How Muscles Contract (Sliding Filament)
A muscle shortens because thousands of tiny protein filaments slide past one another. Inside each sarcomere, a nerve signal dumps calcium (Ca²⁺) out of the sarcoplasmic reticulum; the calcium shifts tropomyosin aside so myosin heads can grab the thin actin filaments and — burning ATP — ratchet them inward, dragging the two Z-discs closer and narrowing the H-zone and I-band. Press ⚡ Stimulate (or hold Tetanus) and watch the Ca²⁺ and ATP meters, the sarcomere length in micrometres, and the live tension curve respond — keep it firing and the muscle fatigues as ATP runs low.
What's happening
The Science in Plain Language
1. The building block is the sarcomere. A muscle fiber is packed with repeating units called sarcomeres, stacked end to end. Each one is bounded by two Z-discs. Thin actin filaments anchor to the Z-discs and reach inward; a thick myosin filament sits in the middle, studded with hundreds of little heads. When a sarcomere shortens, the whole muscle shortens.
2. The filaments don't shrink — they slide. This is the key insight of the sliding-filament theory. The actin and myosin filaments stay the same length. The myosin heads simply grab actin and pull it inward, so the filaments overlap more and the Z-discs are dragged closer together. Nothing gets shorter except the gaps.
3. Calcium is the “go” signal. At rest, a protein called tropomyosin lies across actin like a lid, covering the spots where myosin would attach. When a nerve fires, the sarcoplasmic reticulum dumps calcium (Ca²⁺) into the muscle — cytosolic calcium leaps from about 0.1 µM at rest to roughly 1–10 µM. Calcium binds troponin, which yanks the tropomyosin lid aside and exposes the binding sites. No calcium, no contraction — the sites stay covered.
4. The cross-bridge cycle does the pulling. Once a site is open, a myosin head (pre-loaded with ADP + Pₓ) attaches to actin, forming a cross-bridge. It releases first Pₓ, then ADP, and pivots — the power stroke — ratcheting the actin filament a few nanometres toward the M-line. A fresh molecule of ATP then binds the head, which makes it let go. The head splits (hydrolyzes) that ATP back to ADP + Pₓ, which re-cocks it, ready to grab further along and stroke again. Thousands of heads do this out of sync, so the pull is smooth and continuous, like a tug-of-war team whose hands are always re-gripping.
5. Why you need BOTH calcium and ATP. They do different jobs. Calcium unlocks the door (exposes the binding sites). ATP powers the machine — and it's needed twice per cycle: once to release the head from actin, and once (via hydrolysis) to re-cock it for the next stroke. A muscle with calcium but no ATP can attach but can never let go; a muscle with ATP but no calcium can never attach in the first place.
6. Relaxing is an active, energy-using job too. To relax, the calcium must be pumped back into the sarcoplasmic reticulum by ATP-driven SERCA pumps. Once calcium falls, tropomyosin slides back over the sites, the heads can no longer attach, and the elastic filaments spring back — the sarcomere lengthens again.
7. Rigor mortis: the machine jammed. After death, cells run out of ATP. Remember, ATP is what lets a myosin head release from actin. With no ATP, every attached head stays locked onto actin and can't detach, so the muscles stiffen into the rigid state called rigor mortis. It gradually fades only as the proteins themselves break down. This is the clearest everyday proof that ATP is required for muscles to let go, not just to pull.
8. What the bands tell you. Under a microscope the sarcomere is striped. The A-band is the full length of the thick (myosin) filament — it never changes, because myosin doesn't shorten. The I-band is the actin-only zone next to each Z-disc, and the H-zone is the myosin-only patch in the centre (around the M-line). When the muscle contracts, actin slides inward, so the I-band and H-zone both narrow while the A-band stays the same. Watch the labelled brackets in the animation shrink as the caliper reading drops from about 2.5 µm toward 1.9 µm.
9. Twitch, tetanus, and fatigue. A single stimulus produces one brief twitch — tension rises and falls as calcium is released and then pumped back. Fire stimuli rapidly enough and the twitches fuse into a smooth, sustained tetanus at maximum force. But holding tetanus is expensive: the cross-bridges and the calcium pumps together burn through the cell's ATP (buffered by phosphocreatine). As that reserve falls, cross-bridges cycle sluggishly and force fades — that is muscle fatigue. Rest, and aerobic metabolism plus phosphocreatine rebuild the reserve. Try it: hold Tetanus and watch the ATP meter drain and the tension curve sag, then release and watch it recover.