Purpose of this study was to examine changes in contribution of genetic and environmental effects to isometric knee extensor strength and leg extensor power among 63 to76-year-old female twins over a three-year follow-up. At baseline in 2000 the sample comprised 206 monozygotic (MZ) and 228 dizygotic (DZ) twin individuals and at follow-up in 2003 149 MZ and 164 DZ twin individuals. Genetic modeling showed that genetic effects explained 58% (95% CI 46-68%) of the variance in muscle strength at baseline and 56% (95% CI 41-68%) at follow-up with no occasion-specific genetic effect. Non-shared environmental effects accounted for 42% (95% CI 32-54%) of the variation at baseline and 15% (95% CI 7-26%) at follow-up. In addition, new non-shared environmental effects explained the remaining variance, 29% (95% CI 22-37%) of muscle strength at follow-up. For muscle power, same genetic effects accounted for 67% (95% CI 57-74%) of the variation at baseline and 48% (95% CI 34-61%) at follow-up. Non-shared environmental effects in common at both measurement points explained 33% (95% CI 25-43%) of the total variation at baseline and 11% (95% CI 5-21%) at follow-up. Remaining variance of muscle power at follow-up was accounted for by time-specific environmental effects. Results indicated that contribution of genetic effects to isometric muscle strength was stable, whereas for leg extensor power the proportion of genetic effects decreased during the follow-up. We observed new specific environmental effects underlying follow-up muscle strength and power, which effects could be due to the onset of new disease processes or changes in lifestyle.