cTnI-ND increases the response of myofilament Ca ²⁺ sensitivity to passive tension other than SL. The uniform cross-sectional area of cryosection-derived mouse papillary muscle strips empowers a reliable measurement of resting tension and compare between cTnI-ND,Tnni3^−/− and WT cardiac muscles with precisely controlled SL. (A) Plotting pCa90 as a function of SL demonstrated a classic length dependent increase of Ca²⁺ sensitivity, in which cTnI-ND,Tnni3^−/− muscle showed a greater response. (B) Plotting resting tension against SL found no difference between the two groups. (C) However, plotting pCa90 against resting tension reproduced the higher response of cTnI-ND,Tnni3^−/− muscle than that of WT control. Values are mean ± SEM. Each dot represents one muscle strip. n = 5 mice with six strips for WT and 4 mice with four strips for cTnI-ND,Tnni3^−/− groups. *P < 0.05 vs. WT in Student’s t test.