Lever arm in its power strokeActin (thin)Ca²⁺ on troponinATP / ADP·Pi on a free headOne sarcomere of frog skeletal muscle, cut to a single plane of its filament lattice. Ruler: 1 tick = 100 nm; lengths along the filaments are true (thick 1.6 µm, thin 0.95 µm), widths, lattice spacing and myosin heads are drawn 5× larger. Ca²⁺ from the pipette binds troponin (green dots); tropomyosin (the grey strand) rolls aside, and myosin heads that face an open site bind, swing their lever arm toward the M line (lit), release ADP, take a new ATP, let go and re-cock. Each stroke pulls the thin filament ~5–10 nm; asynchronous heads make the slide smooth and both Z discs move in against their load springs. Remove Ca²⁺ and the springs pull the sarcomere back out to 2.5 µm. Only one head pair per 43 nm repeat is drawn. Shown about 20× slower.
2.0–2.25 µm全重叠的肌节长度（蛙）sarcomere length at full overlap (frog)1.6 / 0.95 µm粗肌丝 / 细肌丝长度thick / thin filament length~5–10 nm每个横桥一次做功冲程working stroke per cross-bridge~2–20 /s每个肌球蛋白头的循环（等长→快速缩短）cycles per myosin head (isometric → fast shortening)pCa ~5.8肌钙蛋白半激活（约 1.6 µM Ca²⁺）half-activation of troponin (~1.6 µM Ca²⁺)