Tip of the Month -- April 2006 

Muscle Fiber Types and Performance

By Angelo L. Todaro

Human Muscle Fibers

The muscles in the human body are composed of muscle fibers working together to produce force. These muscle fibers can be categorized into two main types, Type I (slow twitch) and Type II (fast twitch).  Type I muscle fibers are very resistant to fatigue because they have high mitochondria and capillary density, which allows them to use oxygen to produce energy. The slow twitch fibers can fuel repeated and extended muscle contractions such as those required for endurance events like a marathon. Type II fiber produce the majority of the force needed for high force activities, like doing a heavy squat. The Type II fibers have the ability to produce much higher force at higher velocities then the Type I fibers. 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. On the other hand, the slow twitch fibers fire less rapidly, but can go for a long time before they fatigue. Type II muscle fibers are usually divided into two subcategories, Type IIa and Type IIb. Type IIa have more mitochondria and higher capillary density then the Type IIb and are also more resistant to fatigue. The Type IIb have lower mitochondria and capillary density and get fatigued rather easily. The Type IIb fiber is also known as the “couch potato fiber” because sedentary people have a much high number of these fibers then trained individuals. According to the current research, with resistance training the Type IIb fiber transform into the Type IIa fibers. Most humans generally have about 50% type one fibers and 50% type two fibers.

Fiber Type and Performance

The ratio of Type I to Type II is predetermined by the genetics of the individual. Some athletes are genetically inclined for certain sports because they have more Type I than Type II of more Type II then Type I. Olympic caliber sprinters and power lifters have been shown to possess about 80% fast twitch fibers while those who excel in the marathon may have 80% slow twitch fibers.

How can athletes improve performance?

You have to understand that at the elite levels of sport performance there are drastic differences in muscle fiber composition. But with progressive resistance training, you can improve your performance by training specific to your sport and following the basic principles of resistance training.

Following the principle of progressive overload is the basis of resistance training, and with resistance training you will be able to increase the potential to improve sport performance. With consistent endurance training, muscle fibers can develop more mitochondria and surrounding capillaries. This way your muscles will be able to utilize oxygen better and this will improve performance in aerobic activities. When doing heavy resistance training, both the Type I and II muscle fibers will adapt to the training by a process called hypertrophy, an increase in the cross sectional area of the individual fibers. These hypertrophied fibers have adapted to the resistance training and will be able to produce more force.

Prediction of performance just based on fiber type is not very precise, even among elite endurance athletes. Success in sports and athletic activities are based of a number of other factors such as mental preparedness, proper nutrition, hydration, getting enough rest, and having appropriate equipment and conditioning.

References

Kraemer, W.J. , Steven J. Fleck. Designing Resistance Training Programs. 2004