Experimental protocol for simultaneous determination of Ca²⁺-activated force and the rate constant of force redevelopment (k_tr) in skinned myocardium. The bottom panel depicts the changes in force recorded before, during, and after a step change in length (top panel) of skinned myocardium. Once isometric force reached steady state in Ca²⁺-activating solution (e.g., pCa 4.5), muscle length was rapidly slackened by 20%. After ∼15 ms of unloaded shortening, the preparation was restretched to its original length. Ca²⁺-activated force at pCa 4.5 (P₀) was determined by subtracting the resting force measured at pCa 9.0 from the total force generated at pCa 4.5. k_tr was estimated by linear transformation of the half-time of force redevelopment after a rapid release–restretch maneuver.