The Science of Motor Unit Recruitment Part 1

Understanding the science of motor units is absolutely essential for getting results, whether you want to gain muscle, build strength, or torch body fat. Building muscle fast requires you to recruit, and fatigue, all your motor units so they’ll grow. To reach elite levels of strength you must train your body to tap into the biggest, strongest motor units as fast as possible. And to burn fat, it’s essential to boost the metabolic cost of the exercises in your workouts as high as possible. How do you skyrocket the metabolic cost? You guessed it, by recruiting more motor units.
Take a close look at the picture above. The yellow area is your brain, spinal cord, and a bunch of nerves that travel throughout your body. It’s the output from this electrical mainframe that controls your muscles.
That’s why, over the next week, I’m going to cover everything you need to know about the science of motor unit recruitment, along with some intriguing new research that could change the way we think about motor units. So let’s start at the beginning.
“What the heck is a motor unit, anyway?” That’s probably a question you’ve asked yourself once or twice. As defined by my grad school bible, Principles of Neural Science: “The axon and the muscle fibers it innervates constitute a motor unit.”
Clear as mud, eh? In layman’s terms, the motor unit is a collection of muscle fibers along with the nerve that tells those muscle fibers to contract. Each group of muscles is innervated by only one nerve. A nerve can can talk to 100 muscle fibers (small, eye muscles), or thousands of muscle fibers (large, hamstrings muscles) or somewhere in between.
Before you curl a dumbbell an electrical signal starts in your brain and travels down your spinal cord to the lower cervical region where it activates a different nerve that goes out to your biceps. This nerve that exits your spinal cord and reaches your biceps is a motor neuron. (“Neuron,” by the way, is just another name for “nerve.”) The activated motor neuron releases the neurotransmitter, acetylcholine, which attaches to receptors on your muscles that create a cascade of events that contracts your biceps. (Of course, this all happens in fractions of a second.)

In the past, most trainers just focused on the muscles – how to make them sore, fatigued, etc. But it’s the nerves that control the muscles, and that’s why we need to shift our focus further up the chain of command. After all, a muscle will only do what the motor neuron tells it to do, and that’s why it’s imperative to understand this science.
The way I train my clients, and the parameters I outline for boosting performance or altering body composition, all hinge on a key piece of research from 1965 by Harvard physiology professor, Dr. Elwood Henneman. Dr. Henneman is the man credited for discovering that the size (diameter) of a motor neuron determines its recruitment order (J Neurophysiol, 28:560-580, 1965).
The smallest motor neurons are recruited first, followed by a progressively larger motor neurons. This is the basis of the “size principle.”
Why are the smallest motor neurons recruited first? Because their physiological properties make them more easily excitable. Think of a small motor neuron as being a 15-year old boy, whereas a large motor neuron is Hugh Hefner. That 15-year old kid gets excited just by seeing the cover of Playboy. To get Hugh Hefner excited, however, requires the actual presence of the scantily-clad centerfold.
In other words, a small motor neuron – much like the 15-year old kid – requires significantly less input from the brain to “turn it on.”
Over the years I’ve talked in great lengths about the size principle, and you might’ve thought I was specifically referring to the size of muscle fibers. I wasn’t (well, I sort of was, as you’ll learn in a minute). The size principle is actually based on the diameter of the nerve that talks to the muscle.
Now, here’s where the size principle ties together with the muscles. The smallest motor neurons are connected to the smallest muscle fibers and the largest motor neurons are connected to the largest muscle fibers. So, by default, that means the smallest muscles are recruited first, followed by the largest muscle fibers. Remember, the combination of the motor neuron and the muscle fibers it innervates is a motor unit. That’s why it can be said that the smallest motor units are recruited first, followed by progressively larger motor units.
Ok, now you understand the how the size principle works. The question you’re probably asking at this point is, “How can I use this information to build a leaner, stronger, more muscular body?”
The key to transforming your body in record time depends on making the most out of every rep, set, and workout. The way you do this is by adhering to the science of motor unit recruitment. After all, the more motor units you recruit, the faster you’ll get results. It’s as simple as that. It doesn’t matter what your training goal is: you must recruit as many motor units as possible to get the fastest results.
Up to this point, I’ve given you plenty to think about. In part 2 I’ll outline more pertinent research and uncover the essential components that you must adhere to in every workout to maximize motor unit recruitment.
Stay focused,