Muscle contraction is powered by millions of myosin heads independently binding actin, pulling a tiny power stroke, and releasing — repeated fast enough that the aggregate looks like smooth force.
Force ~ n_bound * power_stroke
cycle: bind -> power stroke -> ATP-driven release
- Myosin heads — motor domains on the thick filament that generate force each cross-bridge cycle.
- Actin binding sites — thin-filament sites exposed to myosin once calcium moves tropomyosin aside.
- ATP availability — fuel required to detach myosin from actin and reset the head for another stroke.
- Calcium activation — Ca2+ released from the sarcoplasmic reticulum that exposes actin's binding sites via troponin.
This is the molecular basis of both voluntary movement and rigor mortis: without ATP, myosin heads stay locked onto actin because they can't detach — exactly what exercise physiologists model when studying fatigue.