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a. Oxidative capacity refers to a muscle’s ability to generate energy through the aerobic
metabolism of fats and glycogen (see fuel metabolism section).
b. Glycolytic capacity refers to a muscle’s ability to generate energy through the anaerobic
metabolism of glycogen and ATP and CP.

Source: “Physiology of Sport and Exercise” Jack H. Wilmore and David L. Costill. Human
Kinetics Publishers 1994. ; “Exercise Physiology: Human Bioenergetics and it’s applications”
George A Brooks, Thomas D. Fahey, and Timothy P. White. Mayfield Publishing Company 1996. ;
Eric Hultman “Fuel selection, muscle fibre” Proceedings of the Nutrition Society (1995) 54: 107-
121.

Overview of fiber types

Type I muscle fibers are endurance fibers. They utilize primarily free fatty acids (FFA) for
fuel, fatigue slowly but can’t generate much force. They are used primarily during low-intensity,
long-duration activity such as walking. Type I fibers have the least capacity for muscle growth of
the fiber types. Type I fibers are sometimes called red fibers because of their reddish color under
a microscope.

Type IIb muscle fibers are high force fibers. They generate more force but fatigue quickly.
Type IIb fibers use glycogen as their primary fuel, generating lactic acid as a byproduct. Type IIb
fibers are primarily used for high-intensity, high-force, short-duration activities such as weight
training and sprinting. They have the greatest capacity for growth and appear white under a
microscope.

Type IIa fibers are intermediate fibers. They have medium-force capacity and fatigue
characteristics. They can derive energy either from glycogen or fat depending on the type of
activity that is being done. They have a growth capacity between Type I and IIb fibers and
appear pinkish under a microscope.

Most people have roughly equal numbers of Type I and Type II fibers although elite
athletes may have extreme distributions of fibers (2). Typically, endurance athletes have a
preponderance of Type I fibers while elite strength athletes are Type II fiber dominant.

Generally speaking, fibers do not change from one type to another but the overall
characteristics of the fibers can shift towards more aerobic or anaerobic (1,5). The effects of
different types of training on fiber characteristics appears in figure 1.

Figure 1: Possible interconversion of muscle fiber characteristics with training

Type of activity Endurance training Interval training Weight training

Fiber Characteristics Type I Type IIa Type IIb

Section 2: Muscle Fiber Recruitment

The concept of muscle fiber recruitment is important to understand for this reason:
different types of exercise rely on each muscle fiber type to a greater or lesser degree. The