Muscle Fibers and the 80% Test

Today we're going to discuss an easy way to test for fiber dominance in your major muscle groups.

First, let's review why this is useful information. Human muscles have two basic fiber types: slow twitch and fast twitch (it's actually more complicated than this, but using these two types serves our purpose). All of your muscles will have some combination of slow and fast twitch fibers, but it's possible (and common) to have a higher ratio of one type or the other in different muscles. A fast twitch dominant muscle will respond best to a different type of training than a slower twitch muscle (fast twitch muscles are best trained with heavier weights while slow twitch muscles respond better to lighter weights and longer sets).

A great way to determine your muscle fiber dominance is the 80% Test. To perform the 80% Test, you'll choose a weight that is 80% of your one rep max (the heaviest weight you can lift for one repetition) and perform as many repetitions as possible. The number of repetitions you perform will tell your fiber dominance in the tested muscle(s).

Here is how to interpret your result:
  • 1-10 repetitions: You are fast twitch dominant and should perform high load and high acceleration exercises. (The closer you are to one repetition, the more fast twitch dominant you are.) Focus your training on loads that challenge you for 2-6 repetitions.
  • 11-13 repetitions: You have a balanced fiber ratio and should perform a combination of high acceleration and slower tempo training using moderate loads. Focus your training on loads that challenge you for 6-12 repetitions.
  • 14+ repetitions: You are slow twitch dominant and should perform longer duration sets with lighter loads. (The more repetitions you perform, the more slow twitch dominant you are.) Focus your training on loads that challenge you for 10-15 repetitions.

Your dominant fiber type will tell you how to structure the bulk of your training, but everyone should perform a combination of heavy-, moderate- and light-load exercises with varying repetitions to get the best training effect. I would focus 60-70% of your workout on fiber-specific training and use the rest of your time performing other load and repetition ranges.

Different muscles groups may have different muscle fiber makeups, so you should test each of your major muscle groups with a different exercise.

The following exercises are recommended:
  • Squats: For testing your quads and glutes
  • Leg Curl: For testing your hamstrings
  • Flat Dumbbell Bench Press: For testing your chest and triceps
  • Seated Dumbbell Shoulder Press: For testing your shoulders and triceps
  • Cable Rows: For testing your back and biceps
  • Machine Calf Raise: For testing your calves.

It's not perfect, but the 80% Test will give you a good idea about your fiber types. This is a great tool to use when you're deciding how to structure your next training program.