Of the many different types of muscles in the human body, skeletal muscles are the ones that come to most people’s mind when someone mentions muscle-building. The skeletal muscles are the ones related to movement and physical strength, which is why they are the most familiar to the majority of people. However, not everyone’s muscles are the same. Everyone has different types of muscle fibers with varying degrees of efficiency and power. This difference in muscular ability leads to different muscle contractions exhibited by either one type of fiber or another.

Type I muscle fibers are more commonly known as “slow-twitch” muscles. They are red in color and are highly efficient in their use of oxygen to fuel muscular activity without lactic acid buildup. These are lower-power, high-endurance muscles, and one could expect to find more of them in a marathon runner than a sprinter, who would likely have more of the next type of muscle fiber.

Type IIa muscle fibers are somewhat of a cross between Type I and Type IIbIIb fibers, getting the best of both worlds. Type IIa muscle fibers are very strong and powerful due to their rapid rate of contraction, but are still very efficient and resist fatigue. Although these are arguably the “best” type of muscles for athletics, they are not very common in humans (cheetahs, for example, would be more likely to have them).

Type IIb muscle fibers are at the other end of the spectrum from Type I fibers. These white fibers are more commonly known as “fast twitch” muscles. Muscle contractions of Type IIb fibers are quick, resulting in fast and explosive strength, but little endurance. For this reason, this type of muscle fiber is not well suited for a long distance event, but is great for short sprints.

People have different concentrations of each type of muscle fiber in their bodies. Those more suited to endurance related athletics would logically be more likely to have a higher concentration of Type I, or slow twitch fibers. For this reason, it is not surprising to find that Olympic marathon runners and Ironman participants do, in fact, have significantly more of this type of muscle. Similarly, athletes who excel in short fast-paced races or dashes, such as running backs in football or Olympic sprinters, have much more Type IIb, or fast twitch muscle than others.

If one was looking to train themselves for certain events, they could do exercises that are more inclined to build one type of muscle than another. For example, doing more reps of an exercise builds slow twitch endurance muscle, while doing more weight and fewer reps will result in the building of more fast twitch muscle.

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