Contractile force and decay during repetitive stimulation in the presence or absence of Ca²⁺ influx. (A) Representative superimposed specific force traces elicited during the 1st (left), 20th (middle), and 40th (right) stimulus train in EDL muscles from CASQ1-null mice conducted in either standard Ringer’s solution containing 2.5 mM Ca²⁺ (black), nominally Ca²⁺-free Ringer’s (blue), or standard Ringer’s solution supplemented with 10 µM BTP-2 (green). (B and C) Time course analyses of specific force (B) and end/peak ratio (C) under the three different experimental conditions. Data are shown as mean ± SEM; *, P < 0.05. Numbers in parentheses (n) indicate the number of EDL muscles analyzed. Number of mice: n = 6 CASQ1-null (2.5 mM Ca²⁺); n = 6 CASQ1-null (Ca²⁺-free); n = 7 CASQ1-null (2.5 mM Ca²⁺ + BTP-2).