How to Build Muscle: Henneman's Size Principle Explained

Clem Duranseaud
Jul 12
10 min read

Updated: 24 hours ago

Every time you pick up a weight, your brain has to answer one simple question:

How many muscle fibres do I need to lift this?

That decision happens in a fraction of a second, but it determines how much force you can produce and, ultimately, how much muscle you can build.

This process is governed by Henneman's Size Principle. One of the most influential concepts in exercise physiology.

Once you understand the Size Principle, concepts like progressive overload, training to failure, heavy versus light weights, and hypertrophy all start to make sense.

In this article, we'll break it down in plain English and show you how to use it to build more muscle and get stronger.

Table of contents

What is a motor unit
The size principle
Mechanical tension
Training to failure
Heavy vs light weights
Progressive overload
Common misconceptions

What is a motor unit

A motor unit consists of one motor neuron and all of the muscle fibres it controls.

When that motor neuron sends a signal, every muscle fibre within that motor unit contracts at the same time. It's an all-or-nothing response, those fibres are either switched on or they're not.

Small motor units have 10-50 muscle fibres
Large motor units can have up to 2000 muscle fibres
There are motor units have all sizes within that range

Small motor units control only a handful of muscle fibres, allowing for precise movements like writing or playing the piano. Others control thousands of fibres, producing the large amounts of force needed for exercises like squats and deadlifts.

Think of motor units as the individual teams that make up your muscles. Instead of asking every team to work on every task, your brain recruits only the teams it needs. As the demand for force increases, more teams join in.

This raises an interesting question.

How does your brain decide which motor units to recruit first?

That's exactly what Henneman's Size Principle explains.

Henneman's Size Principle

Now that we know what a motor unit is, the next question is:

How does your brain decide which motor units to recruit?

The answer is surprisingly simple: it recruits the smallest ones first.

This is known as Henneman's Size Principle.

Whenever you perform a movement, your nervous system recruits motor units in a predictable order, from the smallest and least powerful to the largest and most powerful.

It never starts with the biggest motor units unless the task demands it.

Imagine picking up a coffee mug.

It's light, so your brain only recruits a handful of small motor units. The movement feels effortless because you simply don't need much force.

Now imagine loading a heavy barbell for a set of squats.

Suddenly, those small motor units aren't enough. To produce more force, your nervous system progressively recruits larger and larger motor units until enough muscle fibres are working to lift the weight.

This process happens automatically. You don't consciously choose which motor units to activate, your nervous system makes that decision in a fraction of a second.

Why This Matters for Muscle Growth

Here's the important part.

The largest motor units control the largest, strongest muscle fibres, the fibres with the greatest potential for strength and hypertrophy.

If you never recruit these high-threshold motor units, you're leaving some of your muscle-building potential untapped.

Fortunately, there are two ways to recruit them:

Lift a heavy weight. Heavy loads require a lot of force from the very first repetition, so your nervous system quickly recruits the largest motor units.
Take a lighter weight close to failure. At first, only the smaller motor units are needed. But as those fibres fatigue, your brain recruits progressively larger motor units to keep the weight moving. By the final few repetitions, you're recruiting many of the same high-threshold motor units as you would with a heavy load.

This explains one of the most misunderstood concepts in strength training.

Muscles don't know how much weight is on the bar. They only know how much force they have to produce.

Whether that force comes from lifting a heavy weight for five reps or a lighter weight for twenty reps, the goal is the same: recruit as many motor units as possible.

That's why both heavy and light training can build muscle, provided the effort is high enough.

Understanding the Size Principle changes how you look at almost every aspect of training.

It explains why progressive overload, training close to failure, and mechanical tension are considered the foundations of muscle growth.

Let's look at how they all fit together.

Training Close to Failure

If Henneman's Size Principle teaches us anything, it's this:

Your muscles don't grow because you complete a certain number of repetitions. They grow because you recruit the motor units capable of producing high amounts of force.

This is where training close to failure becomes so important.

Muscular failure is the point where, despite giving your maximum effort, you can no longer complete another repetition with good technique.

But here's the key point: you don't have to reach failure to recruit your largest motor units. You simply need to get close enough.

Let's use a set of 20 repetitions as an example.

During the first few reps, the weight feels easy. Your body only recruits the smaller, low-threshold motor units because they produce enough force to move the load.

As those muscle fibres begin to fatigue, your brain has a problem to solve: the same weight still has to move, but the fibres doing the work are becoming less effective.

The solution is to recruit additional, larger motor units.

With every repetition, more motor units are called into action. By the final few reps, your nervous system is recruiting the same high-threshold motor units that would have been activated almost immediately with a much heavier weight.

That's why the last few repetitions of a hard set feel so different from the first few.

They're the repetitions where the greatest number of muscle fibres are working.

So, Should You Train to Failure?

Not necessarily.

Research consistently shows that training within about 0–3 repetitions in reserve (RIR) produces very similar muscle growth to training to complete failure, while often causing less fatigue and allowing for better recovery over the course of a training program.

For most exercises, stopping with one or two good repetitions left in the tank is an excellent balance between stimulating muscle growth and managing fatigue.

There are exceptions, of course.

Exercises such as bicep curls, lateral raises, or leg extensions are generally safe to take to failure because the risk of injury is relatively low.

On the other hand, repeatedly taking heavy compound lifts like squats, bench presses, or deadlifts to absolute failure can dramatically increase fatigue, break down technique, and make recovery much more difficult.

The Takeaway

Don't chase failure for the sake of it.

Instead, chase effective effort.

If you're consistently training hard enough that only one, two, or at most three good repetitions remain, you're recruiting the motor units that have the greatest potential for muscle growth.

Now that we understand why effort matters, another question arises:

If light weights taken close to failure recruit the same motor units as heavy weights, why bother lifting heavy at all?

Let's answer that next.

Light Weights vs Heavy Weights

One of the biggest misconceptions in the gym is that heavy weights build muscle, while light weights are only good for toning.

The Size Principle tells us that this simply isn't true.

Remember, your nervous system recruits motor units based on how much force is required, not the number printed on the dumbbell.

When you lift a heavy weight, say a load you can only manage for five reps, your body has no choice but to recruit high-threshold motor units almost immediately. Producing that much force requires your largest and strongest muscle fibres from the very first repetition.

With a lighter weight, the story is different as we have seen in the previous section.

Both heavy and light weights can stimulate similar muscle growth, as long as the lighter set is taken close enough to failure.

So Why Lift Heavy?

If both approaches recruit the same motor units, why do strength athletes spend so much time lifting heavy?

Because muscle size and strength aren't exactly the same thing.

Building muscle is largely about exposing your muscle fibres to enough mechanical tension.

Building strength also requires your nervous system to become better at producing force and coordinating movement.

Think of it this way.

A bodybuilder's goal is to make the engine bigger.

A powerlifter's goal is to make the engine produce more horsepower.

There's plenty of overlap, but the emphasis is different.

Heavy weights teach your nervous system to:

Produce force more efficiently.
Improve coordination between muscles.
Refine technique under heavy loads.
Become comfortable lifting maximal weights.

That's why, if your primary goal is to increase your one-rep max, heavy lifting is essential.

So Why Lift Light?

Lighter loads have their own advantages.

Because they create less joint stress and systemic fatigue, they can be an excellent way to accumulate training volume while still recruiting high-threshold motor units.

They're also ideal for:

Isolation exercises like lateral raises and bicep curls.
Training around injuries or joint pain.
Higher-volume hypertrophy phases.
Beginners who are still learning technique.

Many experienced lifters combine heavy and light training throughout the week to get the best of both worlds.

The Best Approach

For most people looking to build muscle, the answer isn't heavy or light.

It's both.

Use heavier loads on your big compound lifts, such as squats, bench presses, rows, and overhead presses, to develop strength and reinforce good movement patterns.

Then use moderate or lighter loads on accessory exercises to safely accumulate more quality training volume.

As long as your sets are challenging and you apply progressive overload over time, your muscles don't particularly care whether you reached that effort with six repetitions or sixteen.

They only care that you've given them a reason to adapt.

Mechanical Tension: The Trigger for Muscle Growth

If recruiting motor units is the first step toward building muscle, mechanical tension is the signal that tells your muscles to adapt.

Mechanical tension is simply the force experienced by your muscle fibres while they're producing force.

The greater the tension placed on a muscle, and the longer it experiences that tension, the greater the stimulus for muscle growth.

This is why Henneman's Size Principle is so important.

Remember, the largest motor units control the muscle fibres with the greatest potential for hypertrophy. Whether you recruit them by lifting a heavy weight or by taking a lighter weight close to failure, those fibres are exposed to high levels of mechanical tension.

And that's exactly what stimulates them to grow.

Mechanical Tension Is More Important Than Weight

Many people assume that muscle growth comes from lifting the heaviest weight possible.

Not quite.

Imagine two people performing a set of leg extensions.

The first person uses a heavy weight but only moves it through half the range of motion, bouncing each repetition and relying on momentum.

The second person uses a lighter weight, controls every repetition, moves through a full range of motion, and finishes the set with only one repetition left in reserve.

Who's creating more mechanical tension?

In many cases, it's the second person.

That's because muscles respond to the force they produce, not simply the amount of weight attached to the machine.

Good technique keeps tension where you want it: on the target muscle.

Three Things That Maximise Mechanical Tension

Fortunately, creating mechanical tension isn't complicated.

Focus on three things:

1. Train Close to Failure

As we've already seen, getting close to failure forces your nervous system to recruit larger and larger motor units.

Those high-threshold fibres experience the greatest mechanical tension.

2. Use Good Technique

Momentum might help you lift more weight, but it often reduces the amount of work your muscles actually perform.

Control both the lifting and lowering phases, use a full range of motion where appropriate, and let the target muscle do the work.

3. Progress Over Time

Your muscles adapt remarkably quickly.

To keep exposing them to meaningful mechanical tension, you need to gradually increase the challenge through heavier loads, more repetitions, additional sets, or improved execution.

That's exactly what progressive overload achieves.

The Big Picture

Many training methods claim to be the "secret" to muscle growth.

Drop sets.
Supersets.
Blood flow restriction.
Tempo training.

While these techniques can all have their place, they're simply different ways of increasing one thing:

Mechanical tension.

At the end of the day, your muscles don't care whether that tension comes from a barbell, a dumbbell, a machine, or a resistance band.

They only care that you've challenged them enough to justify getting stronger and bigger.

Progressive Overload: Giving Your Muscles a Reason to Grow

By now, one thing should be clear:

Your body is incredibly good at adapting.

The first time you bench press 60 kg, your nervous system recruits enough motor units to complete the task. A few weeks later, after repeated training, that same 60 kg feels much easier. Your muscles have grown, your nervous system has become more efficient, and fewer high-threshold motor units are needed to lift the same weight.

From your body's perspective, there's no reason to keep adapting if the challenge never changes.

This is why progressive overload is the cornerstone of every successful strength and muscle-building program.

Progressive overload simply means gradually increasing the demands you place on your muscles over time.

As the workload increases, your nervous system is forced to recruit more motor units and expose your muscles to greater mechanical tension, encouraging them to grow stronger and larger.

Progressive Overload Isn't Just Adding Weight

One of the biggest misconceptions is that progressive overload means putting more plates on the bar every week.

That's certainly one way to do it, but it's far from the only one.

You can progressively overload your workouts by:

Adding weight while maintaining good technique.
Performing more repetitions with the same weight.
Completing more sets over the course of your workout.
Improving your technique or range of motion, making each repetition more demanding.
Reducing your repetitions in reserve (RIR) by working closer to failure with the same load.
Increasing training frequency, allowing each muscle group to receive more quality work each week.

All of these methods increase the training stimulus and encourage your body to continue adapting.

Small Improvements Add Up

Progressive overload doesn't need to happen every workout.

In fact, trying to add weight every session is one of the quickest ways to stall your progress.

Instead, think long term.

Adding one extra repetition, increasing the weight by 2.5 kg, or improving your technique slightly may seem insignificant today. But over months and years, those small improvements compound into remarkable strength and muscle gains.

Consistency will always beat dramatic jumps in training load.

The Goal Is Better, Not Different

Many lifters fall into the trap of constantly changing exercises because they mistake variety for progress.

Your muscles don't grow because your workouts are different every week.

They grow because they become better at the same movements over time.

That's why it's worth sticking with a well-designed program for several months before making major changes. The longer you practice an exercise, the easier it becomes to measure progress and apply progressive overload.