Fast-Twitching Muscles & Bench Presses

Man working out

Some people are born to be sprinters or weightlifters. They seem to have muscles made out of springs, they're so powerful. You might also notice that someone this explosive also gets tired quickly.

Your muscle fiber type has a lot to do with how powerful your muscles are, and how quickly they tire out. Someone who is naturally strong and can bench press a lot of weight has a high concentration of fast twitch muscle fibers in the chest and other muscles that contribute to the bench press.

Motor Units

Your muscle is made out of thousands of muscle fibers, little strands of muscle tissue than come together to form one big muscle. Your muscle fibers are grouped into motor units. A motor unit is a bunch of muscle fibers, usually around 300, that are controlled by one nerve.

This nerve sends an electric signal down into the muscle fibers and makes them contract. There are different types of motor units: slow twitch, fast twitch and fast intermediate.


Slow-twitch motor units are composed of darker-looking muscle. That means that the dark meat you see in your Thanksgiving turkey is composed of slow twitch muscle fibers! They appear darker because they have a better blood supply than fast-twitch muscle fibers. They need more blood because they rely on oxygen for their energy, and more blood brings in more oxygen.

Slow twitch muscle fibers are used more for endurance. They don't contract very quickly but they can keep contracting for a long time. That's because they use oxygen for energy, which keeps the muscle going for a while.


Fast twitch fibers use short-term energy supplies like glycogen for energy. Glycogen is a much less plentiful resource than oxygen, so fast-twitch muscle fibers get tired quickly. They appear whiter than the slow twitch fibers because they have less blood supply.

There are actually two types of fast twitch fibers: regular and intermediate. The regular fast twitch fiber is completely dependent on things other than oxygen for its energy. An intermediate fiber is a step between completely slow twitch and completely fast twitch fibers.

Fast Intermediate Fibers

The fast intermediate fiber, or type IIB fiber, is a hybrid. It has the fast, powerful quality of a fast twitch muscle fiber but it has more endurance. Slow twitch fibers use oxygen for energy and fast twitch fibers use molecules like glycogen for energy, but an intermediate can use both. Having these fast intermediate fibers will help your bench press because you can maintain the power of a fast twitch fiber for a longer period of time.

Fibers Used in Bench Pressing

In most weightlifting exercises, you use all of your muscle fiber types, especially with a heavy weight. In any exercise, your slow twitch fibers are recruited first, according to an article from the American Council on Exercise.

If your slow twitch fibers can't get the job done, your fast-twitch fibers jump in to help. If the weight in the bench press is heavy enough, you will use almost all of your muscle fibers in your attempt to push the weight up. Unless you're using very light weights, you will be relying on your fast twitch muscle fibers to bench press.

Muscle Fiber Changes from Bench Press

If you bench press, you will slowly convert your pure fast twitch fibers to fast intermediate fibers. You might think that this would make you weaker since your fast twitch muscles are becoming more like slow twitch muscles. That's not the case.

In a research review published in Sports Medicine in 2004, researcher Andrew Fry analyzed over 70 studies related to muscle fiber adaptations to strength training. He was trying to figure out how lifting weights affected your muscle fibers.

Young Man In Gym Exercising Chest On The Bench Press

Fast twitch muscle fibers help you lift more weight.

He found that more intense training sessions converted fast fibers to fast-intermediate more quickly than lower intensity training. That means that the more intensely you train, the faster your fibers turn into fast intermediate. Since fast intermediate fibers maintain their strength longer than traditional fast fibers, your bench press will be stronger and you will be able to maintain that strength throughout more sets, allowing you to tax your muscles even more.

In Neuroscience, 2nd. edition, a neuroscience textbook, the authors note that sprinters have more fast-twitch muscle fibers than marathon runners. Since slow twitch fibers can't turn into fast intermediate or pure fast twitch muscle fibers, the difference comes from genetics. Some people are born with more fast twitch fibers than others. These people will be gifted in the bench press, a strength exercise. If you are not gifted, you can keep training and making the fast twitch fibers that you do have stronger.