Motor Unit Recruitment and Henneman's Size Principle: How Heavy Do You Really Need to Lift?

Henneman’s size principle lifting rule in one line: your nervous system recruits small motor units first and large, growth-relevant ones only when the demand is high enough. Demand gets high in two ways, a heavy weight or a lighter weight taken close to failure, which is why you probably do not need to lift 90% of your max to grow. This article is the nerve-and-muscle physiology behind that idea, part of our complete muscle hypertrophy guide for natural lifters.

What is Henneman’s size principle?

A motor unit is one motor neuron plus every muscle fibre it controls. Small units drive a handful of slow, fatigue-resistant fibres. Large units drive many fast fibres that produce more force and tire quickly.

Henneman’s size principle says units are recruited in order of size. Picking up a pen uses the small ones. A near-maximal squat needs the large ones as well. The brain does not choose individual fibres; it turns up the drive and the units switch on in that fixed-ish order.

Here is the physiology in plain terms:

• Recruitment: more units join as demand rises.
• Rate coding: units already active can fire faster to add force.
• Threshold: the point of demand at which a given unit switches on. Large units have high thresholds.

One human lab study on the soleus (calf) muscle recorded 42 motor units in five people and found units were recruited progressively from rest up to contractions close to 95% of maximum, with low-threshold units firing slower and peaking lower than late-recruited high-threshold units (Oya et al., 2009). Small sample, isometric contractions, but a clean picture of the principle in humans.

Why do high-threshold motor units matter for muscle growth?

The high-threshold units control the fast-twitch fibres, which are generally thought to have the most growth potential. If a set never asks for those units, those fibres get little stimulus. That is the logic behind the old advice that you must lift heavy to build size. For how fibre types differ in training response, see our guide on slow vs fast-twitch fibres.

Notice the wording, though. The principle explains which units are active. It does not by itself prove that every active unit grows equally, or that recruitment is the only driver of hypertrophy. Mechanical tension, effort and volume all matter too, covered in mechanical tension vs muscle damage.

Do I need to lift 90% of my 1RM to recruit them?

No. This is the part most lifters miss. Heavy loads recruit large units straight away because the force requirement is high from rep one. But that is one route, not the only route.

The second route is fatigue. As the small units tire, they cannot hold the force on their own, so the nervous system recruits more units to keep the bar moving. In a lab study of 20 people holding a light (15% of maximum) sustained contraction until failure, motor unit recruitment thresholds dropped as the task went on, by 48% and 30% in two groups of units during the more demanding position task (Baudry et al., 2009). In other words, units that needed more drive early on were being activated at lower drive later. This was a sustained isometric elbow task at very low intensity, not a gym set, so it supports the mechanism rather than proving it for lifting.

This is why a light set can feel like nothing for 15 reps and then suddenly brutal. The first reps are handled by small units. The last ones are where the pool opens up. Our deeper comparison of the programming side is in low vs high load recruitment and acidosis, and the fatigue source itself is explained in central vs peripheral fatigue.

What does the muscle growth evidence say?

If recruitment is the key, then the load should not matter much as long as sets are hard. That is what the outcome studies largely show.

A 2017 systematic review and meta-analysis of 21 studies compared low-load (60% 1RM or less) with high-load training, all sets to momentary failure, over at least 6 weeks. Muscle hypertrophy was similar between conditions, while 1RM strength was greater with high loads (Schoenfeld et al., 2017). The authors conclude maximal strength benefits from heavy loads while hypertrophy can be achieved across a spectrum of loads. A 2021 review argues the old repetition continuum, where heavy builds strength, moderate builds size and light builds endurance, is not well supported and proposes adaptations can be obtained across a wide spectrum of loads (Schoenfeld et al., 2021).

What about people who already train? In a 12-week trial of 49 resistance-trained men, a group using about 30-50% 1RM for 20-25 reps and a group using about 75-90% 1RM for 8-12 reps, all to failure, had similar gains in lean mass and in type I and type II fibre size, though bench press 1RM rose more with heavier loads (14 vs 9 kg) (Morton et al., 2016). An 8-week study of 18 well-trained men found similar quadriceps thickness gains (9.3% with heavy loads vs 9.5% with light loads of 25-35 reps) but a larger squat strength gain with heavy training (19.6 vs 8.8%) (Schoenfeld et al., 2015).

Notice both of those trials share a condition: sets were taken to failure. That is the catch, and it is the practical bit.

How close to failure do light sets need to be?

This is where the evidence is less tidy, so I will give both sides.

A 2023 meta-analysis of 15 studies found a trivial advantage for training to set failure over non-failure for hypertrophy (effect size 0.19, 95% CI 0.00 to 0.37), with no moderating effect of relative load. But when the analysis compared momentary muscular failure with non-failure specifically, there was no advantage either way (effect size 0.12, 95% CI -0.13 to 0.37) (Refalo et al., 2023). The authors suggest a potentially non-linear relationship between proximity to failure and hypertrophy.

Why do the findings look mixed? Studies define “failure” differently, include different loads, and many are short. My cautious reading: you do not need to grind out every rep to absolute failure, but you should not be miles away from it either, especially on light sets where the nervous system needs fatigue to reach the big units. Heavier sets can end 1-3 reps short and still have high recruitment from the start. You can estimate how close you are with our RIR/RPE calculator.

Training style	Where recruitment of big units comes from	Practical stopping point
Heavy (about 75-90% 1RM, 5-12 reps)	The load itself, from rep one	Roughly 1-3 reps short of failure
Moderate (about 60-75% 1RM, 10-15 reps)	A mix of load and rising fatigue	Roughly 1-3 reps short
Light (about 30-50% 1RM, 20+ reps)	Mostly fatigue late in the set	Close to failure, 0-2 reps short

The ranges in the table are practical rules of thumb, not figures taken from a single study.

Is the size principle a rigid law?

Mostly, but not always. In a 2022 study of one rhesus macaque, researchers found motor unit activity changed depending on the behaviour, and described the pattern as flexible rather than rigid, which challenges the idea that cortex only supplies a single common drive (Marshall et al., 2022). That is animal work in one monkey, so be careful about extending it to your bench press.

For lifters, the size principle remains a solid working model for slow, grinding sets. Just do not treat it as a switch you can flip with a particular rep range.

What are the limits of the evidence?

• EMG is imperfect. One classic study found the size principle could not be reliably detected at individual sites for the lowest-threshold units using conventional needle EMG, because the electrodes sample too small an area (Ertas et al., 1995). Surface EMG, used in most lifting studies, has its own limit: it records a blended signal and cannot count individual motor units.
• Mechanism vs outcome. The recruitment story explains why light training could work. The training trials above show that it does work in the groups tested, mostly young men.
• Short trials. Most last 6-12 weeks. Longer-term differences are less clear.
• Joints and fatigue. Heavy sets stress joints more, light sets accumulate fatigue and discomfort. These practical issues matter more than the physiology for many people.

How to use this: a practical split

You do not need to pick a side. A mixed approach covers both routes.

1. Heavy compounds (squat, press, row) in roughly the 5-10 rep range, stopping 1-3 reps short. This builds strength and gets high recruitment immediately.
2. Moderate isolation work in the 10-15 range for most accessories.
3. Light finishers (15-30 reps) on joint-friendly movements, taken close to failure, especially useful if you train at home with dumbbells or a limited stack.

On a budget or in a crowded gym where the heavy dumbbells are always taken, this is useful: a lighter weight pushed hard still counts. If you want a structured version, our push, pull and legs plans already mix rep ranges this way.

If you have joint pain, a recent injury or a medical condition, talk to a doctor or physiotherapist before choosing heavy or near-failure training, rather than guessing from an article.

The bottom line

Henneman’s size principle explains why big units need a high demand to switch on. Heavy loads give you that demand immediately, and light loads can give it to you through fatigue, if you take them close enough to failure. Trials in trained people so far show similar muscle growth either way, and heavy lifting wins for maximal strength. Use both, finish sets honestly hard, and keep progressing.