The purpose of this study was to examine changes in the contribution of genetic and environmental effects to isometric knee extensor strength and leg extensor power among 63- to 76-year-old female twins over a 3-yr follow-up. At baseline in 2000 the sample comprised 206 monozygotic (MZ) and 228 dizygotic (DZ) twin individuals, and at follow-up in 2003 the sample comprised 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. Nonshared 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 nonshared environmental effects explained the remaining variance, 29% (95% CI: 22-37%) of muscle strength at follow-up. For muscle power, the same genetic effects accounted for 67% (95% CI: 57-74%) of the variation at baseline and 48% (95% CI: 34-61%) at follow-up. Nonshared 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. The remaining variance of muscle power at follow-up was accounted for by time-specific environmental effects. Results indicated that the 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.