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Malti💪 Muscle Contraction: Sliding Filament Theory
Interactive 3D sarcomere simulation of the sliding filament theory. Watch myosin cross-bridge heads cycle through the real ATP-driven power stroke and explore the force-length curve, calcium-troponin regulation and rigor mortis.
About the Sliding Filament Simulator
This simulation renders a single sarcomere in true 3D — myosin thick filaments, actin thin filaments, their myosin cross-bridge heads, and the Z-discs that bound the sarcomere. Each head independently cycles through the four real stages of cross-bridge chemistry: it attaches to actin and performs the power stroke, releases ADP and inorganic phosphate, detaches when a fresh ATP molecule binds, and is re-cocked to its high-energy position by ATP hydrolysis before attaching again. Crucially, the thick and thin filaments themselves never change length in this model — only the spacing between the Z-discs (the sarcomere length) shrinks, because the filaments slide past one another rather than contracting.
The sarcomere length slider sets Z-disc spacing directly and therefore how much actin and myosin overlap; force output follows the real, non-monotonic Gordon–Huxley–Julian force–length relationship, shown live on the accompanying plot. The ATP slider controls how many cross-bridges can complete their cycle — at 0% ATP, heads that are bound stay bound permanently, the physiological basis of rigor mortis. The calcium toggle switches troponin-tropomyosin regulation on or off: without Ca²⁺, tropomyosin physically blocks the myosin-binding sites on actin and no cross-bridges can form at all.
Frequently Asked Questions
What is the sliding filament theory?
It is the accepted model of how skeletal muscle shortens: thick (myosin) and thin (actin) filaments do not themselves contract. Instead, myosin cross-bridge heads pull the thin filaments past the thick filaments, sliding them deeper into the sarcomere and shortening the distance between Z-discs without changing either filament's own length.
What are the four stages of the cross-bridge cycle shown here?
Attachment and power stroke (the myosin head binds actin and swings, pulling the thin filament), release of ADP and phosphate at the end of the stroke, detachment when a new ATP molecule binds the head, and re-cocking of the head to its high-energy position as that ATP is hydrolysed — after which it can attach again.
Why doesn't ATP make the muscle contract directly?
ATP's main job in this cycle is to detach the myosin head from actin and re-cock it, not to power the stroke itself. The power stroke is driven by the conformational change of a myosin head that already hydrolysed ATP earlier in the cycle. This is why zero ATP does not relax muscle — it freezes it.
What is rigor mortis, physiologically?
After death, cellular ATP production stops. Myosin heads that are bound to actin can no longer detach, because detachment requires a fresh ATP molecule to bind the head. The result is that essentially all cross-bridges lock in the bound state, producing the widespread muscle stiffness known as rigor mortis. Set ATP to 0% in the simulator to see the heads freeze mid-cycle.
Why does force depend on sarcomere length?
Force output depends on how many myosin heads can physically reach an actin-binding site, which depends on the degree of filament overlap. Too little overlap (sarcomere stretched) leaves few heads within reach; too much overlap (sarcomere very short) causes thin filaments from opposite ends to interfere with each other. Force peaks at an intermediate, optimal overlap.
What is the ascending limb, plateau and descending limb?
These are the three regions of the classic force–length curve first mapped by Gordon, Huxley and Julian in 1966. On the ascending limb (short sarcomeres), force rises as overlap increases. On the plateau, overlap is optimal and force is maximal. On the descending limb (long sarcomeres), force falls as filaments pull apart and fewer cross-bridges can form.
What does the calcium toggle actually control?
Calcium released from the sarcoplasmic reticulum binds troponin, which shifts tropomyosin away from the myosin-binding sites that otherwise wrap around and cover the actin filament. With calcium off, those sites stay covered and no cross-bridge can form, so no force is produced regardless of how much filament overlap exists.
Is this a cartoon or a physiologically accurate model?
It models genuine skeletal muscle physiology: fixed filament lengths, a real four-stage cross-bridge cycle, calcium-troponin-tropomyosin regulation, and the textbook non-monotonic force–length relationship. It is simplified for clarity — for example cross-bridges are shown at a schematic density and don't model individual myosin kinetics stochastically — but every mechanism depicted is real, established muscle physiology.
Interactive 3D sarcomere simulation of the sliding filament theory — watch myosin cross-bridges cycle through the real ATP-driven power stroke and explore the force-length relationship, calcium regulation and rigor mortis.
2D · HTML5 Canvas 2D · 60 FPS target · runs fully client-side, no install