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SIMONEN, R. L., L. PÉRUSSE, T. RANKINEN, T. RICE, D. C. RAO, and C. BOUCHARD. Familial aggregation of physical activity levels in the Québec family study. Med. Sci. Sports Exerc., Vol. 34, No. 7, pp. 1137–1142, 2002.Familial aggregation of physical activity phenotypes was investigated in 696 subjects from 200 families of the Quebec Family Study. The mean age of offspring and parents was 27 and 53 yr, respectively.The levels of physical activity were estimated using a 3-d diary and a questionnaire dealing with physical activity during the past year.An ANOVA performed on the age and sex adjusted physical activity phenotypes revealed that there were 1.40–1.52 times more variation in physical activity levels between families than within families (0.001 <P < 0.0001), suggesting that physical activity levels aggregate in families. Maximal heritabilities (SEGPATH), adjusted for the degree of spouse resemblance, reached 25%, 16%, 19%, and 17% for the degree of inactivity, time spent in moderate to strenuous physical activities, total level of daily activity, and weekly time spent in the main activity during the previous year, respectively.These results suggest that physical activity level is characterized by a significant degree of familial resemblance, and that inactivity has a slightly higher heritability level than moderate to strenuous physical activity or total physical activity phenotypes. The pattern of familial correlations suggests that shared familial environmental factors along with genetic factors are also important in accounting for the familial resemblance in physical activity level.