The effect of aging on the in vitro contractile properties of the patagialis (PAT) muscle of 35 young adult (YA; 8 weeks of age) and 35 aged adult (AA, 110 weeks of age) Coturnix quails was examined after 0-30 days of stretch-overload. Overload was achieved by placing a weight equivalent to 12% of the birds' body weight on one wing. The contralateral wing served as the intra-animal control. Overload increased the weight of the PAT by 45.1+/-2.1% in YA, and 24.1+/-2.6% in AA. Twitch contraction time increased with loading from 43.2+/-1.2 to 67.3+/-2.2 ms in YA birds and 57.2+/-1.7 to 77.4+/-1.9 ms in AA birds. Unloaded shortening velocity (Vo) decreased by 40.1+/-2.2 and 38.8+/-3.2% in YA and aged birds, respectively. The decrease in fast myosin expression was greater in overloaded muscles of YA (20%) as compared to AA birds (12%). However, this was accompanied by a greater decrease in total muscle ATPase activity in aged birds (61%) compared to YA birds (40%). Myosin isozyme Ca(2+)-ATPase activity was 26% lower in FM1 but not other fast myosins in YA birds, but it was approximately 30% lower in all fast myosins in PAT muscles of aged birds. These data show that the reduction of Vo and the increase in twitch duration with aging may be due in part to reductions in ATPase activity in all myosin isoforms, as compared to myosin isoforms isolated from YA birds.