The contribution of genetic influences to measures of lower-extremity function in older male twins

Tests of balance, gait and endurance were administered to 95 monozygotic: ( MZ) and 92 dizygotic (DZ), white male twins aged 68 to 79 years who had bee n born in the United States. Within-twin-pair correlations were calculated for each individual task and for an overall summary performance score. Thes e were subjected to structural equation modeling to determine the contribut ions of genetic and environmental influences to individual differences in p erformance scores. MZ intraclass correlations were significant and greater than DZ correlations for the g-foot walk and the repeated chair stands task , but not for the standing balance task. The heritability of the lower-extr emity summary score was 57%, of which 39% was due to additive genetic effec ts and 18% due to nonadditive effects. In addition, we found that genetic i nfluences contributed primarily to twin similarity in the poorest quartile of performance, whereas shared environmental influences contributed to twin similarity in the best quartile.