What it does
A skeletal muscle fibre is packed with repeating units called sarcomeres, each a couple of micrometres long. Each sarcomere has thin actin filaments anchored to a Z line at each end and thick myosin filaments in the middle. When a muscle contracts, nothing in it gets shorter: the thin filaments are pulled along the thick ones towards the middle, so the filaments overlap more. This is the sliding filament theory.
This page draws one sarcomere to scale. Drag the length slider, or press Contract and Relax, and watch the bands change. The A band (the length of the thick filaments) never changes; the I band (thin filaments only) and the H zone (thick filaments only) shrink as the muscle shortens. Below, a second picture walks through the cross-bridge cycle: calcium unlocking the binding sites, the myosin head binding, the power stroke, and ATP making the head let go and re-cock.
It fits Class 11 Biology (locomotion and movement: the structure of contractile proteins and the mechanism of muscle contraction). See how the nerve signal that starts it all travels in nerve impulse lab, and how ions cross membranes in fluid mosaic membrane.
How to use it
- Slide Sarcomere length between 1.8 and 3.6 µm, or press Contract (it shortens to 2.0 µm) and Relax (back to 2.6 µm).
- Read the brackets under the picture and the numbers: A band, I band, H zone, the overlap of thin and thick filaments in each half, and the change in length.
- The big line says how many of the myosin heads can reach actin. With no overlap, none can, and the muscle cannot pull.
- In The cross-bridge cycle, press Next step and Back. Tick No ATP left to see why the heads stay stuck without ATP.
Worked example
Start at 2.6 µm. The A band is 1.60 µm. The I band is the rest: 2.6 − 1.6 = 1.00 µm (half at each end). The thin filaments reach 1.0 µm in from each Z line, so the middle part with no thin filament, the H zone, is 2.6 − 2 × 1.0 = 0.60 µm. Each half has 0.50 µm of overlap, and about 71 % of the myosin heads can reach actin.
Press Contract and let it reach 2.0 µm. The A band is still 1.60 µm. The I band is 0.40 µm and the H zone is 0: the thin filaments now meet in the middle. The sarcomere is 23.1 % shorter than at the start. Every one of the thousands of sarcomeres in a row does this at once, which is why the whole muscle shortens by a useful amount.
Tips
- Remember the rule: the A band stays the same; the I band and H zone get shorter; the Z lines come closer.
- Calcium is the switch, ATP is the fuel. Calcium uncovers the binding sites; ATP lets the head go and re-cock it.
- If ATP runs out, the heads cannot let go and the muscle locks: rigor mortis after death is this.
FAQ
Why does the A band not change? It is defined by the thick filaments, and they do not change length. Only how far the thin filaments slide over them changes.
What happens if a sarcomere is stretched too far? At about 3.6 µm in this model the thin and thick filaments no longer overlap, no cross-bridges can form, and the sarcomere cannot produce force.
Is the cycle really this slow? No. Each head cycles many times a second, and millions of heads work at once, out of step, so the pull is smooth.
Where the numbers come from
- Sliding filament theory, the sarcomere bands (A band, I band, H zone, Z and M lines) and the cross-bridge cycle with Ca²⁺, troponin, tropomyosin and ATP: general physiology.
- Lengths (thick filament 1.6 µm with a 0.2 µm bare zone, thin filament 1.0 µm) are rounded general values for vertebrate skeletal muscle, used to draw the model to scale; real values vary a little between muscles and species.
- Checked in the tests against hand geometry: at 2.6 µm the I band is 1.0 µm, the H zone 0.6 µm and the overlap 0.5 µm per half; the H zone vanishes at 2.0 µm and overlap ends at 3.6 µm. Teaching model only. Nothing you enter is stored or sent anywhere.
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Last updated 2026-11-09.