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 · Lateral rectus

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.

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)

See it in 3D

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

Duchenne muscular dystrophy

← All of 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 (OpenStax (Rice University), 2022); Anatomy, Bony Pelvis and Lower Limb: Vastus Lateralis Muscle (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).

Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake

Representative model, for educational purposes only.