What muscles do
Muscle tissue has one special ability: it can contract, getting shorter and pulling on whatever it is attached to. The human body has more than 600 skeletal muscles, which make up about 40% of body mass. They move the skeleton, hold your posture, keep joints stable and produce heat.
Three types of muscle
| Skeletal | Cardiac | Smooth | |
|---|---|---|---|
| Where | Attached to bones | Wall of the heart | Walls of the gut, blood vessels, bladder, airways, uterus |
| Control | Voluntary | Involuntary | Involuntary |
| Appearance | Striated; long cells with many nuclei | Striated; branched cells, one nucleus, intercalated discs | Not striated; spindle-shaped cells, one nucleus |
| Speed and fatigue | Fast, powerful, tires quickly | Rhythmic, does not tire | Slow, sustained |
The 3D, fibre and lever views focus on skeletal muscle; the last view compares all three.
Major skeletal muscles
Each muscle starts at an origin on a bone that stays fairly still and ends at aninsertion on the bone it moves. The biceps bends the elbow and thetriceps straightens it. The deltoid lifts the arm sideways, thepectoralis major pulls it across the chest and the latissimus dorsi pulls it down and back. In the leg, the quadriceps straightens the knee, the hamstrings bend it, the gluteus maximus straightens the hip and the gastrocnemius (calf) lifts the heel. The rectus abdominis and obliques bend and twist the trunk.
From muscle to sarcomere
A muscle is made of bundles called fascicles, each containing many muscle fibres. A fibre is a single, very long cell with many nuclei. Inside it are hundreds of myofibrils, rods made of repeating units called sarcomeres, each about 2.5 µm long. A sarcomere runs from oneZ-disc to the next. Thin filaments of actin are anchored to the Z-discs, and thick filaments of myosin sit in the middle, overlapping them. The regular overlap of dark and light bands is what makes skeletal muscle look striped.
How a fibre contracts
When a nerve impulse reaches a fibre, it releases calcium ions from stores inside the cell. Calcium binds to troponin, which moves the protein tropomyosin away from binding sites on actin. Now thecross-bridge cycle can run:
- An energised myosin head attaches to actin, forming a cross-bridge.
- The head tilts in the power stroke, pulling the actin filament towards the centre of the sarcomere.
- A new ATP molecule binds and the head lets go.
- Splitting the ATP re-cocks the head, ready to attach again further along.
Millions of heads repeat this cycle, so actin slides past myosin and every sarcomere shortens. The filaments do not shrink; they overlap more. This is the sliding filament model. When nerve impulses stop, calcium is pumped back into its stores (which also uses ATP), tropomyosin covers the binding sites again and the fibre relaxes.
Controlling force
A single fibre contracts fully or not at all. Each motor neuron controls a group of fibres called a motor unit, and the nervous system grades force in two ways. It recruits more motor units for a bigger effort, and it sends impulses faster. One impulse gives a brief twitch; if the next arrives before the fibre has relaxed, the twitches add together (summation). At high rates they fuse into a smooth, maximal contraction called tetanus.
Muscles, bones and joints
Muscles can only pull, so they are arranged in antagonistic pairs: while one contracts, the other relaxes and is stretched. Muscles are joined to bones by tendons, and bones are held together at joints by ligaments. Together they form levers. The elbow is a hinge joint acting as the fulcrum; the biceps pulls on the radius just 4 cm away, while the hand is about 33 cm away. Because moments must balance, the biceps must pull with several times the force of the load. In return, a small shortening of the muscle moves the hand a long way, quickly.
Muscles and health
Regular exercise makes muscle fibres thicker and improves their blood supply, while disuse makes them waste away. Muscle fatigue happens when fibres cannot keep up with demand during intense activity. After death, ATP runs out, myosin heads cannot detach and the body stiffens: this isrigor mortis.
Key terms
- Sarcomere
- The repeating contractile unit of a myofibril, from one Z-disc to the next.
- Cross-bridge
- The link formed when a myosin head binds to actin.
- Motor unit
- One motor neuron and all the muscle fibres it controls.
- Tetanus
- A smooth, sustained contraction caused by rapid repeated stimulation.
- Antagonistic pair
- Two muscles that move a joint in opposite directions.
Common misconceptions
- “Muscles push bones.” Muscles can only pull; movement in the opposite direction needs another muscle.
- “The filaments shorten when a muscle contracts.” Actin and myosin keep their length and slide past each other.
- “Relaxing needs no energy.” ATP is needed for myosin to let go and to pump calcium away.
Educational model only. This simulator uses simplified shapes and a simple contraction model with illustrative values. It is not a medical or training tool and must not be used to diagnose or treat muscle injuries or conditions, or to plan exercise or rehabilitation. Always seek advice from a qualified healthcare professional. See our educational and medical disclaimer.
Key takeaways
- There are three types of muscle: skeletal (voluntary), cardiac and smooth (both involuntary).
- Muscle → fascicle → fibre → myofibril → sarcomere, built of actin and myosin filaments.
- Calcium triggers contraction; ATP powers the cross-bridge cycle and relaxation.
- Force is graded by recruiting motor units and by faster stimulation (summation and tetanus).
- Muscles pull on bones through tendons and work in antagonistic pairs across joints that act as levers.
Model note: the 3D figure is built from simple shapes and shows muscle positions approximately. Fibre timings are based on a fast-twitch human fibre and shown five times slower than real time. Lever forces use typical adult forearm distances and ignore other elbow flexors.