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[Physiology](https://durgical.com/en/physiology/) · [Nerve and muscle](https://durgical.com/en/physiology/#s-nerve_muscle)

# Motor units, recruitment and summation

- Concept
- Exam priority: medium

A muscle grades its force by switching on more nerve–fiber groups and by firing them faster until the twitches fuse into a steady pull.

Updated 2026-11-01

Also called: Size principle, Tetanus, Twitch

**See it in 3D:** [Vastus lateralis](https://durgical.com/en/anatomy/vastus-lateralis-muscle/) · [Lateral rectus](https://durgical.com/en/anatomy/lateral-rectus-muscle/)

## Key points

- Motor unit = one α motor neuron + its muscle fibers; all fibers of a unit are the same fiber type.
- Size principle (Henneman): small, slow, fatigue-resistant units are recruited first; large, fast units join only for high forces.
- Small units (about 10 fibers or fewer) in eye and hand muscles allow fine control; large limb muscles have units of many hundreds to over a thousand fibers.
- Temporal summation: a second spike before relaxation adds force because Ca²⁺ stays high; at high frequency twitches fuse (fused tetanus).
- Treppe (staircase): successive twitches gain a little force as Ca²⁺ handling warms up.

## What it is

A motor unit is one α motor neuron plus all the muscle fibers it supplies. Because each fiber obeys all-or-none, the nervous system grades force in two ways: recruiting more motor units (spatial summation) and firing them faster so twitches overlap (temporal summation, up to tetanus). Small units are recruited first, which gives fine control at low force.

## Key facts

- A single twitch lasts from about 10 ms (eye muscles) to over 100 ms (soleus).
- Normal voluntary contractions are asynchronous: different units fire out of phase, so force is smooth even at sub-tetanic rates.
- Muscle tone at rest comes from low-level motor unit activity driven by spinal reflexes and descending input.

## How it works, step by step

- A weak voluntary command excites the smallest α motor neurons first (they have the highest input resistance).
- Their slow, oxidative units produce small, steady force.
- As drive increases, firing frequency rises and twitches summate.
- Further drive recruits larger motor neurons with fast, glycolytic units: big jumps in force.
- At maximal effort nearly all units fire at high rates, near fused tetanus.

## Approximate values

| Parameter | Value | Note |
| --- | --- | --- |
| Fibers per motor unit, extraocular muscle | ≈ 10 or fewer | |
| Fibers per motor unit, large limb muscle | hundreds to > 1000 | |
| Frequency for fused tetanus | ≈ 30–80 Hz | depends on fiber type |

Approximate values for a healthy adult at rest; each laboratory sets its own ranges.

## Clinical notes

- After lower motor neuron loss (for example in polio or ALS), surviving neurons sprout to adopt orphaned fibers: EMG shows large, long motor unit potentials and fasciculations.
- In myopathies (for example Duchenne), units have fewer working fibers: EMG shows small, short potentials and early recruitment.

## Memory aid

Small first, fast later: the muscle calls the quiet workers before the heavy lifters.

## Test yourself

Think of the answer, then tap the question to see it.

## Why can a single motor neuron not produce a graded contraction of its own fibers, and how is force graded instead?

Each spike fires all fibers of its unit all-or-none. Force is graded by recruiting more units and raising firing frequency so twitches summate.

## Why do small motor neurons get recruited before large ones?

A smaller cell body has higher input resistance, so the same synaptic current depolarizes it more and it reaches threshold first (size principle).

## Why does a second action potential arriving before relaxation produce more force than a single twitch?

The muscle spike is much shorter than the twitch. A second spike releases more Ca²⁺ before SERCA has cleared the first, so more cross-bridges stay active and series elastic elements are stretched further.

## What EMG change is expected in a muscle weeks after partial loss of its motor neurons?

Surviving neurons sprout to reinnervate denervated fibers, making fewer but larger motor units: high-amplitude, long-duration potentials with reduced recruitment.

[Add to my cards (4)](https://durgical.app/estudiar/?add=/en/physiology/motor-units-recruitment-and-summation/)

## See it in 3D

The structures of Durgical's 3D anatomy where it happens.

- [Vastus lateralis](https://durgical.com/en/anatomy/vastus-lateralis-muscle/)
- [Lateral rectus](https://durgical.com/en/anatomy/lateral-rectus-muscle/)

## Related topics

- [The neuromuscular junction](https://durgical.com/en/physiology/the-neuromuscular-junction/)
- [Fast and slow fibers, energy and fatigue](https://durgical.com/en/physiology/fast-and-slow-fibers-energy-and-fatigue/)
- [Upper versus lower motor neuron lesions](https://durgical.com/en/physiology/upper-versus-lower-motor-neuron-lesions/)
- [Length–tension and force–velocity relationships](https://durgical.com/en/physiology/length-tension-and-force-velocity-relationships/)

## Related conditions

- [Duchenne muscular dystrophy](https://durgical.com/en/conditions/duchenne-muscular-dystrophy/)

[← All of physiology](https://durgical.com/en/physiology/)

Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: [10.4 Nervous System Control of Muscle Tension](https://openstax.org/books/anatomy-and-physiology-2e/pages/10-4-nervous-system-control-of-muscle-tension) (OpenStax (Rice University), 2022); [Anatomy, Bony Pelvis and Lower Limb: Vastus Lateralis Muscle](https://www.ncbi.nlm.nih.gov/books/NBK532309/) (StatPearls (NCBI Bookshelf), 2024); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 6). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/motor-units-recruitment-and-summation/