MODULE SUPPLEMENT: MUSCULO-SKELETAL SYSTEM
Muscle Fibers

To understand these changes, it's helpful to consider the types of muscle fibers and their characteristics. As noted in the figure on the previous page, muscle fibers come in two major forms: Type I and Type II.

• Type I fibers are slow twitch, high oxidative, endurance fibers that are slow to fatigue.
• Type II fibers are fast twitch, high glycolytic, rapid acting fibers that are quick to fatigue.

For additional background on muscle anatomy and physiology refer to:

The Muscle Fiber
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The predominant loss that occurs with age appears to be in type II fibers. Loss of the motor units and innervating axons influences this process because it is believed to cause 'denervation--renervation' remodeling. That is, some muscle fibers lose their neural connections and become "denervated". Denervation or loss of type II (fast twitch) muscle fibers, results in increased innervation from collateral sprouting of axons from the type I, slow-twitch motor units.

Basically, we end up with a predominately type I muscle fiber pattern. The reduction and atrophy of Type II motor units, along with a decrease in nerve conduction velocity and intrinsic changes in muscle fiber quality, results in a decline in peak muscle power with age.