How do your muscles move you?
More than 600 muscles, a third of your body, and every one can only pull. Peel a body down to its muscles, zoom into a sarcomere to watch myosin heads tug on actin with ATP, work out how hard your biceps pulls, and race the three energy systems.
▶ Play with it · Read the 60-second explainer · Watch the 40-second video
- More than 600 motors under your skin. Skeletal muscles are about a third of your body mass, joined to bones by tendons: the deltoid, biceps and triceps, pectorals and abs, glutes, quadriceps, hamstrings and calves. The diaphragm, a dome under the lungs, does most of your breathing.
- Three kinds of muscle. Skeletal muscle is striped and voluntary. Cardiac muscle is striped, branched and runs on its own pacemaker, beating about a lakh times a day without tiring. Smooth muscle in the gut and vessels has no stripes and squeezes slowly for seconds.
- Millions of tiny tugs. A muscle is bundles of fibres, fibres are packed with myofibrils, and myofibrils are chains of 2.5 µm sarcomeres. A nerve signal releases calcium, myosin heads grab actin and pull, one ATP per stroke, and the filaments slide so each sarcomere shortens to about 2.0 µm.
- Muscles only pull. So they work in opposing pairs: biceps and triceps, quadriceps and hamstrings. The biceps grips the forearm about 5 cm from the elbow while your hand is 33 cm away, so it pulls about seven times the weight you hold.
- Fuel, fibres and fatigue. Phosphocreatine lasts about 10 s, glycolysis a minute or two, and aerobic respiration for hours. Sprinters are mostly fast-twitch, marathoners mostly slow-twitch. Lactate doesn't cause next-day soreness, and training thickens fibres rather than adding new ones.
- Keeping them strong. Strains usually heal with rest; see a doctor for a pop or if you can't walk. After 30, muscle falls about 3–8% a decade, but strength training twice a week (WHO) and enough protein help at any age.
| Term | Meaning |
|---|---|
| Skeletal muscle | Striped, voluntary muscle joined to bones by tendons. |
| Tendon | A tough cord of collagen that joins a muscle to a bone. |
| Sarcomere | The repeating unit of a muscle fibre, about 2–2.5 µm long, from one Z-disc to the next. |
| Actin and myosin | The thin and thick filaments; myosin heads pull on actin to make the filaments slide. |
| Cross-bridge cycle | Grab, pull, release, re-cock: one ATP per stroke for each myosin head. |
| Antagonist pair | Two muscles that pull a joint opposite ways, like biceps and triceps. |
| ATP | The energy molecule every muscle stroke spends, remade by phosphocreatine, glycolysis and aerobic respiration. |
| Slow- and fast-twitch fibres | Type I fibres are steady and tireless; type II are quick and powerful but tire fast. |
| Sarcopenia | Loss of muscle mass and strength with age. |
From Galen's pairs and Borelli's levers to a frog in a syringe and two papers that changed biology on the same day.
- 170 · Galen: every muscle has an opposite (Galen of Pergamon, Rome)
- 1543 · The muscle men of the Fabrica (Andreas Vesalius, Padua and Basel)
- 1663 · The frog muscle that did not swell (Jan Swammerdam, Amsterdam)
- 1680 · Borelli: the body is a machine of levers (Giovanni Alfonso Borelli, Rome)
- 1791 · Frog legs twitch with electricity (Luigi Galvani, with Lucia Galeazzi Galvani, Bologna)
- 1922 · A Nobel Prize for muscle heat (Archibald Vivian Hill and Otto Meyerhof, London and Kiel)
- 1927 · Phosphocreatine and ATP are found (Cyrus Fiske and Yellapragada Subbarow (and independently Karl Lohmann, and Philip and Grace Eggleton), Boston)
- 1942 · Threads of actomyosin contract (Albert Szent-Györgyi, Brunó Straub and Ilona Banga, Szeged)
The full story, with 30 moments, charts, people and 56 sources: glassbox.how/e/muscleclear/history. The data lives in history.json.
Made with the Glassbox studio from this box's storyboard (window.glassbox.director). Free to reuse under CC BY 4.0.
| File | What | Size |
|---|---|---|
glassbox/reel.mp4 |
Reel / Short, with captions and soundtrack | 1080×1920 |
glassbox/video.mp4 |
YouTube video, with captions and soundtrack | 1920×1080 |
glassbox/slide-1…10.jpg |
Instagram carousel | 1080×1350 |
glassbox/thumb.jpg |
YouTube thumbnail | 1280×720 |
glassbox/cover.jpg |
Share card and repo social preview | 1200×630 |
glassbox/history-reel.mp4 |
“History in 10 moments” Reel / Short | 1080×1920 |
glassbox/history-slide-*.jpg |
History carousel | 1080×1350 |
glassbox/post.json |
Post copy and schedule used by the publish kit |
This box has no accounts and no ads, and it ships its own fonts and libraries. When you run it yourself it sends nothing anywhere. On glassbox.how, the site's /bar.js also loads Glassbox's analytics: Google Analytics to count visits (it asks first in the EU, UK and Switzerland, and stays off when your browser sends Global Privacy Control or Do Not Track) and ClickTrust to detect bots.
It remembers a few things in your own browser only, and never sends them anywhere:
| Browser storage key | What it holds |
|---|---|
muscleclear.v1 |
Which chapters you have opened, your best quiz scores, and sound on or off. |
Exactly what each one sees is at glassbox.how/privacy.
- Code: MIT. Use it, change it, ship it.
- Explanations, text, images and videos (
glassbox.json,glassbox/): CC BY 4.0. Credit “Glassbox, glassbox.how/e/muscleclear”. - Third-party parts keep their own licences: three.js (MIT), Geist, Instrument Serif (SIL OFL 1.1).
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Found a mistake? Open an issue. Corrections happen in public.
It's plain HTML, CSS and JavaScript. No build step and no dependencies. Run locally, it contacts no other website.
python3 -m http.server 8000Three.js and the fonts ship in vendor/ and fonts/, so it also works offline.
Then open http://localhost:8000.
| File | What |
|---|---|
index.html, css/app.css |
The page and its styles |
js/app.js, js/stage.js, js/ui.js, js/kit.js |
The shared Glassbox 3D engine: chapters, 3D stage, controls, quiz, video director |
js/muscle.js |
The ghosted body with its muscles and bones, plus the shared physiology: length–tension curve, energy systems, fibre types, the forearm lever and muscle loss with age |
js/chapters/*.js |
One file per chapter: the 3D model, controls, text, key terms, quiz and video scenes |
glassbox.json |
Title, question, explainer beats, key terms, browser storage and credits shown on glassbox.how |
reel in each chapter |
The storyboard the Glassbox studio records into short videos |
glassbox/ |
The published video, slides, thumbnail and post copy |
fonts/, vendor/three/ |
Self-hosted Geist and Instrument Serif (SIL OFL 1.1) and three.js (MIT) |