Representative stretch activation response following a rapid 2% stretch in ML. Highlighted are the important phases of the force response and the associated dynamic XB parameters in a WT myocardial preparation following a 2% stretch in ML (see Materials and methods for details). Phase 1 represents the immediate increase in force when a sudden stretch in ML is applied and represents the number of strongly bound XBs at the instance of the imposed stretch. P1 is the magnitude of the immediate force response and is measured from the prestretch isometric steady-state force (set to zero before the stretch) to the peak of phase 1. This is followed by phase 2, which represents a rapid decline in force development with a dynamic rate constant k_rel and is an index of the rate of XB detachment. The minimum point of phase 2 is represented by P2, which indicates the magnitude of XB detachment. Phase 3 denotes the delayed force development with a dynamic rate constant k_df and is an index of the rate of XB recruitment. P_df is the difference between P2 and P3 and represents the magnitude of XB recruitment in response to the imposed stretch.