The Size Principle: Which Muscle Fibers Work, and When
Quick Intro
The size principle, described by the neuroscientist Elwood Henneman and colleagues in the 1950s and 1960s, states that the nervous system recruits muscle fibers in a set order, from small to large. Small units that produce little force are called on first, and larger units that produce more force are added as the demand grows. It explains how a muscle can move a light object gently and a heavy one forcefully.
The Idea in Plain Terms
A muscle is controlled by motor units. Each unit is one motor neuron, a nerve cell from the spinal cord, plus all the muscle fibers it controls. Units vary:
| Unit type | Size | Force | Fatigue resistance |
|---|---|---|---|
| Low-threshold | Small | Low | High, mostly slow-twitch fibers |
| High-threshold | Large | High | Low, mostly fast-twitch fibers |
When you need only a little force, such as lifting a cup, the nervous system uses the small units. As the demand rises, it recruits larger and larger units in order. The sequence is orderly because smaller neurons are easier to excite.
The principle has a useful consequence for training. The large, high-threshold units, which have the most potential for growth, are recruited only when the demand is high. This happens in two ways:
- Heavy loads. The units are called up at the start because the force needed is high.
- Fatigue. With a lighter load, the small units tire over a set, and the nervous system recruits larger ones to keep going. Near the end of a set taken close to failure, the high-threshold units are working.
This is why a light load taken close to failure can build muscle. It is also why sets that stop far from failure with a light weight give a weak signal, since the large units may never be fully recruited.
Real muscle is more complex. Recruitment order can vary somewhat with the speed and type of movement, and the principle is a rule of thumb that holds well in most situations.
A Worked Example
A set of curls with a light weight:
| Rep range of the set | Units in action |
|---|---|
| Reps 1 to 10 | Mostly small and medium units |
| Reps 15 to 20, nearing failure | Larger units join as small units tire |
With a heavy weight, large units are involved from the first rep.
What the Research Shows
Henneman’s original work, published in Science in 1957 and in later papers, showed the orderly recruitment in animal studies. Human studies using recordings of muscle electrical activity have confirmed the order in most voluntary contractions, with some variation in fast, ballistic movements.
Training studies relate to the principle. A 2016 study by Morton and colleagues found that low-load and high-load training produced similar growth when sets were taken to failure, consistent with the idea that fatigue leads to high recruitment. A 2017 meta-analysis found the same pattern across studies.
The idea does not mean light training near failure is identical to heavy training. Strength gains are larger with heavy loads, partly through nervous system adaptations.
Who It Suits
This concept helps:
- Lifters using light loads because of injury or equipment.
- Anyone confused about why both heavy and light sets can work.
- Coaches explaining effort.
How to Use It
- Use heavy loads for strength.
- For lighter loads, take sets within a few reps of failure.
- Do not stop sets with many reps left and expect full effect.
- Mix rep ranges for strength and size.
- Keep technique safe near failure.
- Prefer stable exercises for the last reps.
Common Misunderstandings
It is not that fast-twitch fibers are only used in heavy lifting.
It is not that light weights cannot build muscle.
It is not a reason to grind every set to failure.
FAQ
Do I need to lift heavy to use fast-twitch fibers? No. Fatigue brings them in.
Why do fast reps matter for power? Intent to move fast changes recruitment in some cases.
Is it true for all muscles? It holds well in most.
Does it explain training to failure? Partly. It explains the effect of fatigue.
Where This Comes From
Henneman E, 1957, relation between size of neurons and their susceptibility to discharge (Science, 126, 1345 to 1347). Morton RW and colleagues, 2016, neither load nor systemic hormones determine resistance training-mediated hypertrophy or strength gains in resistance-trained young men (Journal of Applied Physiology, 121, 129 to 138). Schoenfeld BJ and colleagues, 2017 (Journal of Strength and Conditioning Research, 31, 3508 to 3523).
Related to: mechanical tension, myo-reps, rest-pause training and the three mechanisms of hypertrophy.