Sarcomere Length-Tension Simulator

Sarcomere Geometry

Sarcomere length 2.20 μm

Ca²⁺ activation 100%

Live Readouts

Overlap force

100%

Active tension

100%

Overlap zone (each half)

0.70 μm

Plateau

How it works

A sarcomere is bounded by two Z-discs, anchoring thin (actin) filaments that point inward; thick (myosin) filaments sit centered on the M-line. Force-generating cross-bridges can only form where a thick filament's myosin heads overlap a thin filament. Stretching or compressing the sarcomere slides the Z-discs apart or together without changing filament length — it only changes how much they overlap.

This gives the classic Gordon–Huxley–Julian length-tension relationship, piecewise in sarcomere length (SL):

SL ≤ 1.27 μm   → F = 0        (thick filaments jam against Z-discs)
1.27–1.67 μm   → F rises      (opposing thin filaments overlap & interfere)
1.67–2.00 μm   → F keeps rising toward maximum overlap
2.00–2.25 μm   → F = 100%     (plateau: full myosin-head engagement)
2.25–3.65 μm   → F falls linearly to 0 (filaments pull apart)
SL ≥ 3.65 μm   → F = 0        (no thick/thin overlap left)

Overlap alone does not guarantee force — the thin filament's myosin-binding sites are gated by tropomyosin, which Ca²⁺ (via troponin) must move out of the way. Active tension is therefore approximated as:

Active tension ≈ F(overlap) × Ca²⁺ activation fraction

Sarcomere length slider — slides the Z-discs, changing filament overlap and re-plotting your position on the curve below.
Ca²⁺ activation slider — scales how many myosin heads are cycling at any instant, independent of geometry.
Cross-bridge cycling — heads within the overlap zone attach, swing through a power stroke, and detach in a repeating ATP-driven cycle (shown schematically, not to exact timescale).

Thin filament (actin) Thick filament (myosin) Myosin head (cross-bridge)

⚠ Couldn't load the 3D engineThree.js failed to load from the CDN. Check your connection and reload.