Muscle Fiber Make-Up and Determining the Reps
Generally speaking, slow-twitch athletes tend to have more endurance while fast-twitch athletes tend to have more strength and power.
In regards to weight training, a slow-twitch athlete should be able to perform a higher amount of repetitions at any given intensity compared to a fast-twitch athlete.
Bench Press at 85% of 1RM
Slow-twitch athlete: 7 reps
Fast-twitch athlete: 3 reps
On average, 7 reps represents 80% of 1RM while 3 reps represents 90% of 1RM. It is then clear that if your training prescription is based on specific percentage progressions, then the actual repetitions should reflect the athletes muscle fiber profile.
The issue with this approach is that no athlete has the same fiber make-up in every muscle and every movement.
An isolated movement like the Scott Curl can give you a score of 12 reps at 85% of 1RM even if your 85% maximum rep Bench Press score is 3 reps. The same athlete who scores 12 reps on the Scott Curl can score 4 reps on a multi-joint movement like the Chin-Up. This makes it impossible to assess for general programming purposes.
Is the athlete fast-twitch or slow-twitch? Should we favor higher reps or lower reps?
A more practical way to determine the general muscle fiber make-up of an athlete for programming purposes is to test the vertical jump. A fast, forceful, and high vertical jump will be a good indication of a generally fast-twitch athlete while a deep, slow, and moderate vertical will be a good indication of a generally slow-twitch athlete.
With this information, you can now program your training intensities according to the appropriate fiber profile of the trainee. As a rule of thumb, the average training intensity variation from slow-twitch to fast-twitch athletes should vary by about 5%.
3 x 10-12
4 x 6-8
4 x 8-10
5 x 4-6
Cycle average intensity: 76% (rep average: 8.6)
3 x 8-10
5 x 4-6
4 x 6-8
6 x 2-4
Cycle average intensity: 81% (rep average: 6.4)
Understanding the muscle fiber make-up of the athlete is a great tool for program individualization, but the differences in program design on a wider scale are quite subtle.