Fast vs. Slow Twitch Muscle Fibers

By Ben Rorem 7th Hour


Skeletal muscles are what they sound like. They're the muscles that attach to and move bones. Skeletal muscles fibers come in two major types - slow twitch and fast twitch fibers. Slow twitch muscle fibers are for endurance activities and everyday things like standing. Fast twitch fibers perform rapid movements and anaerobic activities like sprinting and jumping. There are intermediate muscle fibers also which have properties of both fibers. They look like fast twitch (pale), but have similar endurance of slow twitch. The reason they're called twitch muscles is because of the time it takes for them to contract.

Positive and Negative Effects

If you train for slow twitch muscles, you wont have as many fast twitch muscles and vice versa. So if you're practicing a lot of long distance activities, then you probably can't lift as much weight as a person who lifts everyday. This is also vice versa. So there are advantages and disadvantages to training for one or the other. On average, a person has 50% of each type.


People didn't always know there were two types of muscles. So this is an advancement in itself. The ratio of fast v slow twitch muscles is thought to be a genetic trait. Scientific studies have figured out what happens to muscles when a person trains for one type or another. Ex: Slow twitch muscle training. Muscles get larger, improved nutrient and waste movement to and from cells, more mitochondria, and much more myoglobin (brings oxygen to mitochondria). Also, I found out why chickens have dark and white meat. The dark meat is slow twitch muscles (legs and thighs for standing). They are dark because slow twitch muscles are oxygen rich. The white meat is fast twitch muscles (breasts for quick flight). Fast twitch muscles don't have much oxygen so they're paler.
Slow twitch muscles can produce large amounts of energy slowly. Fast twitch muscles produce small amounts of energy very quickly.
Fast Twitch Fibers and Slow Twitch Fibers