Figure 3.

Simplified cross-bridge cycle in contraction detailing myosin’s discrete sub-states (A-E) and their correspondence to lever-arm orientation. The rigor cross-bridge binds actin (Ac) in the A-state (M_A), exerts force F_rt, and maintains the PAGFP (green cylinder with arrow) tagged HCRLC (red) in the A-state orientation. ELC is shown in blue. ATP addition relaxes the fiber by binding to myosin and detaching actin. The relaxed cross-bridge assumes the E-state (M_E) with the lever-arm re-primed for a new power stroke. Ca²⁺ addition activates the fiber producing force F_at. In isometric contraction single lever-arms assume one of their intermediates (A-E) depicted here as M_B which is a high force producing intermediate having a characteristic orientation indicated by Φ.