muscle fibers from type I to type II. The answer is no. There is no substantial evidence showing that a human can transform a Type I muscle fiber into a Type II. A Type I muscle fiber will always be a Type I and so forth.

The ratio of muscle fibers a person has is determined by genetics and is influenced through pre- and post natal differentiation, growth and development of the individual.¹ Additionally each muscle will vary in its ration of slow twitch to fast twitch muscle fibers. Postural muscles such as the rectus abdominis (abdominals) generally have more Type I fibers than the quadracepts group.

Although muscle fibers will not convert into one another they do become better at utilizing the energy sources available to them.² Research suggests that through training one muscle fiber can become more like another. For example with lots of strength and power based training a type IIa muscle fiber will become more like a type IIb fiber. Again the muscle fiber does not magically transform into another fiber type is simply becomes more efficient at working under the physiological conditions upon which the body is place. Research has also shown that when training stops a muscle fiber will go back to its original state. 

Understanding the characteristics, qualities and how slow and fast twitch muscle fibers adapt to exercise is vitally important for anyone who wants to train with specificity. If you are involved in a sport that is predominately fast twitch then you need to train with anaerobic, strength and power based exercises. If you are involved in a sport predominately slow twitch you need to aerobic, endurance based training.

Specific muscle groups should also be trained according to their specific function and role as it applies to the human body. Postural muscles such as the rectus abdominis, soleus and peroneal's need to be trained for endurance, the musculature of the forearms also respond better to endurance based training. The larger muscles should be trained depending upon the specific activities of the individual.

Use this information to design better training programs and your athletic ability will improve dramatically.

Good Luck and Good Training,

¹Foss, M. Keteyian, S. (1998) Fox's Physiological Basis for Exercise and Sport 6th Edition. (pp 151-152) USA:  McGraw Hill 

²Pearson D. (2004) Strength and Conditioning Journal, Vol 26, # 1. Muscle Fibers. (p. 45) USA: National Strength and Conditioning Association.