The Size Principle

Dr. Henneman’s discovery of “the size principle” in 1957 that states motor units are recruited in order of size from low threshold motor units containing 1-10 muscle fibers to high threshold motor units containing hundreds to thousands of muscle fibers is a principle discussed in exercise science classrooms around the world. In the world of strength and conditioning, our discussions tend to center around hypertrophy, force production, and the rate of that force production. There’s one common physiological response required for any of these adaptations to occur, and that response is motor unit recruitment. More specifically, high threshold motor unit recruitment is necessary for skeletal muscle hypertrophy, maximum force production, and for high rates of force production. Today we are going to discuss the principle that will help you better understand the process of motor unit recruitment: ‘the size principle’.

By Travis Mash

The Size Principle

Content menu:

What is the Size Principle?

In 1957, Dr. Elwood Henneman discovered that motor units are recruited in order from smallest to largest depending on the intensity of the force being applied. More simply stated, when an athlete begins any particular low intensity movement like walking or a slight jog, the brain sends a signal down the spine recruiting smaller motor units containing a small amount of muscle fibers to accomplish the desired movement. For maximal effort movements requiring high amounts of intensity like maximal sprints, all out vertical leaps, or maximum back squats the body recruits the high threshold motor units with hundreds to thousands of muscle fibers to accomplish these high intensity tasks.

Before I go any further, I want to make sure all of you understand what a motor unit is. 

What is a Motor Unit?

Image 1 Motor Unit

Skeletal muscle is controlled by alpha motor neurons that originate in the spinal cord for the most part. Alpha motor neurons consist of the body, the dendrites, and the axon. The axon is the cable that carries the electrical impulses exciting the skeletal muscle fibers to shorten and in turn creating movement. Alpha motor neurons that innervate muscle fibers are large in diameter and are surrounded by a myelin sheath. The myelin sheath consists of a lipid and protein substance that, along with the large diameter of the neuron, acts like the rubber around power lines increasing the rate of the impulse.

A motor unit consists of the alpha motor neuron explained above and all the muscle fibers it innervates. Motor units can innervate anywhere from 1 to 1,000 muscle fibers. Muscles requiring very fine movements like the muscles that control eye movements might only have one muscle fiber per motor neuron. However, bigger muscles like the quadriceps have hundreds to thousands of muscle fibers per alpha motor neuron. The other important characteristic is the homogeneity of muscle fibers in a motor unit.

Importance of Homogeneity of the Muscle Fiber in a Motor Unit

Neuromuscular Junction

Individual motor units innervate muscle fibers of similar contraction rates (Type Ia, IIb, IIx, and hybrids). This is where the importance of the Henneman Principle is pronounced. Low threshold motor units innervate smaller type Ia fibers that aren’t capable of producing massive amounts of force but are highly oxidative meaning they aren’t very fatigable. High threshold motor units innervate faster twitch fibers capable of producing high amounts of force, but are less oxidative making them highly fatigable. 

High force is required to recruit high threshold motor units. Therefore, if you want to produce high force and force at high rates, you have to recruit high threshold motor units with the larger fast twitch fibers that also innervate a greater amount of muscle fibers. However, this is where things take a turn towards specificity, and this is where monitoring athletes is a must. 

Relationships of the size principle

Whether an athlete is performing a maximum one repetition back squat or squat jumps with a light load, as long as the athlete exhibits maximum effort, high threshold motor units are recruited. Maximum effort is required for maximum high threshold motor units to be recruited. However, the velocity of the stimulus dictates the adaptation. Let me explain!

Force-Velocity Relationship

Maximum force is inversely proportional to the velocity of a contraction. Therefore, a slow velocity recruits the maximum amount of high threshold motor units, but also the slow velocity ensures maximum cross bridges between actin and myosin. You can refer back to my article “Repetition Velocity: the Adaptation is in the Speed” to understand this process more clearly.

Hypertrophy and Strength

Maximum hypertrophy and maximum force production of skeletal muscle requires:

  1. Maximum effort recruiting the high threshold motor units, which can be defined as the maximum possible velocity for a given load.
  2. A slow velocity to enable a maximum number of cross bridges, so experiencing near failure in a given set of repetitions maximizing effort.

There is one difference between maximum hypertrophy and maximum force production in terms of stimuli required. The biggest difference is the amount of load used. Hypertrophy can be maximized with light or moderate loads and with large loads as long as near failure is reached, ensuring a maximum amount of mechanical loading on the individual fibers. A muscle’s ability to produce maximum force requires higher loads. Once again, you can refer back to my former article on repetition velocity for all the details. 

Power and Rate of Force Development

For the first couple of years in an athlete’s development, absolute strength and hypertrophy is the primary concern along with perfecting functional movements and positional awareness. However, after those first few years, specificity will rule the decision making process from then on. Maximum force production isn’t as important as high amounts of force produced at higher rates. 

A maximum back squat is performed at around 0.3m/s, but a vertical leap is happening at over 2.7m/s and sprinting is even faster. That’s why maximum force production isn’t proportional to maximum velocity. Explosive athletic endeavors require force production at incredibly high rates linked to: rate coding, the elements within a stretch reflex, elasticity, passive force production, strain energy within tendons, and fiber type makeup of muscle fibers. 

If you are training to simply be bigger and stronger, you are probably training to be slow as well. Maximum power and maximum rates of force development require:

  1. A maximum effort recruiting high threshold motor units just like for hypertrophy and strength.
  2. Movements performed at maximum velocities
  3. Movements performed at higher velocities requiring moderate to light loads
  4. A look at amortization phases of movements and ground contact times within bounding exercises

Now that we’ve explained the differences between hypertrophy, strength, and rate of force development, let’s take a closer look at ensuring athletes are maximizing the proper adaptations. We will look at monitoring athletes and suggest some methods for maximizing the desired adaptations.

Using Velocity to improve intent

This is where my saying “you are either measuring or guessing” comes into play. Whether you want maximum strength, maximum hypertrophy, or maximum rate of force development, you need to monitor to ensure that the intended adaptations are taking place. Let’s look at a few parameters of each.

Strength needs maximum effort

Strength is the measurement of force production at a given rate. Bosco created a few qualities of strength that take place at various velocities, and Dr. Bryan Mann further defined those. Therefore, if you want to maximize absolute strength, the velocity of contraction must be lower than 0.5m/s. If you want to improve an athlete’s maximum bench press, they need loads that end with maximum efforts producing as close to 0.18m/s as possible.

 However, if an athlete is lifting a weight at 0.75m/s, they aren’t getting better at performing one repetition maximums. I suggest performing force-velocity profiles for each athlete at the various movements they will perform. That’s the main way we can ensure that each athlete is performing at a maximum effort within a given load.

For example, if an athlete can front squat 85% at 0.45m/s, yet on a given day they’re only moving the load at 0.37m/s, then there are only two conclusions: 

First, the athlete isn’t lifting with maximum intent on each repetition, which will limit potential adaptations. Compensatory acceleration is a term that Dr. Fred Hatfield made popular, which stated simply means to lift a weight as fast as possible throughout the entire range of motion to maximize all adaptations.

Second, the athlete is too fatigued to maximize the adaptations from the workout. Fatigue management has been discussed in the last decade, and most science based coaches and practitioners have focused their decision making on the daily readiness of individual athletes, especially top performers. Fatigue is a measurable symptom of stress applied to an organism. Velocity has been shown to be an effective measurement of fatigue and in turn stress. If the velocity of any particular movement is substantially slower than normal, nothing of value can happen during that fatigued state.

Either way, the coach needs to take action to ensure the athlete maximizes the benefits of the session. Also if an athlete exhibits a substantial amount of fatigue, the athlete could set themselves back even more with continued exercise. Yes, the coach could be putting the athlete at risk, and at minimum the coach could be suppressing future adaptations.

Hypertrophy needs maximum effort and velocity loss

For maximum hypertrophy to take place, the athlete must perform each repetition with maximum effort and at a maximum velocity. Finally, velocity loss must take place of around 40-50% ending near the movement’s maximum velocity threshold. What does this mean? Regardless if the athlete is bench pressing 40% or 75%, each repetition needs to be performed at a maximum velocity and the velocity between the first repetition and final repetition will be between 40-50% less and nearing the maximum velocity a particular velocity can be performed. For example, a back squat one-repetition maximum occurs at a velocity around 0.3m/s for the average athlete. For maximum hypertrophy to occur, the final velocity of a squat should be at least 0.5m/s or lower to ensure maximum cross bridges and the recruitment of high threshold motor units.

Power and RFD need maximum effort and velocity

The rate at which an athlete can develop force requires specific work being performed at a particular velocity. If you’re a weightlifter, you have to produce insane amounts of force at incredibly high velocities. That’s why it baffles me when I see high level weightlifters trying to improve his or her one repetition maximums in the squat. Back squatting a massive load at 0.18m/s is meaningless to an athlete’s ability to clean or snatch a bigger weight. However, if an athlete improves the amount that they can front squat at 1.0m/s, that’s directly proportional. 

When it comes to the rate of force development, we measure the following parameters:

  1. Increasing the eccentric velocities of any particular movement or decreasing the time of the contraction
  2. Improving loads at particular velocities in a movement
  3. Ground contact times and heights for depth jumps

Increasing eccentric velocity

This one is tricky because athletes will sacrifice position to increase eccentric velocity, and that’s not what we are after at all. If an athlete can move a particular load at a higher velocity during the eccentric contraction, that shows an improved ability to decelerate coming from improved elasticity, strengthened connective tissue, and strengthened proteins especially titin that resist stretch and lead to increased passive force. Not to mention, higher velocity eccentric contractions are linked to greater hypertrophy in the faster twitch fibers. It’s my opinion that we have looked way too much at concentric velocities and not nearly enough at eccentric velocities. 

Improving loads at a particular velocity

Instead of getting caught up in the 1RM race that I see in most organizations, I recommend defining the important velocities of individual athletes, and focus on measuring improvements of force production within those velocities. For example, my weightlifting athletes perform the majority of their squat velocities at 0.6m/s or faster and very rarely test 1RMs below 0.5m/s. We have found solid relationships between 1RMs at 0.5m/s and readiness from tapers. Simply put, if one of my athletes hits a 1RM front squat at 0.5m/s or faster, I know they are ready to perform in the competition after a taper. 

I am not saying that 1RMs don’t have their place. Early on, improvements in absolute strength are highly proportional to improvements in all qualities of strength. However, the more advanced an athlete becomes requires more and more specificity.

Methods along with velocity to improve RFD

We use our GymAware RS and FLEX units to measure all of our movements, but there are a few other methods that might aid in your quest of force produced at higher velocities. Here are a few:

  • Bands- I love bands along with VBT to increase the eccentric velocity from the pull of the bands, and bands allow athletes to accelerate throughout an entire range of motion without worrying about the weight flying off his or her shoulders.
  • Weight Releasers- this allows us to overload the eccentric contraction creating more elasticity, and we have recorded higher overall velocities from the post-activation potentiation.
  • Depth Jumps- this allows our athletes to strengthen all of the physiological components that occur from the force absorption like strengthened connective tissue and strengthened titin protein filaments. This is a great way to measure an athlete’s stretch reflex in a very specific manner.
  • Flywheel- I can’t overstate the importance of using a flywheel properly and measuring those contractions with linear velocity. More to come on that in the near future.


In 1957, Dr. Elwood Henneman discovered the size principle which states that motor units are recruited in order from smallest to largest depending on the intensity of the force being applied. This principle is important in regards to strength, hypertrophy, and rate of force development. Maximum recruitment of high threshold motor units is necessary for either of those three adaptations to take place. However, I have shown that there are differences required to maximize each adaptation. 

  • Maximum Strength requires maximum effort, maximum velocity for a particular load and exercise, and very high loads for improved absolute strength, which can all be measured with VBT especially with GymAware RS and FLEX units.
  • Maximum hypertrophy requires maximum effort, maximum velocity for a particular load, velocity loss of 40-50% regardless of initial load, and a final velocity near the exercise’s maximum velocity threshold.
  • Maximum improvements in the rate of force development require maximum effort, maximum velocity for a particular load, and a substantial amount of time and volume spent at a particular velocity zone.

As always, the proof is in the details. This is why almost all coaches that teach the basics correctly will still get results. It’s not hard to progressively overload athletes to create improvements in strength and hypertrophy. However, if the goal is to maximize a particular adaptation, precision and knowledge is required. You can’t accidentally help someone achieve greatness. You have to know the details, and you have to measure those details. My goal as a teacher is to help those solid coaches teaching the basics to take their knowledge to the next level, and in turn I can indirectly help all of those athletes in the world to reach their hopes and dreams at a higher rate.

As always, feel free to email me at with any questions or topics you would like for me to discuss. 

Video presentation:


  1. Mendell LM. The size principle: a rule describing the recruitment of motoneurons. J Neurophysiol. 2005 Jun;93(6):3024-6. doi: 10.1152/classicessays.00025.2005. PMID: 15914463.
  2. Bawa PN, Jones KE, Stein RB. Assessment of size ordered recruitment. Front Hum Neurosci. 2014 Jul 28;8:532. doi: 10.3389/fnhum.2014.00532. PMID: 25120446; PMCID: PMC4112781.
  3. Zayia LC, Tadi P. Neuroanatomy, Motor Neuron. [Updated 2022 Jul 25]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 Jan-. Available from:
  4. Arantes, Victor & Paulucio, Dailson & Alvarenga, Renato & Terra, Augusto & Koch, Alexander & Machado, Marco & Pompeu, Fernando. (2020). Skeletal muscle hypertrophy: molecular and applied aspects of exercise physiology. 50. 195-207. 10.1007/s12662-020-00652-z. 


Download: How to get started with velocity based training [use case]

* indicates required
Do you own a GymAware / FLEX device? *

By submitting my email address:

You can unsubscribe at any time.

Coach Travis Mash

Travis Mash

Being a World Champion in powerlifting, Travis competed at a world-class level in Olympic weightlifting and has coached professional Olympic weightlifters alongside Don McCauley and Glenn Pendlay at Team MDUSA. Now Travis coaches the most successful weightlifting team in the USA.