Fast-twitch and slow-twitch muscle fibbers

There are, as you already know, two main categories of fibbers in the skeletal muscles of our bodies. The first type is known as slow twitch fibbers (or slow oxidative fibbers, or red fibbers, or Type I fibbers) and the second type is known as fast twitch fibbers (or white fibbers, or Type II fibbers). These two types distinguish one from another on how fast they can contract. Another important difference between them is in their capacity for endurance and power work.

Slow twitch fibbers have more endurance because they have a greater capacity for aerobic metabolism. They have more myoglobin (the substance that transports oxygen across the muscle cell), more aerobic enzymes that catalyze the release of energy during aerobic metabolism, and more mitochondria (the protein structures within muscle cells where aerobic metabolism occurs).

On the other hand, fast twitch fibbers have greater contractile speed than the slow twitch fibbers and have more anaerobic enzymes that catalyze the release of energy when oxygen is not available, and they produce more lactic acid than the slow twitch fibbers at equivalent workloads and therefore fatigue more quickly. They also use their glycogen faster. Fast twitch fibbers are fully utilized at power work.

Effects of training

Endurance training will increase the aerobic capacity of both types of muscle fibbers, although trained fast twitch fibbers never reach the level of aerobic capacity of trained slow twitch fibbers. Saltin and his colleagues found that an athlete can increase the aerobic capacity of fast twitch fibbers to a level that surpasses that of untrained slow twitch fibbers. Conversely, strength and sprint training will increase the size and contractile speed of both types of muscle fibbers as well as their potential for rapid energy release. Fast twitch fibbers, however, possess a greater potential than slow twitch fibbers for such increases. In support of this statement, the fast twitch fibbers of a trained person are usually much larger than their slow twitch fibbers. Although an athlete can increase contractile speed and force in slow twitch fibbers that have been sprint trained, they never reach the level of even untrained fast twitch fibbers.

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