Fast Twitch vs Slow Twitch Muscle Fibers

After years of debate on this subject sports science has now accepted everywhere there are two types of muscle fibers. Slow twitch fibers also called Type I muscle and fast-twitch or Type II muscle fibers, the fast twitch muscle fibers are then categorized farther into Type IIa muscle fibers and Type IIb muscle fibers.

Type I Muscle Fibers:

All Type I muscle fibers will have a slower contractile speed. Type I muscle fibers also have a smaller cross-sectional area than type II. Slow twitch muscles fibers have a very high oxidative (aerobic) capacity, along with a very low glycolytic (anaerobic) capacity. This is why they are called red muscle fibers, because of the abundant supply of oxygen they appear red.

Type I or slow twitch contract slowly, enabling the muscle to hold a steady pace, twitching for long durations with no fatigue setting in. Type I or slow twitch muscle fibers are used with any endurance activity like long distance runners, cyclists and swimmers use mostly Type I fibers.

Type II Muscle Fibers:

Type IIb or fast-twitch muscle fibers will have a fast contractile speed covering a large cross-sectional area, this is where you get muscle size. But conversely type II muscle fibers have a low oxidative capacity with a very high glycolytic capacity.

Both Type IIa and Type IIb enable short and fast quick bursts of power like powerlifting or sprinting and bodybuilding. Your Type IIa muscle fibers are the intermediate where some hold a slightly less contractile ability which is able to continue firing for a fraction longer than type IIa.

Type IIb muscle fibers are very gycolytic which means they will hypertrophy a lot more than your type I fibers. Type IIb muscle fibers have a lot less blood in them causing them to appear white, hence white muscle fibers. The process of contraction of these muscle fibers is complex.

Even the smallest muscle group in your body has got well over 100,000 muscle fibers. Each of these muscle fibers is connected to a motor neuron which sends a message/impulse from your brain/spinal cord to your muscle fibers. Each motor neuron will control about 2-2,000 muscle fibers.

The single motor neuron which is attached to 2000 muscle fibers is called a motor unit. Each motor unit will fire in a frequency that is only accepted by the like-fibers it stimulates. I.e. Slow twitch motor neurons, Type I will cause the muscles that it is attached to contract slowly. Fast twitch muscle fibers, Type IIb unit will fire-up quickly.

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