Muscles generate heat and force for movement, help us breathe, and keep our bodies upright. Skeletal muscle tissue is composed of two fibers, actin (thin fibers) and myosin (thick fibers).
These two fibers give the muscle a striated appearance. In order for muscle to contract it must first be stimulated by nerves called motor neurons. A single motor neuron and the muscle fibers stimulated by it is called a motor unit. The recruiting motor units play a large part in the force of the muscle during contraction. The more motor units (muscle fibers) recruited, the stronger the force of contraction.
Muscle fibers are classified as Type I, Type IIa and Type IIb fibers. “Fast” and “slow” twitch are also two other classifications for muscle fibers. Type I fibers (slow twitch) fibers are more resistant to fatigue than Type IIa or IIb fibers and have a high capacity for aerobic metabolism, fatigue faster and are mainly anaerobic.
Slow twitch fibers are mainly for endurance while fast twitch are for speed and performance. A muscle will generally have an equal amount of both fast and slow twitch muscle fibers. In regards to hypertrophy (muscle growth), fast twitch fibers grow faster and larger than slow twitch. Within the fast twitch muscle fibers, type IIa fibers are considered intermediate between fast and slow twitch fibers in relation to speed and contraction.
For example, Type IIa fibers can become more glycolytic or aerobic depending on the type of training an athlete performs. If an endurance runner were to stop running and start weight lifting, then his or her Type IIa fibers would become more glycolytic in order to handle the stress of the activity.
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