A Brief Introduction to Muscle Fiber Types and Motor Unit Recruitment

What kind of muscle fibers would you say are predominant in this individual? On a side note, if in order to squat heavy you need to wear what this guy is wearing, I'm all in!

There are two broad categories of muscle fiber types that have been identified: slow twitch (type I) and fast twitch (type II).  In humans, approximately 50% of our skeletal muscle fibers are slow-twitch and are characterized as having great aerobic endurance qualities.  Fast-twitch fibers can be divided into two sub categories: fast-twitch type “a” (type IIa) and fast-twitch type “x” (type IIx).  These fibers produce more force than slow-twitch fibers, but fatigue more rapidly.  It should be noted that type IIa fibers are found in both endurance and strength athletes, and can be classified as “intermediate” fibers, as they have both aerobic and anaerobic properties.  In general, slow-twitch fibers are recruited during low-intensity activities, and fast-twitch fibers are recruited as tension requirements increase.  The peak power output for fast-twitch fibers is approximately four times that of slow-twitch fibers.  In general, endurance athletes will have a greater amount of slow-twitch fibers, and strength/power athletes will have a greater number of fast twitch fibers.  Genetics play a huge role in fiber type composition, but we’ll discuss this in more detail another time.

A motor unit contains the alpha motor neuron and all the muscle fibers it innervates.  These motor neurons and nerves originate in the spinal chord and help us move our muscles.  In small muscles that require fine precision such as the eye, a single motor neuron may innervate as few as three muscle fibers.  In larger muscles such as the quadriceps or gluteus maximus, as many as 800 muscle fibers may be controlled by one motor neuron.  Slow-twitch motor units will normally consist of 10-180 fibers, whereas the larger fast-twitch units may include 300-800 fibers.

Let’s apply the above information to something we here at CrossFit West know all too well: the back squat.  For this lift, let’s think of the quads, hamstrings, and glutes as the main muscles being used (although it is said that during a back squat, over 200 muscles are actually working.  No wonder it’s the King of all lifts!).  Motor unit recruitment will start with the smaller slow-twitch fibers (think warming up with the bar).  Then as we add more weight and the tension increases, the firing rate of these slow-twitch fibers will increase.  Greater tension is then developed as the larger, more powerful fast-twitch fibers are recruited (this happens as the weight gets HEAVY).  This process will continue until maximal force is achieved and all the available motor units are recruited and are firing at their maximal rate.

So what am I trying to get at here?  For one, I’m trying to teach you a little exercise physiology because it is pretty fun and exciting stuff!  And number two, I’m trying to get you to think a little about what is going on in your muscles as you lift.  Many of us have never given it any thought.  It may seem like common knowledge, but the way to get stronger and more powerful is to develop these type II muscle fibers.  The only way to do this is to spend some time lifting at maximal or near maximal weights.  Otherwise, these fibers are not recruited and therefore not developed.  So, next time you go after your 5×2 back squat, think a little bit about those tiny muscle fibers and the motor neurons that are making the magic happen.

I’ll leave you with a poem that we can all appreciate:


Down this road, in a gym far away,
a young man was heard to say,
“no matter what i do, my legs won’t grow”
he tried leg extensions, leg curls, and leg presses , too
trying to cheat, these sissy workouts he’d do.

from the corner of the gym where the big men train,
through a cloud of chalk and the midst of pain
where the noise is made with big forty fives,
a deep voice bellowed as he wrapped his knees.
a very big man with legs like trees.

laughing as he snatched another plate from the stack
chalking his hands and monstrous back,
said, “boy, stop lying and don’t say you’ve forgotten,
the trouble with you is you ain’t been SQUATTIN’. “



Front Squat 2RM



21 deadlifts (225/155)


15 thrusters (95/65)

15 pullups

9 squat cleans (135/95)

9 ring dips

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