Muscle Fibers Defined

Slow Twitch (Type I)
The slow muscles contain more mitochondria and myoglobin which make them more efficient at using oxygen to generate more fuel (ATP) for continuous, extended muscle contractions over a long time. In this way slow twitch fibers are great at helping athletes run marathons and bicycle for hours.

Fast Twitch (Type II)
Becasue fast twitch fibers use anaerobic metabolism to create fuel, they are much better at generating short bursts of strength or speed but fatigue more quickly. Fast twitch fibers generally produce the same amount of force per contraction, but fast twitch fibers are able to fire more rapidly than slow twitch, hence their name. Having more fast twitch fibers can be an asset to a sprinter because they need to generate lots of force in a little time. The slow twitch fibers, on the other hand, fire more slowly, but can go for a long time before they fatigue.

Type IIa Fibers
This type of fast twitch muscle fiber is called intermediate fast-twitch fibers and they can use both aerobic and anaerobic almost equally to create energy. In this way they are a combination of Type I and Type II muscle fibers.

Type IIb Fibers
These fast twitch fibers use anaerobic metabolism to create energy and are the 'classic' fast twitch muscle fibers that excel atproducing quick, powerful bursts of speed. This muscle fiber has the highest rate of contraction (rapid firing) of all the muscle fiber types, but they also have a much faster rate of fatigue and can't last as long before they need rest.

Fiber Type and Performance
Our muscle fiber type may influence what sports we are naturally good at or whether we are fast or strong. Olympic athletes tend to fall into sports that match their genetic predispositions. Olympic sprinters have been shown to possess about 80 percent fast twitch fibers while those who excel in marathons tend to have have 80 percent slow twitch fibers.

Great post Marx! It is always the opposite of what you think it should be between fast and slow twitch.

__________________
MIND+POWER=GROWTH

That is what I found interesting too! Striving to learn bro-