Henneman was an OG when it came to explaining the neural adaptations that occur when we get stronger and more powerful.
When we perform any concentric* movements, our nervous system recruits motor units and activates the muscle fibres in a smooth, stepwise fashion. Starting with the smallest type I motor units working its way up to the biggest type II motor units in ascending order of size.
*Eccentric muscular contractions behave differently to concentric, being more energy efficient, relying on elastic components of the muscle fibre and often working in reverse of the Henneman principle (3).
When we contract our muscles the muscle fibres are always recruited from smallest to biggest
If a movement has only a low intensity (say typing on a keyboard) then only a fraction of the motor units in our forearms and hands will be engaged for the task. Whereas performing a higher intensity movement (say hanging from a monkey bar by one hand) will lead to activation of more motor units in our forearms so that we don't slip and fall.
This evolutionary wiring system 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.
How to activate your biggest motor units
In sport and in strength training, we are most often focused on developing our bodies overall strength and power, it's these qualities that give us the ability to sprint, change direction, jump, strike and tackle. It's therefore natural that we will want to be performing training activities that activate and recruit our biggest type II motor units. The same motor units responsible for the explosive actions needed to be a successful athlete.
There are four key ways to work through the Henneman site principle to get to your biggest motor units, but they aren't all created equally.
Train into fatigue
As we complete more and more repetitions in a set and the closer we come to the point of failure, we gradually fatigue our smaller type I motor units. As a result, our larger type II fibres are effectively forced to begin activating as the exhausted type I motor units can't go on (2).
This method however is the least effective way to become a more explosive and stronger athletes. 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 can also lead to extra type I muscle fibre hypertrophy, something that might be desirable in bodybuilding but less ideal for athletes needing to maintain a high power to weight ratio.
Bodybuilding style training leads to preferential hypertrophy of type I muscle fibres, whereas strength and power training leads to increased ratios of type II fibres (Fry).
As you train with progressively heavier and heavier weights, not only do you increase muscle mass, improve lifting technique, increase neural output and potentially shifting your muscle composition toward type II motor units, your brain also learns to unlock and recruit more of your biggest type II motor units to lift the weights. Effectively enhancing your ability to activate and move through the Henneman size principle rapidly. It's like taking the handbrake off a car.
This unlocking of the motor units is a large reason why people new to training in the gym can make such rapid gains in strength in their first few months. It's not that they are able to pack on muscle or become significantly better lifters in that time. It's mainly just that they are unlocking strength they already possessed but that their brains could not access fully.
This is also part of the reason why more experienced lifters only really start developing strength when they train above 80-85% of their 1RM for a given exercise, anything lighter just doesn't activate enough of the powerful type II motor units.
The third way to increase your bodies ability to harness the biggest and most explosive type II motor units is by being fast. Explosive activities enhance the rate at which your body is able to recruit these bigger, more explosive, type II motor units. This applies to both plyometric type contractions and standard exercises that are done fast, each having their own benefits for accelerating our bodies recruitment ability.
However, the actual speed of your contractions might not be as important as once thought when it comes to Henneman's size principle...
Train with intent to move
Intent to move is the idea that it's not just about how fast the bar moves but also about how fast you intended to move the bar. It's the nervous systems request for movement velocity that dictates a lot of great nervous system changes, including improved recruitment of the largest motor units.
Intent to move effectively merges the benefits of heavy lifting and fast movements, giving us benefits from both exercise modes and enhancing our training adaptations
Here is an example. If you're lifting heavy weights and you're in a strength training block, the bar is not going to move very fast. Sets of heavy deadlifts are going to be at a slow relative velocity, say around 0.3m/s.
But intent to move says you should try and lift as fast as you possible can anyway. Even if the weight is so heavy that it prevents you from actually achieving a velocity faster than say 0.32m/s. As long as that 0.32m/s is indicative of your best intended effort, you will be forcing your body to upregulate the recruitment and efficiency of your type II motor units.
It's the intended output from your brain, not the input of the actual movement velocity that creates the positive adaptations.
Using VBT to harness the size principle
So how can we utilise velocity based training to take advantage of the Henneman size principle? Here are some of my favourites.
Ascending warm ups
Ok this isn't really a velocity based training approach, but it's still super useful. Ascending warm up sets are a great short and long term strategy for performance, they prime our motor units by progressively overloading them without ever trying to smash them with a maximum effort while cold. There is no hard and fast rule when doing these warm ups but the objective is that no set ever be a surprise in how heavy it is, and each gap should get progressively smaller as you approach the working weight for a given day.
Try to beat last weeks velocities
A little motivation and competition is one of the best ways to leverage velocity in the gym, and what better way to compete than with your own former scores? To do this, simply record your velocities across every set of a given exercise, and then next time you do that exercise, try to beat each velocity. I personally like to use the velocity of your best rep and mean propulsive velocity is my go-to metric. Ideally you should beat your former velocity for a given weight on more weeks than you don't, and as an added bonus, this motivation tactic can also become an easy autoregulation strategy for those weeks when your velocity is way down.
Set power targets
With strength training, it's easy to see if you are making progress, more weight or more reps is usually a good indication that you are moving in the right direction, but what if you are trying to improve your power output?
This is where velocity tracking can come in, instead of worrying about how much weight is on the bar, focus on how much power or velocity you are able to generate. This gives you an objective target week on week as you try to increase your wattage every session and it shifts the focus away from how much weight you can lift. When power becomes the goal the weight you lift will auto regulate itself and help you optimise training for maximum power output.