Skeletal muscle consists of fibers, also called muscle cells. Forming these long, slim fibers are bundles of protein filaments with the ability to contract. The contraction speed of muscle fibers separates them into two categories: fast-twitch and slow-twitch muscle fibers. The type of muscle fibers used depends on the intensity and duration of your exercise. Endurance activity calls for slower contraction, and slow-twitch is the main type of muscle cell used.
Muscle Fiber Types
The two categories of muscle cells or fibers further break down into three classes. Type I, which are slow twitch, and type IIa and type IIb, which are fast twitch. Each type has different capabilities regarding oxygen and energy transfer. The amount and type of muscle cells vary for every person and each muscle. Athletes of low-intensity sports, like distance running tend to have more slow-twitch fibers whereas the muscles of regular, high-intensity exercisers usually contain more fast-twitch fibers.
Slow-Twitch Muscle Fibers
Slow-twitch fibers offer little force output but resist fatigue and injury well. Although not as strong as type II fibers, they produce the dependable, repetitive contractions necessary for endurance running. They are superior to fast-twitch fibers at delivering oxygen and removing carbon dioxide, making them more efficient.
Fast-Twitch Muscle Fibers
Fast-twitch muscle fibers can be categorized as type IIa or type IIb. Anaerobic activities, which do not need oxygen, use these fibers. Type IIa muscle fiber contract quickly, yielding short bursts of energy. They are larger than type I fibers and much stronger. Type IIa fibers exhaust quickly. Their primary purpose is for short duration, high-intensity exercise. These muscle cells assist at the initiation of all exercise but as the muscles continue into aerobic activity, the slow-twitch fibers take over. Type IIb fibers are extremely strong with almost no resistance to fatigue or injury. Type IIb fibers are so tough and susceptible to damage that sudden use often leads to irreparable harm. Sedentary individuals have primarily type IIb muscle fibers.
Muscle Activity and Endurance Running
Endurance running is an aerobic activity. The term aerobic implies the need for and use of oxygen. The intensity level for endurance runs remains low so the work done by your muscles is steady and continual requiring a lot of oxygen. The muscles do not have to apply much power but better oxygen transport is necessary when compared to anaerobic activity. Endurance running has additional benefits like burning fat, improved cardiovascular function and increased overall energy levels.
References
- University of California Muscle Physiology Laboratory: Skeletal Muscle Fiber Structure
- "Fitness: The Complete Guide"; Frederick C Hatfield PhD; 2008
- "Nutrition: The Complete Guide"; John Berardi PhD; Ryan Andrews MS/MA, RD; 2009
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