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With few exceptions (most of which would never occur during normal training), it is
impossible to recruit Type II fibers before Type I fibers. The only case where Type II fibers are
recruited without Type I fibers being recruited is during eccentric muscle actions (see chapter
20).

In summary:

At low intensities of exercise (i.e. walking), only Type I fibers are recruited.

At moderate intensities (i.e. jogging, weight training), Type I and IIa fibers are recruited.

At maximal intensities (i.e. sprinting, weight training), all fiber types are recruited.

Section 3: An overview of exercise

and energy production

Exercise is classified into two main categories: aerobic or anaerobic. These categories
differ in intensity, duration and the way that energy is produced by the body. Generally speaking,
aerobic exercise relies on energy pathways that require oxygen (oxidative metabolism).
Anaerobic exercise relies on energy pathways that do not require oxygen (glycolytic metabolism).

Energy production during exercise

The only fuel source that can be used directly by muscles is a compound called adenosine-
tri phosphate (ATP). During exercise, ATP is broken down to adenosine-diphosphate (ADP) in the
muscles and must be regenerated. However, there is only enough ATP stored in the muscle for
roughly 6 seconds of effort. There are four different energy systems which regenerate ATP during
exercise (2), and each of the energy systems is examined in more detail in subsequent chapters.
The contribution of each energy system during exercise depends primarily on the intensity and
duration of the activity, but other factors such as gender, diet and training status also play a role.

1. ATP-CP system: From one to twenty seconds of activity, the body relies on stored ATP and
   creatine phosphate (CP) to provide fuel. This energy pathway is used primarily during maximal
   intensity exercise such as sprinting and low rep weight training. This reaction does not require
   the presence of oxygen (i.e. it is anaerobic).


2. Anaerobic glycolysis: From twenty to sixty seconds, the body breaks down carbohydrate stored
   in the muscle (called glycogen) for energy resulting in the production of lactic acid. Lactic acid
   causes a burning sensation and may be one cause of fatigue during exercise. Anaerobic glycolysis
   will predominate during near maximal intensity exercise such as a 400 meter sprint or medium
   rep weight training (6-20 reps). Anaerobic glycolysis does not require oxygen to proceed.