SL dependence of tension, loaded shortening, and power output in permeabilized cardiac myocytes from Control, phospho-mimetic cMyBP-C t3SD, phospho-null cMyBP-C t3SA, and cMYBP-C KO mice. (A) Top: A photomicrograph of representative mouse permeabilized cardiac myocyte preparation and a schematic showing the permeabilized myocyte attachment. Bottom: Length and force traces during maximal Ca²⁺ activation at the beginning of the experiment, followed by representative length traces during a series of force clamps at both long and short SLs during half-maximal Ca²⁺ activation, and a final length and force trace during maximal Ca²⁺ activation. (B) Summary data of normalized SL–tension relationships during approximately half-maximal Ca²⁺ activation from the four groups. Data points are mean ± SEM. (C) Representative normalized force-velocity and power-load curves at approximately half-maximal Ca²⁺ activations at long and short SLs from one myocyte preparation from each group.