cTnI-ND increases the SL-dependent increase of myofilament Ca ²⁺ sensitivity. (A) Cryosection-derived detergent skinned LV papillary muscle strips mounted between force transducer and length lever arm provide uniform thin preparations with clear striations under light microscope for an accurate determination of SL in contractility studies. (B) Isometric force–pCa curves of skinned papillary muscle sections normalized to maximum force at pCa 4.5 showed left shifts in WT and cTnI-ND,Tnni3^−/− groups when SL was increased from 2.0 to 2.15 and to 2.3 μm. cTnI-ND,Tnni3^−/− muscle showed higher increases in Ca²⁺ sensitivity than WT control at SL of 2.15 and 2.3 μm. The effect of cTnI-ND is larger at higher [Ca²⁺], resulting in a trend of increase in the cooperativity of myofilament activation (the inset table). (C and D) Resting tension at pCa 9 (C) and maximum active force developed at pCa 4.5 (D) did not show significant difference between cTnI-ND,Tnni3^−/− and WT groups (the trends of lower passive tension and higher active tension in cTnI-ND,Tnni3^−/− muscle strips at longer SL did not reach statistical significance). (E) The pCa50 values demonstrate the anticipated higher Ca²⁺ sensitivity at longer SL in both cTnI-ND,Tnni3^−/− and WT muscle strips, in which cTnI-ND,Tnni3^−/− group showed a trend of greater response. (F) The differences became statistically significant when pCa90 was compared, indicating a mechanism that is more effective on activated myofilaments. Values are mean ± SEM. n = 5 strips from three mice each in cTnI-ND,Tnni3^−/− and WT groups. *P < 0.05, **P < 0.01, ***P < 0.001 vs. WT in paired Student’s t test, and ^#P < 0.05 vs. SL at 2.0 or 2.15 μm in two-way ANOVA with post-hoc method of Tukey’s test.