Should You Train Fast v Slow: Fast Twitch vs. Slow Twitch Breakdown

By Presser

December 21, 2021

5 min read

Here is a breakdown on if you should train fast or slow.

So the skeletal muscles are comprised of three types of fibres, there is type I, also known as slow-twitch and type II also referred to as fast twitch, and there are types of type II (which I will explain below).

Slow Twitch (Type I)

Slow twitch fibres are geared to supporting aerobic training and prolonged exercise, so exercises such as long distance running, swimming, cycling and other forms of endurance training.

The Slow Twitch fiber contracts much slower and is more resistant to fatigue and has a quicker recovery time.

Contraction type: slow

Speed: 100 milliseconds

Fast Twitch (Type II)

There are actually two types of fast twitch fibres, there is Type IIA and Type IIX (also referred to as Type IIB).

Type IIA

Essentially this is an intermediate muscle fiber and a hybrid of the two others,as it is a mix of Type I and Type IIX(IIB), so it is great for aerobic and anaerobic training such as weight training, powerlifting, plyometric movements and agility training.

This fiber contracts moderately and doesn’t become fatigued as quickly or take as long to recover.

Contraction type: fast

Speed: 50 milliseconds

Type IIX /IIB

Fast twitch fibres are geared to supporting anaerobic training and produce more power and force focused movements such as weight training, sprinting, and agility training.

Type IIx is also the purest form of fast twitch fiber, the quick contractions also means that it gets fatigued much quicker and has a longer recovery time.

Contraction type: very fast

Speed: 25 milliseconds

So now we know what muscle fibres are available to us, should this affect how you should train? Should different muscle groups be trained differently?

squats, medicine ball

How to train Type I

The best and most effective way to train Type I is typical endurance training, and when in the gym this would be higher reps with lighter weights.

How to train Type II

Type II is definitely the fiber you want to hit if you want to increase strength, and the best way to do that is heavier weights and less reps.

But things are really not that cut and dry, as all of your muscles are comprised of both fast and slow twitch muscle fibers.

So let’s break it down.

What percentage of fast twitch and slow twitch are present in different muscle groups, the table below with provide more details:

Muscle Group	Fiber Type	Rep Range
Chest	Approx 60% fast twitch fibers	Low to medium rep range
Triceps	Approx 67% fast twitch fibers	Low rep range and go heavy
Shoulders	Approx 60% slow twitch dominant	Medium to high rep range
Quads	Vastus Lateralis muscle is about 69% fast-twitch and 32% slow-twitch fibers.	Low rep range and go heavy
Biceps	60% fast-twitch and 40% slow-twitch	Low rep range and heavier weights
Abdominals	55-58% slow-twitch, and 42-45% fast twitch.	MIx it up, vary between light weight and high reps to heavy with low rep range
Calf (gastrocnemius)	The Gastrocnemius (calf) muscle is about 50% fast-twitch fibers and about 50% slow-twitch fibers.	MIx it up, vary between light weight and high reps to heavy with low rep range
Calf (Soleus)	80% slow twitch	Medium to high rep range
Back	54-58% slow twitch	Low to medium rep range
Deltoid	Over 50% slow twitch	MIx it up, vary between light weight and high reps to heavy with low rep range

One study concluded that ‘the percentage of type I fibres was higher than 50 in the deltoid and gastrocnemius, lower than 50 in the rectus femoris and about 50 in the biceps brachii.’, so these numbers are a guide and not concrete by any means, this can change per individual.

In conclusion, these figures are great to have but essentially muscle fibers can be trained to switch as one study concluded that a ‘shift from type II to type I fibers may occur under longer duration, higher volume endurance type events.’

And age also plays a part in switching muscle fibers as the percentage of fast twitch muscle fibers in your body starts to decrease after age 30.

The key point here is that when training, ensure you keep a wide variety of rep ranges, and vary your weights from light to heavy, rather than specifically targeting only fast or slow twitch fibers.

So until next time, keep pumping!