Skeletal muscle is the body’s main consumer of glucose derived from food.

Muscle Fiber Types
Skeletal muscle is composed of two types of muscle fiber: Type I (slow-twitch) and Type II (fast-twitch).
Type I fibers contract slowly and can maintain contraction over long periods of time. They are rich in mitochondria and myoglobin and are well vascularized. These fibers rely mostly on aerobic metabolism to make the ATP that fuels cells. Type I muscle fibers are fatigue-resistant and efficient—great for supporting posture, distance running, cycling and any activity that needs steady output.

Type II fibers contract quickly, produce more force and power, but also fatigue more quickly. They have fewer mitochondria and less vasculature and rely more on anaerobic pathways like glycolysis (using glucose without oxygen).

Type I muscle fibers are smaller in diameter and generate less peak force but excel at endurance and heat management. Type II fibers are typically larger, produce more force and speed and handle explosive tasks like sprinting, jumping or heavy lifting.

Most muscles are a mix of fiber types, and genetics sets the starting ratio of Type l to Type ll, but fibers are adaptable. With aging and disuse, Type II fibers tend to atrophy more, which is one reason that power declines faster than endurance.

Another distinction important for this story: Type I fibers are more insulin-sensitive than Type II fibers. Additionally, these fiber types differ in different body types.

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