Contractile force during repetitive stimulation in the presence or absence of extracellular Ca²⁺ influx. (A–C) Time course of peak specific force recorded in EDL muscles from WT and CASQ1-null mice during 40 consecutive stimulus trains in the presence of either standard Ringer’s solution containing 2.5 mM Ca²⁺ (A), nominally Ca²⁺-free Ringer’s solution (B), or standard Ringer’s solution supplemented with 10 µM BTP-2 (C). (D–F) Quantitative analyses of the relative decay during the second stimulus train (ST_2nd/ST_1st) and the peak of the bump phase (ST_peak/ST_2nd) from the corresponding data shown in A–C. Data are shown as mean ± SEM; *, P < 0.05. Numbers in parentheses (n) indicate the number of EDL muscles analyzed. Number of mice: n = 8 WT, n = 7 CASQ1-null, A and D; n = 7 WT, n = 6 CASQ1-null, B and E; n = 7 WT, n = 6 CASQ1-null, C and F.