(A) Bright-field and fluorescent images of a young LAL muscle fiber loaded with fluo-4 and stimulated with single stimulus (twitch) and 2s, 30Hz stimulus train. (B) Representative traces showing kinetics of fluo-4 fluorescent responses to twitches and a 2s, 30Hz stimulus train in young, old and Sod1−/− LAL muscle fibers. The inset is a magnified single fluo-4 twitch response from young LAL. (C) Average fluo-4 fluorescent response amplitudes to twitch (left) and 2s, 30Hz stimulus train (right) in young, old and Sod1−/− LAL muscle fibers. (D) Twitch calcium clearance time constants (time to 50% of the peak) calculated for young, old and Sod1−/− LAL muscle fibers. (E) Average fluo-4 fluorescent responses in muscle fibers induced by application of 2 mM caffeine to young, old and Sod1−/− LAL. Error bars are SEMs.