(A) The traditional view of myofilament regulation is that binding of Ca²⁺ to troponin C (TnC) induces shifting of tropomyosin (Tm) to expose myosin binding sites on the actin filament. This implies a static relationship between the Ca²⁺ transient and twitch tension, with Ca²⁺ signaling as the primary determinant of twitch dynamics. (B) Our new evidence shows that posttranslational modification of thick filament proteins (e.g., Mlc2v phosphorylation) alters Ca²⁺ sensitivity of the filaments, highlighting a previously unappreciated adaptable relationship. Mlc2v phosphorylation simultaneously increases myosin binding and myosin lever arm stiffness, altering kinetics of crossbridge cycling (shown by dashed blue boxes) to increase crossbridge duty ratio. This phosphorylation-dependent behavior of myosin can positively cooperative to influence calcium-dependent activation and kinetics of the thin filament by allowing crossbridges to cooperatively activate neighboring binding sites on actin (dashed blue arrow). (C) Mlc2v phosphorylation can regulate ventricular torsion because Mlc2v phosphorylation levels vary through the LV wall. Higher phosphorylation in the left-handed helical fibers of WT epicardium enhances their twitch tension and consequently peak torsion compared with DM mice. Because elevated Mlc2v phosphorylation also lengthens twitch duration, diastolic untwisting in WT mice is dominated by epicardial fibers. Without opposition by right-handed endocardial fibers, the untwisting rate is increased relative to DM. Meeting hemodynamic demand without the benefit of epicardial Mlc2v hyperphosphorylation elevates endocardial workload in DM mice, contributing to DCM and heart failure (bottom).