Henneman's Size Principle explained - How it can help your lifting


Henneman was an OG when it came to explaining now only how it is that our nervous system drives the expression of strength and power, but also how we can train to make adaptations to the recruitment of our biggest and strongest type II muscle fibres to make gains in the weight room.

What is the Henneman Size Principle in simple terms?

The Henneman Size Principle explains how the nervous system activates and recruits these motor units during exercises.

Our muscles are made up of motor units and muscle fibres of varying sizes, strengths and fibre type compositions. When we try to produce force, say to lift a barbell our muscles and nervous system (neuromuscular system) will activate motor units in a predictable progressive fashion, starting with the smallest and building to the largest as we require more and more force. This is the Henneman Size Principle - Motor units are always recruited (concentrically*) in this smallest to biggest, weakest to strongest fashion.

* Eccentric muscular contractions on the other hand behave differently to concentric, being more energy efficient, relying heavily on elastic components of the muscle fibre such as Titan and often working in reverse (from biggest to smallest) during actions.

How far into the big motor units you go is dictated by the intensity and/or the exertion of an activity.

Illustration explaining the Henneman size principle of motor unit recruitment in strength and power training
As we aim to increase tension, we recruit larger and larger motor units to meet the force requirements. This process always proceeds from smallest to largest, slowest to fastest.

As we aim to increase tension, we recruit larger and larger motor units to meet the force requirements. This process always proceeds from smallest to largest, slowest to fastest.

Why does the size principle of motor unit recruitment exist?

This evolutionary wiring of the muscles serves a few key purposes.

Firstly, it allows us to execute fine motor skills with precision, and secondly it helps conserve energy. There is no advantage to going around all "Hulk Smash" every time you are trying to type out an email, this would be an inefficient use of energy and make for a short lifespan on your laptop!

It also prevents people from tearing their tendons and muscles apart by over-recruiting muscles if they are untrained or in poor condition. On the flip side, out own size principle is highly trainable, we can rapidly improve the rate and degree to which our body can produce force, be explosive and quickly unlock the potential of our muscles biggest motor units.

The size principle in weightlifting & strength training

To enhance strength and power, we want to engage in training that targets and enhances our most powerful motor units — Type II muscle fibres. These are crucial for explosive actions like sprinting, jumping and lifting heavy weights. Because strength and power are as much skills as anything else, we want to focus here on learning to efficiently and rapidly be able to recruit these biggest type II motor units when we need them.

There are really three effective ways to work through the Henneman size principle and active your biggest motor units for developing strength and power; lift heavy or lift fast.

1. Exhaustion training. How not to activate your type II motor units

An important caveat, is that technically training into exhaustion also activates larger type II fibres as the exhausted type I motor units are exhausted first.

This method however is a counterproductive way to become more explosive. Regularly taking sets deep into fatigue can lead to burnout and overtraining, lots of research has shown that training closer to the point of failure leads to suboptimal strength gains, see: training to failure. This style of higher volume training with lighter loads also leads to type I muscle fibre hypertrophy, something that might be desirable in bodybuilding, but is less ideal for athletes needing to maintain a high power-to-weight ratio.

Fry et al. a comparison of muscle fibre type hypertrophy between Bodybuilders, powerlifters and weightlifters

2. Lift heavier: Progressive strength training

As you train with progressively heavier and heavier weights, not only do you increase muscle mass, improve lifting technique, increase neural output and potentially shift your muscle fibre composition toward type II motor units (many motor units are highly adaptable), your brain and nervous system also learns to unlock and recruit more of your biggest type II motor units more efficiently to lift the heavier and heavier weights.

This process is largely where the skill of strength and power training comes in, you are rewiring your brain and down regulating your golgi-tendon organs (the force sensors that rate-limit strength in untrained individuals) to better tap into the biggest motor units in your body on demand - generating more force, more smoothly.

This coordination of force, unlocking of motor units and down regulation of Golgi-tendon organs is a large reason why newbie gains can be so drastic in the first few months of training — It's like taking the handbrake off a car.

For more experienced lifters, the demands of progressive overload is the reason why you can’t really make strength gains training under the 80% of their 1RM threshold — these light loads just doesn't force the activation of your biggest most powerful type II motor units.

Unless, you were to move that light weight faster…

3. Lift faster: Utilise velocity based training

Should you lift weights fast or slow? If you are to take a leaf out of Henneman's book then he would say faster is better!

But that doesn't mean your should strip load off the bar and do only a low % of your 1RM (see rule #1), instead its about lifting with intent and lifting heavy weights, but moving them as fast as you can for that given load.

Beyond lifting heavy, lifting fast(er) can significantly impact the recruitment rate of Type II motor units. High-velocity training, including Olympic lifts, loaded jumps, plyometrics, banded exercises etc all boost the activation of our biggest type II units — which are also the quickest fibres to generate force — helping us generate high amounts of force, quickly.

While the actual speed of your movements is one important element of exercise prescription, perhaps more as important might be the intended velocity of the movement, even if the actual movement speed is not that fast.

Lifting weights faster: The secret to unlocking incredible strength and power

Intent to move is a principle that's often misunderstood as just lifting light weights as fast as you can. In reality, it's about the effort (intent) to lift all weights as fast as possible, regardless of their actual speed.

This concept focuses on requesting your nervous system to move weights rapidly, even if they don't actually move very fast due to their heaviness. By applying intent in your lifts, it is possible to combine the benefits of lifting heavy weights with those of explosive power training, leading to improved strength and power and up-regulation of your biggest and most powerful type II motor units.

For instance, imagine you're working on increasing your maximum strength with heavy weights, training above 90% of your 1RM. These weights will move slowly at a velocity of 0.35m/s.

When you apply intent to move here it doesn't mean reducing the weight to allow for faster bar speeds. Instead, your intention is to lift these heavy weights with as fast as you can — even if the speed increase is marginal or non-existent. Compared with simply going through the motions the bar speed might only increase from 0.35m/s to 0.37m/s. The slight increase in speed, has negligible impact on training outcomes, but the intention and effort put into that tiny speed increase will contribute to significantly better motor unit activation and ultimately greater strength and power gains.

Henneman’s Size Principle workouts

Training to increase the activation and excitability of your biggest motor units is crucial for field sport athletes, powerlifting, weightlifting and even bodybuilders!

By applying the three principles above — lift heavy, lift fast(er), and to a lesser extent, lift in closer proximity to failure — there are many ways to modify and enhance your training program to capitalise on Henneman’s theory of motor unit recruitment.

Bar speed tracking feedback

Utilising a bar speed tracking app like Metric VBT, you can precisely measure the velocity and power output of all your sets, providing you with real-time feedback on how much intent and force your are producing.

Simply trying to set new velocity records, chasing positive trending velocity or even just aiming to beat an arbitrary velocity target to get that satisfying bell during your set can drive motivation and competition in your training and take your gains to another level

Feedback like this has been well proven to boost performance immediately in session and also leads to greater performance gains when done consistently as part of a training program.

Chart. Weakley et al 2019 - feedback improves athletic transfer from the weightroom
Feedback given in real-time improves athletic performance during training.

Ascending warm up sets

Ascending, high-intent warm-ups are invaluable for priming motor units, and getting more value out of your training session by strategically increasing load any allowing the muscles and nervous system to be fully primed by the time you reach your work sets. This approach ensures readiness for peak performance on the work sets, but also turns your warm up sets into their own training stimulus, increasing rate of force development across a range of loads.

To do this, Take your time with deliberate incremental jumps between weights as you work to your days working weight, here is an example for a 150kg back squat

  • 20kg x10
  • 60kg x8
  • 100kg x6
  • 120kg x3
  • 135kg x2
  • 145kg x1
  • Work sets @ 150kg
Ascending warm up sets using velocity based training to maximise strength

These six warm up sets can be completed in pretty quick succession and you will notice that the reps and gap between the load decrease with every set, keeping the volume and fatigue to a minimum.

Compete Against Yourself

One of the most effective ways to utilise velocity tracking in the gym is to consistently aim to beat your past performance velocities. This form of self-competition spurs motivation and can serve as an intuitive autoregulation method.

You could do this by enabling real-time feedback like described above, try to beat last weeks velocity on the same exercise and load combination, use the Trend score to compare performance with your 6-week average performance, or go for the ultimate sign of great intent and set a new velocity or power personal record.

A screenshot from the Metric VBT bar speed tracking app.
Record alerts, progress charts and velocity trends in the Metric VBT app to keep you motivated and accountable.

A power record alert, velocity trend score and mean velocity context as shown in the Metric VBT app. These score cards give valuable context to highlight fatigue and intent in training.

Chase power output instead of load

With strength training, it's easy to see if you are making progress, lifting more weight or completing more reps is usually a good indication that you are making gains, but what if you are trying to improve your power output?

This is where using a bar speed tracker can be a huge advantage in your lifting, instead of worrying about how much weight you can lift for your power exercises, focus on the power you are able to generate on each rep.

Each week try to increase your wattage and focus not on adding load, but instead finding the perfect load that allows you to generate the most power.

By understanding and applying the Henneman size principle along with the use of velocity based training devices like Metric VBT to track and monitor our training output, you can optimise strength and power development for peak athletic performance.

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References and resources

  • Henneman, E. 1957. Relation between Size of Neurons and Their Susceptibility to Discharge.
  • Fry, AC. 2004. The role of resistance exercise intensity on muscle fibre adaptations. Sports Medicine.
  • Moritani, T. 1987. Activity of motor units during concentric and eccentric contractions.
  • Randell et al, 2011. Effect of performance feedback during velocity based resistance training.
  • Weakley, J. 2023. The Effect of Feedback on Resistance Training Performance and Adaptations: A Systematic Review and Meta-analysis.
  • Weakley, J. 2019. The Effects of Augmented Feedback on Sprint, Jump, and Strength Adaptations in Rugby Union Players Following a FourWeek Training Programme

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