Skip to content
Scrollchart
Fitness Rich #motor-units#recruitment#size-principle

Motor Unit Recruitment

Slow units fire first; fast units join only as load or fatigue rises.

A free, animated motor unit recruitment you can read here or embed on any website, from Scrollchart.

Motor Unit Recruitment

Motor Unit Recruitment (Size Principle)Henneman 1957: smaller motor neurons recruited first, larger as force demand risesActive motor unit contribution020%40%60%80%100%0%25%50%75%100%Force output (% max voluntary contraction)

A motor neuron pool diagrammed by size, with corresponding muscle fibers. As effort rises, recruitment marches from smallest (Type I) to largest (Type IIx). Heavy loads recruit all units immediately; light loads recruit larger units only as fatigue accumulates near failure. The animation shades units in sequence as effort climbs.

Good for

  • Articles on rep ranges and recruitment
  • Light-vs-heavy growth content
  • Coach education

Source & accuracy

This motor unit recruitment is an editorial illustration built to represent the concept accurately. Where it shows figures, they are typical or representative values chosen to make the relationship clear, not a single underlying dataset. The diagram and its explainer are reviewed and maintained centrally, and updated over time as understanding improves.

The size principle and orderly recruitment

A motor unit is one motor neuron and all the muscle fibers it controls. The size principle, described by Elwood Henneman, states that motor units are generally recruited from smallest to largest as force demand rises. Small, fatigue-resistant slow units fire first for light tasks, and larger, more powerful fast units are added only as more force is required.

This orderly pattern means low loads recruit mostly slow units, while heavier loads or sustained effort progressively bring in fast units. It is why everyday low-force activity does not strongly tax the highest-threshold fibers.

How fatigue recruits high-threshold units at lighter loads

Fatigue changes the picture. As lower-threshold units tire during a sustained or high-rep set, the nervous system recruits higher-threshold fast units to maintain force, even when the external load is moderate. This is part of why training closer to fatigue can engage fast fibers that a brief light effort would not.

This is general educational information about neuromuscular physiology, not medical or training prescription. For program design specific to your goals or any neuromuscular condition, consult a qualified professional.

Embed this diagram

Add this animated motor unit recruitment to your own site. Copy one line of HTML, or use the embed builder for theme and sizing options.

Reference

What this is
A free, embeddable, animated motor unit recruitment for any website.
Who uses it
Fitness blogs, Coaches & personal trainers, Medical educators.
How to embed
Copy one line of HTML. No signup. No watermark. Works in WordPress, Webflow, Ghost, Substack, plain HTML.
File size
iframe embed, ~80 KB gzipped (loads on demand, does not block your page paint).
License
Free forever. Editorial explainer text included; updated centrally over time.
URL
scrollchart.com/diagram/motor-unit-recruitment

Embed format options

Copy the universal HTML snippet, the WordPress shortcode, or an iframe fallback - see the WordPress plugin page for details. Any format keeps the same Core Web Vitals profile and the same explainer text.

Embed snippet 
<div data-scrollchart="motor-unit-recruitment" data-scrollchart-v="1"></div>
<script src="https://scrollchart.com/embed.js" async></script>

By embedding, you acknowledge the diagram and explainer text are served from scrollchart.com and may be updated from time to time to reflect new research or to link to relevant sources within the explainer.

Frequently asked questions

Where can I get a free animated "Motor Unit Recruitment" for my website?
Scrollchart provides "Motor Unit Recruitment" as a free, embeddable animated diagram you can add to any website with one line of HTML. No signup is required and there is no watermark. The diagram and its explainer text are served from scrollchart.com, so the embed stays current without any maintenance on your end.
How do I embed a motor unit recruitment diagram in a health or wellness blog?
Copy the HTML snippet from the Scrollchart diagram page and paste it into any post or page in your CMS. It works in WordPress, Webflow, Ghost, Substack, and plain HTML without any plugin or account. The diagram renders as animated SVG directly in your page, so search engines can index the accompanying explainer text.
Related diagrams

Other fitness explainers

See all fitness →
Fitness Rich

Type I vs IIa vs IIx Fibers

Three fiber subtypes with different sizes, fatigue rates, mitochondria, and trainability.
Fitness Medium

Training to Failure (the science)

Failure adds fatigue faster than it adds growth, and the cost-benefit shifts by exercise type.
Fitness Medium

RIR Detection Accuracy

Trained lifters underestimate RIR by 1-2 reps; novices by 4+.