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Muscle Fiber Type Test
What's Your Genetic Potential?

One Rep Max doing Curls

Chest and Triceps
Chest and Triceps
One Rep Max on the Bench Press

One Rep Max doing Squats

We're all born with three primary types of muscle - smooth, cardiac and skeletal. Only one of them is important for people working on an advanced training program. Can you guess which one?

Smooth muscle - also known as involuntary muscle, is found within the walls of organs and structures like the bladder, blood vessels, stomach and intestines.

Cardiac muscle - an involuntary muscle is only found within the heart.

Skeletal muscle - also called voluntary muscle, is the muscle that's anchored by tendons to the bone and grows as we workout.

The correct answer is: Skeletal muscle. Here's why it's important.

Skeletal muscle is broken down into three types.

  • Slow Twitch Type I
  • Fast Twitch Type IIa
  • Fast Twitch Type IIb

Both slow and fast-twitch fiber types generally produce the same amount of force per contraction, but slow-twitch fibers fire more slowly (thus the name) so they can go on for longer periods of time.

Slow-twitch fibers (type I) are best for endurance sports where much lower energy levels are required, but over a much longer time. Think marathon runners, triathletes and other aerobic type activities. Slow-twitch fibers contain more myoglobin and mitochondria, making them more efficient at using oxygen to generate ATP without lactate acid build-up.

Fast-twitch muscle fibers (type IIa and IIb) are best for sports that require very high levels of energy expended over a short period of time. They fire more rapidly and help your body produce more force on-demand when it's called on quickly. Think weightlifting, sprinting and other anaerobic type activities.

The average person is born with a fairly even split between fast and slow-twitch muscle. But, if you were born with more of one over another, by adjusting your training, you can maximize that genetic potential.

Contraction Time
Resistance to Fatigue
Type I Fiber Slow High Resistance to Fatigue
Type IIa Fiber Moderately Fast Fairly High Resistance to Fatigue
Type IIb Fiber Very Fast Low Resistance to Fatigue

You can see the different muscle fibers for yourself. Slow-twitch are red and fast-twitch are white. In a turkey, the dark meat (the legs and thighs) are slow-twitch because they walk around a lot and have to be more resistant to fatigue. The white meat (breast) is fast-twitch because they don't fly much, but when they do, they need a quick burst of energy to get away.

There are a couple of ways you can find out approximately how much of each muscle fiber type you have.

A. A muscle biopsy. The doctor takes a biopsy needle and jams it several inches into the muscle in order to cut off a small piece. That piece is then analyzed for fiber-type percentages. Painful? Yes. But fortunately, there is an easier way.

B. Exercise Test. This is best for muscle groups rather than individual muscles because lifting weights requires the use of many muscles at once.

1. Make sure you're sufficiently rested before starting the test. Relaxing for five minutes or more should work. Then you figure out your One Rep Max on the muscles you want to test.

[Click Here and we'll help you calculate your One Rep Max.]

2. Now, perform one set of exercises using 80% of your one rep max weight.

3. Depending on your many reps you can complete, using the scale below can give you a rough estimate of your percentage of slow-twitch or fast-twitch muscle fibers.


1-7 repetitions, the muscle group is likely composed of more than 50% fast-twitch fibers. Your genetic potential is to engage in more weight lifting, anaerobic type activities.

7-12 repetitions, the muscle is probably fairly well balanced with an equal portion of fast and slow-twitch fibers.

12+ repetitions, the muscle group is likely composed of more than 50% slow-twitch fibers. Your genetic potential is for more endurance, aerobic type activities.

Remember, just analyzing fiber type cannot predict athletic performance; it merely indicates what a person has a greater genetic ABILITY for. Even if you're not born with the perfect genetics - proper conditioning, equipment, hydration, nutrition, preparation and rest combined can make a much more dramatic difference than simple genetic capabilities.

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