Single chemically permeabilized gastrocnemius fibers from six elite endurance-trained master runners (RUN group) and five age-matched sedentary controls (SED group) were mounted between a force transducer and a position motor, studied under conditions of maximal and submaximal Ca2+ activation, and subsequently electrophoresed on 5% polyacrylamide gels to determine myosin heavy chain (MHC) composition. For the SED group, peak isometric tension (Pzero) averaged 143 +/- 3, 156 +/- 4, and 170 +/- 4 kN/m2 and maximal shortening velocity (Vzero) averaged 0.43 +/- 0.01, 1.90 +/- 0.08, and 5.59 +/- 0.40 fiber lengths/s for fibers expressing type I, IIa, and IIx MHC, respectively (all comparisons, P < 0.05). Hill plot analysis of relative forces during submaximal Ca2+ activation indicated no SED vs. RUN differences in Ca2+ sensitivity or in the cooperativity of Ca2+ activation. However, at maximal Ca2+ activation, RUN type I and IIa fibers produced 15% less peak absolute force than SED fibers (P < 0.05). This reduction in fiber force was a direct result of the smaller diameter of the RUN fibers (P < 0.05), because Pzero, peak elastic modulus (Ezero), and Pzero/Ezero were not different between SED and RUN groups. RUN type I fibers also displayed a mean Vzero that was 19% higher than the average Vzero of the SED type I fibers (P < 0.05). In separate experiments, quantification of relative myosin light chain (MLC) isoform content revealed a 28% greater ratio of MLC3 to MLC2 in single type I fibers from the RUN group (P < 0.05), suggesting that the elevated Vzero of the RUN type I fibers was related to a greater expression of MLC3. In conclusion, the single fibers from the elite master runners displayed specific morphological and contractile properties that may enhance the performance of these athletes during prolonged muscular activity.