Final answer:
Skeletal muscle fibers are classified into slow-twitch (type I) fibers, which support aerobic activities like distance running, and fast-twitch (type II) fibers, used for anaerobic activities such as sprinting. The fiber types differ in contraction speed, ATP production, and resistance to fatigue. Fast-twitch fibers fatigue quicker than slow-twitch fibers due to their metabolic pathways.
Step-by-step explanation:
Skeletal muscle fibers can be categorized into two major types based on the speed of contraction and how they produce ATP. The first category is slow-twitch, or type I fibers, known for slower contractions and sustained aerobic activity, such as long-distance running. These fibers are dense with capillaries, rich in mitochondria and myoglobin, which gives them the capability to carry more oxygen and sustain aerobic activities.
The second type of fiber is known as fast-twitch, or type II fibers, which are further divided into three kinds: Type IIa (fast oxidative) Type IIx, and Type IIb (fast glycolytic). Fast-twitch fibers are capable of rapid and powerful contractions and are utilized for quick, anaerobic activities like sprinting or lifting weights. However, they fatigue more quickly than slow-twitch fibers because they contain fewer capillaries, mitochondria, and less myoglobin.
The speed at which these fibers contract is influenced by how quickly the enzyme myosin's ATPase can hydrolyze ATP. Fast fibers hydrolyze ATP about twice as quickly as slow fibers, which allows for faster cross-bridge cycling and thus quicker contractions. Moreover, the primary metabolic pathway of a muscle fiber, either oxidative or glycolytic, determines how it produces ATP, with oxidative fibers being more resistant to fatigue due to more ATP production per cycle and glycolytic fibers fatiguing more quickly.