Fluorescence signals (expressed as F/F0) from Ca2+ transients of young myocytes recorded during a twitch (A), caffeine application in NT solution (B) and caffeine in Na- and Ca-free solution (C). Re-lengthening and shortening (expressed as a percentage of the total cell length) are depicted in the lower panel for twitch (D), contracture under caffeine in NT (E) and contracture under caffeine in Na- and Ca-free solution (F). Fluorescence signals (expressed as F/Fo) for older adult myocytes during a twitch (G), contractures induced by caffeine application in NT solution (H) and in Na- and Ca-free solution (I). Shortening traces (expressed as percentage of total cell length) for a twitch (J), CaffNT (K) and Caff00 (L). Note the larger amplitudes and longer duration in older TG-ssTnI myocytes twitch and contractures compare to older NTG, which are opposite the results obtained from the young group of myocytes.