Interactive 3D Muscular System: Find Every Muscle and See How Muscles Contract
Over 100 labeled muscles on a real skeleton. Search one, peel the layers, then bend an elbow or kick a ball and watch which muscles pull. Zoom in from the whole muscle to the sarcomere sliding.
Forces and fibres
Lab notebook
Every movement you run, every stimulation and every measurement is written here. Export a CSV, or turn the session into a PDF lab report.
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Concept library
Click "New problem" to begin.
Click "Start quiz" for 5 questions on the muscles of the body, muscle tissue, the sliding filament theory, movement and fibre types.
1 Overview
Four zoom levels sit at the top left of the view. Body is the muscular system on its real skeleton, in 3D (desktop) or as front and back plates in 2D (phones). Muscle types compares skeletal, cardiac and smooth muscle under the microscope. Inside a muscle opens a skeletal muscle from the whole muscle to the fascicle, the fibre and the myofibril. Sarcomere shows one sarcomere sliding, the cross-bridge cycle and the neuromuscular junction. On the right, Find searches the muscles, Muscle describes the one you chose, Movement explains the movement playing and Graph plots twitches, length–tension, force–velocity, motor units, fatigue and the energy systems.
2 Finding muscles
Click any muscle on the body, or type in Find: a name (deltoid), an everyday name (calf, abs, hamstrings, six-pack), a region (forearm) or a job (flexes the elbow). The muscle turns gold, the camera flies to it and the Muscle tab gives its origin, insertion, action, nerve and roots, antagonist and an exercise. Show it alone fades every other muscle; the layer buttons peel the superficial muscles away to show the deep ones.
3 Movements
Pick a movement on the bar at the bottom of the view and press ▶: a biceps curl, a triceps push, lifting the arm sideways, a kick, a squat, standing on tiptoe, breathing, chewing and smiling, or standing still. The joints bend for real and the working muscles glow; with Roles colouring the agonist is orange, the antagonist blue, synergists yellow and fixators purple. The Movement tab says whether each muscle is shortening (concentric), lengthening under load (eccentric) or holding (isometric), and works out the force for the lever.
4 Muscle types, inside a muscle and the sarcomere
Muscle types shows the three tissues side by side, contracting; click one for its details. Inside a muscle steps from the muscle to a fascicle, a fibre with its nuclei, sarcoplasmic reticulum and T-tubules, and a myofibril with its bands. Sarcomere: press ▶ to fire the nerve; the sarcomere shortens as myosin heads pull actin, and the cross-bridge cycle runs in the magnifier. Change its starting length, take away calcium or ATP (rigor) in the Sarcomere panel.
5 Stimulate a muscle
In the Stimulate panel choose a fibre type, then fire one twitch, two quickly (summation) or a 1-second train at the frequency you set. The Graph shows the force; above the fusion frequency the twitches merge into a smooth tetanus. The effort slider recruits motor units by size and shows the EMG.
6 Experiment mode and the lab report
Six labs: name the muscles, antagonistic pairs, summation and tetanus, length–tension, fatigue in slow and fast fibres, and the forces in a biceps curl. Each shows its aim, what you need and a safety note; the method ticks off as you work and a conclusion question ends it. Then create a PDF lab report.
7 Quiz, units and keyboard
The practice quiz covers naming muscles, their actions, tissue types, structure, sliding filaments, movement, fibre types and lever calculations, each with a worked solution. SI | US switches forces between newtons and pounds-force, masses between kg and lb and lengths between cm and inches. Keys: Space play or pause, 1–4 the zoom levels, F, A, M, G the panels, L labels, O orbit, N start again. Right-click the view to save it as an image.
The muscular system: an interactive 3D muscle model and human anatomy virtual lab
Your body has more than 600 skeletal muscles, and every movement you make, from a blink to a sprint, is muscles pulling on bones. This free interactive 3D muscle model puts over 100 of them on a real human skeleton. Click a muscle, or search for it, to see where it starts and ends, what it does and which nerve runs it; peel the layers to reach the deep muscles; then bend an elbow, kick a ball or squat and watch the muscular system work, pair by pair. Zoom in and the same model takes you inside a muscle fibre to the sarcomere, where the filaments slide. It works as a human anatomy virtual lab too: six practicals (name the muscles, antagonistic pairs, tetanus, length–tension, fatigue and levers) each end in a PDF lab report, with no specimens, gym or equipment needed.
- Muscles
- Over 100 named muscles on a real skeleton (BodyParts3D), searchable by name, region and everyday name
- Movements
- Curl, triceps push, arm raise, kick, squat, tiptoe, breathing, chewing and smiling, standing
- Close-ups
- Three muscle tissues; inside a skeletal muscle; the sarcomere and the cross-bridge cycle
- Modes
- Simulate, Experiment (6 practicals with a PDF report) and Quiz
- Price
- Free, no account or download; works on Chromebooks, tablets and phones
The major muscles of the body, labeled
Turn the body and the muscles of the body are labeled as you go; click any of them for its card. These are the muscles every GCSE, A-level and anatomy course asks you to know, with the movement each one makes:
| Muscle | Where | Main action | Antagonist |
|---|---|---|---|
| Deltoid | Shoulder | Abducts the arm (lifts it sideways) | Latissimus dorsi |
| Rotator cuff | Around the shoulder joint | Rotates the arm and holds the joint together | — |
| Pectoralis major | Chest | Adducts and flexes the arm | Latissimus dorsi, deltoid |
| Latissimus dorsi | Back | Extends and adducts the arm | Deltoid, pectoralis major |
| Biceps brachii | Front of the upper arm | Flexes the elbow | Triceps brachii |
| Triceps brachii | Back of the upper arm | Extends the elbow | Biceps brachii |
| Rectus abdominis | Front of the abdomen | Flexes the spine | Erector spinae |
| Iliopsoas (hip flexors) | Deep in the hip | Flexes the hip | Gluteus maximus |
| Gluteus maximus | Buttock | Extends the hip | Iliopsoas |
| Quadriceps | Front of the thigh | Extends the knee | Hamstrings |
| Hamstrings | Back of the thigh | Flex the knee, extend the hip | Quadriceps |
| Gastrocnemius | Calf | Plantarflexes the ankle (tiptoe) | Tibialis anterior |
| Tibialis anterior | Front of the shin | Dorsiflexes the ankle | Gastrocnemius, soleus |
The three types of muscle tissue
Skeletal muscle moves the skeleton: long, cylindrical, striated fibres with many nuclei, under voluntary control and quick to tire. Cardiac muscle is found only in the heart: short, branched, striated cells joined by intercalated discs, which pass the impulse from cell to cell so the heart beats on its own rhythm and never tires or goes into tetanus. Smooth muscle lines the gut, blood vessels, airways and bladder: spindle-shaped cells with one nucleus and no stripes, contracting slowly and involuntarily. The Muscle types view puts all three under the microscope, contracting.
The structure of skeletal muscle
A skeletal muscle is wrapped in epimysium and joined to bone by tendons. Inside are bundles called fascicles (in perimysium), each made of muscle fibres: single cells up to 30 cm long with many nuclei, packed with myofibrils and wrapped in a membrane, the sarcolemma, that dips into the fibre as T-tubules. The sarcoplasmic reticulum stores calcium around every myofibril. Each myofibril is a chain of sarcomeres from one Z-line to the next: thick myosin filaments in the middle (the dark A band), thin actin filaments from the Z-lines (the light I band), and the H zone where only myosin lies.
The sliding filament theory and the cross-bridge cycle
A motor neuron releases acetylcholine at the neuromuscular junction; the fibre fires an action potential that runs down the T-tubules, and the sarcoplasmic reticulum releases Ca²⁺. Calcium binds troponin, which pulls tropomyosin off the binding sites on actin. Myosin heads bind to actin (a cross-bridge), swivel in a power stroke that pulls the actin toward the M-line, then a new ATP detaches each head and its hydrolysis re-cocks it. The filaments slide; they do not shorten. So the I band and H zone narrow while the A band stays the same width. When the nerve stops, calcium is pumped back into the sarcoplasmic reticulum and the muscle relaxes; with no ATP the heads cannot let go, which is why a body stiffens in rigor mortis. Phosphocreatine regenerates ATP for the first few seconds of hard exercise.
Antagonistic muscle pairs: agonist and antagonist
Muscles can only pull, so they work in antagonistic pairs across a joint. In a biceps curl the biceps is the agonist (prime mover) and the triceps the antagonist, which relaxes; the brachialis is a synergist and the shoulder muscles act as fixators, holding the bone the biceps pulls from. Each muscle has an origin on the bone that stays still and an insertion on the bone that moves. Play a movement and the roles are coloured on the body.
Types of muscle contraction: isotonic and isometric
In an isotonic contraction the muscle changes length: it is concentric when it shortens under load (lifting the dumbbell) and eccentric when it lengthens under load (lowering it slowly, or the hamstrings braking a kick). In an isometric contraction it makes force without moving (holding the weight still, a plank). Eccentric contractions are the strongest, about 1.5–1.8 times the isometric force, and cause most muscle soreness.
Slow and fast twitch muscle fibres
Type I (slow oxidative) fibres are red with myoglobin, packed with mitochondria and capillaries, and resist fatigue: the soleus, a postural muscle, is about 88 % type I. Type IIa fibres are fast and fairly fatigue-resistant, and type IIx fibres are the fastest and strongest but tire in seconds. The nervous system recruits small, slow motor units first and the large, fast ones only for big efforts (the size principle), and fires them faster to grade force. One action potential gives a twitch; stimuli close together add up (summation) and above the fusion frequency merge into a smooth tetanus, which is how every normal contraction is made.
Lever systems in the body
Bones are levers, joints are fulcrums and muscles supply the effort. The elbow in a curl is a third-class lever: the biceps pulls about 4.5 cm from the elbow against a weight 35 cm away, so a 5 kg dumbbell needs about 440 N of muscle force, but the hand moves eight times faster than the muscle shortens. Standing on tiptoe is a second-class lever (load between the ball of the foot and the calf's pull) and nodding the head a first-class lever. The Movement tab works out the force for each.
Experiments and course links
| Lab | What you find out | Course links |
|---|---|---|
| Name the muscles | Locate the major muscles of the body | GCSE PE; IGCSE; A&P |
| Antagonistic pairs | Agonist, antagonist and the type of contraction | GCSE PE; IGCSE; IB B3.3 |
| Summation and tetanus | How twitches add up as the frequency rises | A-level 3.6.3; A&P |
| Length–tension | Force against sarcomere length and filament overlap | A-level; IB B3.3 (HL); A&P |
| Slow vs fast fibres | Fatigue and the size principle | A-level 3.6.3; GCSE PE |
| Biceps curl forces | Third-class levers and mechanical advantage | GCSE PE; A&P; physics |
Who uses this simulator?
Middle and high school students meeting the muscular system (NGSS, IGCSE, NCERT), GCSE and A-level PE students learning the major muscles, antagonistic pairs, contraction types, levers and fibre types, A-level and IB biology students studying the sliding filament theory and the neuromuscular junction, first-year anatomy and physiology, nursing, physiotherapy and sports science students, and teachers who want a free, no-login 3D model for a classroom screen or homework.
Frequently asked questions
What are the three types of muscle tissue?
Skeletal muscle is attached to bones by tendons and moves the body; its long cylindrical fibres have many nuclei and stripes (striations), and it is under voluntary control. Cardiac muscle is found only in the heart; its short branched cells are striated, joined by intercalated discs and contract on their own rhythm without tiring. Smooth muscle lines hollow organs such as the gut, blood vessels and bladder; its spindle-shaped cells have one nucleus, no striations, and contract slowly and involuntarily.
How does the sliding filament theory explain muscle contraction?
A nerve impulse reaches the neuromuscular junction and acetylcholine triggers an action potential in the muscle fibre. It spreads down the T-tubules and the sarcoplasmic reticulum releases calcium ions. Calcium binds to troponin, which moves tropomyosin off the binding sites on actin. Myosin heads bind to actin, forming cross-bridges, and swivel in a power stroke that pulls the actin filaments toward the centre of the sarcomere. ATP binds and detaches each head, and its hydrolysis re-cocks it for the next stroke. The filaments slide past each other without shortening: the I band and H zone narrow while the A band stays the same width.
What is an antagonistic muscle pair?
Muscles can only pull, not push, so they work in pairs across a joint. The agonist (prime mover) contracts to make the movement while the antagonist relaxes and lengthens; to move back, the roles swap. At the elbow the biceps flexes and the triceps extends; at the knee the hamstrings flex and the quadriceps extend; at the ankle the gastrocnemius plantarflexes and the tibialis anterior dorsiflexes. Synergists help the agonist and fixators hold the bone the agonist pulls from steady.
What is the difference between isotonic and isometric contraction?
In an isotonic contraction the muscle changes length and moves a joint. It is concentric when the muscle shortens under load, as the biceps does when you lift a dumbbell, and eccentric when it lengthens under load, as the biceps does when you lower it slowly. In an isometric contraction the muscle produces force without changing length, so nothing moves, as when you hold the dumbbell still or hold a plank.
Is this 3D muscle model free, and does it need a login?
Yes, it is free. There is no account, sign-up, download or plug-in: it runs in any modern browser on a Chromebook, laptop, tablet or phone, and nothing a student does is sent anywhere. Teachers can share a link or a classroom QR code from the share bar.
Explore related simulators
Every muscle here pulls on a bone you can explore in the Interactive 3D Skeleton. The motor neurons that fire them, the reflex arc and the synapse are in the CNS and Neuron Simulator. Cardiac muscle at work drives the Heart and Cardiac Cycle simulator, the diaphragm and intercostals breathe in the Respiratory System Simulator, and the glycogen your muscles burn is regulated in the Blood Glucose Regulation Simulator.
Sources: OpenStax Anatomy and Physiology 2e, chapters 10–11; OpenStax Biology 2e, chapter 38.4; Gray's Anatomy, 42nd edition; Gordon, Huxley & Julian, J. Physiol. 184 (1966); Hill, Proc. R. Soc. B 126 (1938); Henneman, Science 126 (1957); Burke et al., J. Physiol. 234 (1973); Johnson et al., J. Neurol. Sci. 18 (1973); Gastin, Sports Med. 31 (2001). The muscles and bones are BodyParts3D, © The Database Center for Life Science, licensed under CC BY 4.0; latissimus dorsi, rectus abdominis, the inner abdominal muscles, the jaw and face muscles are modelled here on landmarks of those bones. An educational model: not medical or training advice.
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