Table 1. Baseline and follow-up characteristics of 12 twin pairs (9 male and 3 female pairs). Data for continuous variables are mean (SD). Characteristic Inactive co-twins (n=12) Active co-twins (n=12) p value* Baseline in 1975 Age (yrs) 27 (range 18-35) Height†  (m) 1.77 (.09) 1.75 (.11) .33 Weight (kg) 71.0 (17.8) 66.8 (9.8) .36 Body mass index†  (kg·m-2) 22.8 (4.9) 22.1 (2.1) .57 Leisure time MET index‡§ (h·day-1) .1 (.2) 3.1 (2.4) .002 Work-related physical activity (n) .51     Sedentary 3 5     Standing or walking at work 2 3     Light manual work 7 4     Heavy manual work 0 0 Follow-up in 2007 Age (yrs) 59 (range 50-67) Height (m) 1.75 (.10) 1.73 (.10) .32 Weight (kg) 82.3 (20.1) 73.0 (12.8) .09 Body mass index (kg·m-2) 26.7 (3.9) 24.2 (2.3) .08 Body surface area (m2) 2.0 (.3) 1.9 (.2) .08 Leisure time MET index‡ (h·day-1) 1.2 (1.1) 9.4 (3.8) < .001 VO2peak (ml·kg-1·min-1) 28.1 (3.7) 34.4 (4.9) .003 Systolic blood pressure (mmHg) 145.0 (27.2) 141.6 (19.0) .66 Diastolic blood pressure (mmHg) 90.3 (14.4) 86.3 (9.7) .39 Use of beta-blockers (yes; n) 2 1 .32 Smoking status (n) .39     Current smokers 2 0     Former smokers 6 6     Never smoked 4 6 Work-related physical activity (n) .26     Sedentary 3 5     Standing or walking at work 1 2     Light manual work 4 0     Heavy manual work 0 1 n = 11. See the details of definition and calculation in Methods section. Includes both leisure time physical activity and physical activity to and from work. For the difference between the inactive and active co-twins, based on paired samples Student’s t-test for normally distributed continuous variables, Wilcoxon signed ranks test for non-normally distributed continuous variables and the Symmetry test for categorical variables.