Fast Vs Slow Twitch Muscle Fibers

Fast twitch and slow twitch muscle fibers are used for different things.

A martial artist who wants to be superfast, a sprinter who wants to be able to run 100 meters in under 10 seconds, or even a professional boxer would want to utilize their fast twitch muscle fibres.

Likewise people doing a sustained activity that needs endurance - like a gymnast who needs to be able to hold a pose while suspended upside down, or an archer holding their bow steady without moving while they adjust their aim, or a weight lifter who needs to be able to lift a specified weight and then hold it for so many seconds in order for it to count as a new world record.

A little background info:

Muscle is made up of bundles of individual muscle fibers called myocytes. Each myocyte contains many myofibrils, which are strands of proteins (actin and myosin) that can grab on to each other and pull. This shortens the muscle and causes muscle contraction - and it is the contraction of muscle fibres that allow us to do any number of physical activities.

A normal person has roughly 50% of each type of muscle fiber, but athletes of various kinds can train their bodies to have a radically different percentage of each muscle fiber (eg. Bruce Lee, martial artist and movie star, had a significantly higher ratio of fast twitch muscle fibers).

Muscle fiber types can be broken down into two main types:

Type I, Slow twitch muscle fibers.

The slow muscle fibers are more efficient at using oxygen to generate more fuel (known as ATP) for continuous, extended muscle contractions over a long time. They fire more slowly than fast twitch fibers and can go for a long time before they fatigue. Therefore, slow twitch fibers are great at helping athletes run marathons or bicycle for hours - and even more for activities that require just holding the same position, like gymnasts.

Type II, Fast twitch muscle fibers, which itself can be further categorized into Type IIa and Type IIb fibers.

Fast twitch fibers are much better at generating short bursts of strength or speed than slow muscles. However, they fatigue more quickly and use energy at a faster rate. Fast twitch fibers generally produce the same amount of force per contraction as slow muscle fibers, but they get their name because they are able to fire more rapidly. Having more fast twitch fibers can be an asset to anyone who primarily wants speed and isn`t worried about endurance.

Type IIa Fibers

Type IIa fast twitch muscle fibers are also known as intermediate fast-twitch fibers. They can use both aerobic and anaerobic metabolism almost equally to create energy. In this way, they are a combination of Type I and Type II muscle fibers. They are still faster than slow twitch, but not as fast as Type IIb.

Type IIb Fibers

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

Fiber Type and Speed Vs Endurance Performance

We all have different amounts of fast vs slow twitch muscle fibers, and these in turn effect our athletic ability in different activities. Some of us might make really good gymnasts, but would be horribly slow when sprinting (and vice versa). Athletes typically get into sports that match their muscle makeup, but it is not genetics that is the biggest deciding factor, it is the types of previous exercises that determine which kind of muscles a person grows.

For example a person could be born to family of several generations of gymnasts, but if they practiced sprinting instead for many years the type of muscle fibers they build would be different. Your body adjusts the ratio over the longer term based on what types of muscle fibers you are using most.

How much is the ratio you might ask? Well, studies have shown that Olympic sprinters typically have approx. 80% fast twitch fibers - while people who do sports that require lots of endurance have about 80% slow twitch fibers. So the ratio of muscle fibers can vary quite wildly by up to 30% plus or minus.

Our bodies still need a ratio in order to perform every day activities. So even though 100% fast twitch would be great for sprinting, by the time the race is over the person would probably be too exhausted to even walk for a period of time. We still need both types of muscle fibers just to perform our daily routine.

Studies also have shown that different types of muscle fibers can also simply change into other types over the course of training. In theory (no evidence of this yet) Type IIa fibers might be an intermediary stage of muscle fibers that are transforming from one type into the other. Not a lot of research has been done into whether muscle fibers morph back and forth over the longer term.

What is known however is that people who train for endurance or speed gain what they are looking for over the longer term. It really is simply a matter of regular training, good diet, and taking good care of themselves to prevent injuries (because an injury can result in a loss of muscle tone if they cannot exercise during a long period of time).

There are many factors that make a great athlete (mental preparedness, proper nutrition and hydration, getting enough rest, and having appropriate equipment and conditioning). Different types of muscle fibers is just one factor. Some sports require a combination of both fast and slow twitch fibers, so having more of one is not necessarily beneficial.

eg. A football player would want both speed and endurance. Speed for when they need to be running in a hurry, but also endurance because they will be playing for longer periods of time - so they need muscle fibers that can do both.