whole book.4th print

Chapter 17:

Basic muscular physiology

and energy production

The human body contains roughly 600 muscles, ranging from the large muscles of the
quadriceps (front thigh) to the small muscles that control the movement of the eyes. All force
production depends on the ability of these muscles to contract and cause movement within our
body. All forms of exercise ultimately depend on force production by muscles so it is necessary to
briefly discuss some basic muscular physiology and detail about force production. Additionally,
basic concepts regarding energy production is discussed.

Section 1: Muscle Fiber Types

There are three different types of muscle fibers. Each possesses its own specific
characteristics. The distinction between different fiber types is important from the standpoint of
understanding the adaptations which occur during exercise as well as what fuels are used.

Types of muscle fibers

Human skeletal muscle contains three distinct types of fibers (1): Type I or slow oxidative
(SO), Type IIa or fast oxidative glycolytic (FOG), and Type IIb or fast glycolytic (FG).

Each fiber type has distinct physical and physiological characteristics (such as preferred
fuel) which determine the type of activity they are best suited for. Depending on the type of
exercise done, fibers will adapt accordingly (adaptations to specific types of exercise are
addressed in separate chapters). A summary of the fiber types and their primary metabolic
characteristics appears in table 1.

Table 1: Comparison of various characteristics of fiber types

Type I Type IIa Type IIb

Metabolic Characteristic

Oxidative capacity (a) High Medium Low

Glycolytic capacity (b) Low High Highest

Mitochondrial density High Medium Low

Capillary density High Medium Low

Speed of contraction Slow Fast Fast

Resistance to fatigue high moderate low

Time to fatigue 4’+ about 4’ about 2’

Force production capacity low moderate high

Growth capacity low moderate high