Muscle Fibers

Note: More information on each individual muscle group can be found in their respective section (here).

Fast Twitch, Slow Twitch, What's This?

When attempting to make real progress, there is more than just turning up to the gym and sweating. Each muscle group is different and so can be optimally trained in different ways. The information below will help optimize your workouts.

Fiber Types

Generally speaking you have type I, type IIa, and type IIb fibers. Type I fibers are slow-twitch and type II fibers are fast-twitch.

The following table lists the main characteristics of each muscle fiber type.

Smarter Health and Fitness Muscle Fiber Analysis
Contraction Time Slow Moderately Fast Very Fast
Resistance to Fatigue High Fairly High Low
Activity Used For Aerobic Long-Term Anaerobic Short-Term Anaerobic
Maximum Duration of Use Hours Less Than 30 Minutes Less Than 1 Minute
Power Produced Low Medium Very High

Each muscle has a different composition of muscle fibers. Some muscles are fast twitch dominant while others are slow twitch dominant.

Each individual’s composition will differ slightly according to their genetic make-up.

Fast twitch fibers respond best to relatively low volume, long rest intervals, high intensity and low frequency.

Slow twitch fibers respond best to relatively high volume, short rest intervals, low intensity and high frequency.

Perhaps most importantly, fast twitch muscle fibers have significantly greater growth potential than slow twitch fibers.

Even for those that do not train often, fast twitch muscle groups will be larger than slow twitch muscle groups.

The Test

This test is commonly known as the 80% test. Effectively, you find your 1 rep-max for an exercise that isolates a specific muscle and then test how many reps you can do with 80% of that. If you can do less than 8, the muscle is fast twitch dominant. If you can do more than 8, it’s slow twitch dominant.

The upside of this test is that it’s specific to you.

The downside is that it’s impractical as it is hard to find an exercise for each muscle that really isolates it, meaning it’s probably hard to do a 1 rep-max with that particular exercise (i.e. fly).

You also can’t overcome neural factors like bad technique or an inefficient nervous system that will cause you to underestimate your 1 rep-max and make you look more slow-twitch than you really are.

You can use exercises like front squats and dumbbell bench presses to get a general idea of your fiber make-up, but it’s far from perfect.

The good news is that there’s considerable research on muscle fiber type composition, so hopefully you’ve found our contributions helpful!

Below is a summary of how each muscle group should be trained:

Smarter Health and Fitness Muscle Specific Hypertrophy Analysis
Muscle Group Muscle Composition Rep Amount
Abdominals 55% Fast Twitch Low to Medium Reps
Back (Lats) 50% Fast/Slow Twitch Medium Reps
Back (Lower) 56% Slow Twitch Medium to High Reps
Back (Traps) Slow Twitch High Reps
Biceps ~55% Fast Twitch Medium Reps
Brachioradialis 60% Fast Twitch Low to Medium Reps
Chest 60% Fast Twitch Low to Medium Reps
Forearms Slow Twitch High Reps
Legs (Abductors & Other Hip Flexors) 50% Fast/Slow Twitch Medium Reps
Legs (Adductors) 60% Slow Twitch Medium to High Reps
Legs (Calves - gastrocs) 52% Fast Twitch Medium Reps
Legs (Calves - soleus) 90% Slow Twitch High Reps
Legs (Glutes) ~52% Slow Twitch Medium to High Reps
Legs (Hamstrings) 70% Fast Twitch Low Reps
Legs (Quads) 57-65% Fast Twitch Low Reps (But Vary to High)
Shoulders (Excluding Rotator Cuffs) 60% Slow Twitch Medium to High Reps (Perhaps Low to Medium Reps For Heavy Pressing)
Shoulders (Rotator Cuffs) ~52% Fast Twitch Roughly Medium Reps
Triceps 67% Fast Twitch Low to Medium Reps

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