Fiber Type of Muscle

Q: How do you determine the fiber type of a muscle?

A: One way to determine fiber type is to find your 1 repetition maximum (1RM) on an exercise. Then, perform as many reps as possible with 80% of your 1RM. Regardless of who you follow, Dr. Fred Hatfield (estimates 8 reps at 80% 1RM) or Charles Poliquin (7 reps at 80% 1RM), the following table will indicate your fiber type:

Reps Performed with 80% 1RM Fiber Type
less than 7
7 or 8
more than 8
fast-twitch (FT) dominant
mixed fiber type
slow-twitch (ST) dominant

 Poliquin later refined his method using a higher intensity (85% 1RM) - he found that this gave more accurate results:

Reps Performed with 85% 1RM Fiber Type
less than 5
5
more than 5

 FT dominant
mixed fiber type
ST dominant

The above method indicates whether you should perform high, medium, or low reps on a particular exercise. For instance, if you are FT dominant, then you should use heavier loads and lower reps predominantly in your training. ST dominant individuals, on the other hand, will respond better to lighter loads and higher repetitions. This information is really nothing new.

Another way to determine fiber type that many people are not aware of is to use electronic muscle stimulation (EMS). This method was introduced to me by Dr. Mark Lindsay and it goes like this. Since EMS recruits white fibers first*, a FT dominant muscle will activate at lower intensities. (The opposite occurs during a voluntary contraction - red muscle fiber is recruited initially, and as the intensity of contraction increases, white fiber is fired.) Therefore, by simply placing the pads over different muscles, you can get an idea of fiber type distribution throughout the body as well as among individuals. This will influence the training parameters as described above. Robert Colling wrote an excellent review of human muscle fiber type distribution that can be found on the internet.

Of course, you could always have a muscle biopsy performed if you really want to know!

*Francis, C. "Training for Speed." Australia: Faccioni Speed & Conditioning Consultants, 1997.

[This Q&A piece was printed in the Winter 2002 issue of Coaching One-on-One: Newsletter of the Canadian Personal Trainers Network.]