Comprehending Motor Units

You’re scavenging Instagram seeing people with arms that are busting through seams, legs with muscles that are popping through their shorts and leggings with washboard abs, and ask yourself, “How are they able to look like that?”

You know you train hard, follow your program and fuel your body to perform on a daily basis, but are still wondering how you can improve? One aspect of strength training that frequently gets overlooked for the gains that are being made, is what is actually activating your muscles. Keep reading if you want to learn how to maximize your gainz!

Understanding Motor Units (and how they get you SWOLE)

The central nervous system is the control center of all major movements and processes in your body. It consists of the brain and the spinal cord and branching off from the spinal cord is millions of motor neurons that lead to all over the body. These motor neurons innervate many different muscle fibers that are called motor units. These motor units are linked to the muscles through the neuromuscular junction (NMJ) which is the link between the motor neuron and the muscle cells of muscle fibers.

There is a greater amount of muscle fibers in large muscle groups. In the hamstrings, a muscle group that aids in running and knee flexion, there is a large amount of muscle fibers that can generate a large amount of force. In comparison to the extraocular areas, or eyes, that may have only a few muscle fibers in relation to the motor unit that controls them. Motor units are all over your body innervating muscle fibers to create contractions to go through all of your strength and daily movements.

Basic information...your hamstrings have a lot more muscle than the area that controls your eyes!

An important aspect to understand in physiology is that muscle fibers are controlled by the central nervous system which is fed information from peripheral sensory receptors. When the body calls upon muscle fibers to contract, the command is established via the motor units from the spinal cord. This principle is called the “all-or-none principle.”

Every motor unit also has a threshold. Once an action potential is received from the brain or a reflexive response from the body, then that motor unit will contract ALL of the muscle fibers that it innervates. Regardless if they are needed to be used or not, they will all contract; and if that threshold isn't reached, then NONE of the fibers will contract, hence “all-or-none.” As you increase the acceleration of a movement, the motor unit threshold is achieved, which will force the unit to first recruit slow twitch fibers, then quickly transition into fast twitch fibers to increase the force.

There are motor units that have low and high thresholds and they each innervate the 3 different types of muscle fibers. There are Type I fibers, Type IIa, and Type IIx fibers. Type I fibers are slow twitch muscle fibers that can produce low amounts of contractile force, for long periods of time. They are also recruited first through in contraction due to having a low motor unit threshold. Next up, are Type IIa fibers. They have a higher threshold of recruitment and are fast twitch fibers with a duration of about 30 seconds to 2 minutes before fatigue sets in. When you start to move into more advanced movements and increase acceleration and intent of movements, you begin to tap into Type IIx fibers. These fibers have a high threshold level of recruitment with the greatest ability to generate force and to accelerate. The only downfall is they will fatigue rapidly after 10 seconds so these fibers are utilized strictly for high levels of power and force.

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Recap

Understanding how motor units are involved in training puts greater emphasis on the mental intent and velocity that is involved with a movement. They are the brain in control of how we apply force and complete movements. Moving at high velocities will force your body to utilize high threshold motor units to move heavy loads to gain strength, or if doing a set to failure, as motor units begin to fatigue and muscle fibers decrease their force production, then higher threshold motor units will be recruited to continue on and allow you to add reps, ultimately increasing muscle hypertrophy and muscular size! Therefore, the more you train and move as efficiently as possible through technique, intent, and acceleration, then you will be able to recruit and utilize higher threshold motor units and their muscle fibers precisely to increase your gains!