Muscle Fiber Type & Sport Specific Training

Understanding how muscle fiber types are suited to certain purposes can help shape any sport specific training program.

Skeletal muscle fiber types are divided into two main categories: slow contraction, or slow twitch (Type I) muscle fibers and fast contraction time, or fast twitch (Type II) muscle fibers. Beyond these contractile characteristics, there are differences that appear to influence how these respective types of muscle fibers respond to physical activity.

Type I fibers are characterized as relatively small diameter, red, slow-twitch fibers that are at their best for endurance activities but and comparatively poor the purposes of power and strength. The Type IIx, white fast-twitch fibers produce a great deal of force. However, they fatigue rapidly and produce a high amount of lactic acid because they function anaerobically. Type IIa, red, fast-twitch fibers in essence contain attributes of both these types. In human physiology, the type IIb designation was later replaced by the type IIx, because no Type IIb have been found in homo sapiens. These fibers contract quickly as do the Type IIx, they but have the oxidative capacity of the Type I.

The size of the muscle fiber is also related to the size of the motor neuron. The recruitment of these neurons follows a hierarchy according to size, from smaller to larger. This means Type I is followed by Type IIa, which is in turn is followed by Type IIx.

The names of the fiber types also reflect their metabolic features. Type I fibers are also known as slow, oxidative fibers. Type IIx are also known as fast, glycolytic fibers, while Type IIa are also known as fast oxidative, glycolytic fibers.

Why is this important to training? Certain muscle fiber types are specifically designed for certain types of activity and their fuel sources are specifically geared toward certain activities. Morever, the type of training can also change the amount of space occupied by the fiber type in the muscle. This can help inform training decisions, such as training athletes for their specific type of activity with an aim to enhance a certain type of metabolic pathway. For example, a sprinter might well be trained to perform explosive activities that enhance the hypertrophy of Type IIx fibers. For an endurance athlete, such training could take the form of aerobic training with an emphasis on maximizing his or her capacity to spare carbohydrate (by means of anaerobic threshold training) as well as maximizing the speed of fat metabolism such as by means of long, slow, distance training.

Adaptations with Exercise Training

As a person’s aerobic condition improves, several changes occur at the intracellular level that promote fat utilization and minimize lactate accumulation. This includes an increase in both the number of mitochondria and the number of enzymes they contain. The cross sectional area of Type I muscle fibers increase so the mitochondrial number, capillaries per muscle fiber and number of muscle fibers all increase, as well. In addition, the intramuscular triacylglycerol content is enhanced in Type I fibers, and a lowered hormonal response decreases both the amount of glycogen used and blood glucose utilization. This is often known as the “glycogen sparing effect”.

Train Accordingly

Training for strength, power and speed does not affect aerobic capacity but it does cause specific adaptations in the systems that deliver adenosine triphosphate (the usable form of chemical energy for muscular activity), the capacity to buffer against lactic acid, and significant increases in strength or sprint performances.

This is in keeping with sport specific training, so that if the goal is to go fast in an athletic event, it’s preferable to go fast in training. To last in an endurance race, it’s important to go long in training.


1. Personal Trainer Certification Manual, fifth ed. (2008). The National Federation of Professional Trainers, Lafayette, IN.

2. Jeukendrup, A.E. and Gleeson M. (2010). Sports nutrition: An introduction to energy production and performance, 2nd ed. Human Kinetics, Champaign, IL.


These resources are for the purpose of personal trainer growth and development through Continuing Education which advances the knowledge of fitness professionals. This article is written for NFPT Certified Personal Trainers to receive Continuing Education Credit (CEC). Please contact NFPT at 800.729.6378 or [email protected] with questions or for more information.
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