Table 2. Follow-up results for echocardiographic and electrocardiographic variables in 12 twin pairs, whose within-pair difference in leisure time physical activity is 8.2 (SD 4.0) MET h·day. Characteristic Inactive co-twins, mean (SD) Active co-twins, mean (SD) Mean ifference (95% CI) p value* All 12 pairs LVM†  (g) 130.5 (44.5) 147.7 (56.3) -17.2 (-51.4 to 16.9) .28 LVM/BSA†  (g·m-2) 67.1 (19.2) 77.9 (22.4) -10.8 (-26.3 to 4.8) .15 LVM/body height†  (g·m-1) 74.7 (22.7) 84.9 (28.8) -10.2 (-29.0 to 8.6) .25 LVM/body weight†  (g·kg-1) 1.7 (.5) 2.0 (.5) -.3 (-.7 to .0) .08 LVEDD‡ (mm) 58.8 (8.2) 59.4 (10.1) -.6 (-7.9 to 6.7) .85 LVEDD/BSA‡ (mm·m-2) 30.5 (3.0) 31.9 (3.2) -1.4 (-4.5 to 1.7) .33 EF‡ (%) 56.8 (9.8) 62.9 (10.4) -6.1 (-13.5 to 1.3) .10 Resting heart rate (bpm) 68.0 (10.2) 59.2 (5.1) 8.8 (1.3 to 16.4) .03 RR interval (ms) 902.3 (134.1) 1011.7 (80.1) -109.4 (-216.4 to -2.3) .05 P duration (ms) 91.8 (15.8) 103.8 (12.1) -12.0 (-27.8 to 3.8) .12 PR interval (ms) 157.5 (21.9) 166.7 (26.7) -9.2 (-21.2 to 2.8) .12 QRS duration (ms) 88.8 (10.5) 89.2 (14.0) -.4 (-9.5 to 8.8) .94 QT interval (ms) 400.7 (25.6) 422.0 (18.2) -21.3 (-39.2 to -3.5) .02 QTc (ms) 423.8 (25.9) 417.3 (15.8) 6.5 (-9.1 to 21.9) .38 Sokolow Lyon voltage (mV) 2.7 (.7) 2.9 (.9) -.2 (-1.0 to .7) .88 Cornell voltage (mV) 1.2 (.5) 1.5 (.6) -.3 (-.7 to .2) .22 Cornell product (mV x ms) 110.5 (42.0) 136.7 (67.7) -26.2 (-64.8 to 12.6) .17 P axis (degree) 49.3 (18.3) 45.9 (16.4) 3.4 (-7.0 to 13.8) .49 QRS axis (degree) 32.0 (20.2) 40.3 (22.9) -8.3 (-31.7 to 15.0) .45 T axis (degree) 37.8 (13.6) 39.8 (14.3) -2.0 (-15.1 to 11.1) .74 T wave amplitude in lead V1 (mV) .08 (.20) .11 (.12) -.03 (-.20 to .15) .75 T wave amplitude in lead V5 (mV) .42 (.13) .46 (.20) -.04 (-.13 to .06) .42 T wave amplitude in lead II (mV) .26 (.06) .30 (.09) -.04 (-.09 to .01) .08 5 MZ pairs LVM(g) 153.0 (34.9) 186.6 (38.5) -33.6 (-70.0 to 2.9) .06 LVM/BSA (g·m-2) 77.1 (12.3) 93.2 (12.4) -16.1 (-33.2 to .8) .06 LVM/body height (g·m-1) 86.0 (15.8) 104.2 (18.4) -18.2 (-40.0 to 3.5) .08 LVM/body weight (g·kg-1) 1.9 (.3) 2.3 (.2) -.4 (-.8 to .0) .04 LVEDD (mm) 63.2 (9.5) 63.6 (6.3) -.4 (-9.5 to 8.8) .92 LVEDD/BSA (mm·m-2) 32.0 (2.9) 32.1 (1.1) -.1 (-3.4 to 3.2) .96 EF (%) 59.1 (11.6) 62.6 (12.6) -3.5 (-17.9 to 10.8) .53 Resting heart rate (bpm) 69.6 (5.1) 60.0 (6.9) 9.6 (-4.2 to 23.4) .13 RR interval (ms) 858.8 (83.4) 997.6 (102.5) -138.8 (-363.0 to 85.4) .16 P duration (ms) 86.4 (19.7) 110.8 (13.0) -24.4 (-60.4 to 11.6) .13 PR interval (ms) 158.4 (26.2) 167.6 (27.8) -9.2 (-31.5 to 13.1) .32 QRS duration (ms) 90.8 (16.0) 88.4 (13.7) 2.4 (-13.6 to 18.4) .70 QT interval (ms) 394.8 (25.3) 424.4 (24.6) -29.6 (-44.6 to -14.6) .005 QTc (ms) 424.4 (33.2) 422.0 (19.6) 2.4 (-35.4 to 40.2) .87 Sokolow Lyon voltage (mV) 2.8 (.7) 2.9 (.6) -.1 (-1.3 to 1.1) .89 Cornell voltage (mV) 1.1 (.5) 1.5 (.7) -.4 (-1.4 to .6) .32 Cornell product (mV x ms) 102.0 (44.9) 141.0 (75.2) -39.0 (-126.5 to 48.6) .29 P axis (degree) 49.6 (5.5) 39.6 (14.1) 10.0 (-2.3 to 22.3) .09 QRS axis (degree) 34.8 (6.6) 36.2 (25.5) -1.4 (-28.3 to 25.5) .89 T axis (degree) 42.8 (15.6) 39.2 (3.7) 3.6 (-15.1 to 22.3) .62 T wave amplitude in lead V1 (mV) .10 (.27) .12 (.14) -.02 (-.42 to .39) .93 T wave amplitude in lead V5 (mV) .37 (.16) .37 (.15) .00 (-.17 to .17) 1.00 T wave amplitude in lead II (mV) .23 (.07) .27 (.07) -.04 (-.12 to .04) .21 7 DZ pairs LVM§ (g) 108.0 (44.4) 108.9 (43.3) -.9 (-75.5 to 73.7) .98 LVM/BSA§ (g·m-2) 57.2 (20.7) 62.6 (19.8) -5.4 (-40.6 to 29.8) .69 LVM/body height § (g·m-1) 63.4 (24.3) 65.5 (24.2) -2.1 (-43.1 to 38.8) .89 LVM/body weight§ (g·kg-1) 1.4 (.5) 1.7 (.4) -.3 (-1.1 to .6) .49 LVEDD** (mm) 55.2 (5.2) 56.0 (11.9) -.8 (-15.4 to 13.8) .89 LVEDD/BSA** (mm·m-2) 29.3 (2.8) 31.9 (4.4) -2.6 (-8.7 to 3.5) .33 EF** (%) 54.9 (8.7) 63.1 (9.5) -8.2 (-19.8 to 3.4) .13 Resting heart rate (bpm) 66.9 (13.0) 58.6 (4.0) 8.3 (-4.0 to 20.6) .15 RR interval (ms) 933.4 (160.1) 1021.7 (67.0) -88.3 (-245.9 to 69.3) .22 P duration (ms) 95.7 (12.5) 98.9 (9.3) -3.2 (-20.7 to 14.4) .68 PR interval (ms) 156.9 (20.5) 166.0 (28.1) -9.1 (-28.6 to 10.3) .29 QRS duration (ms) 87.4 (4.9) 89.7 (15.3) -2.3 (-17.1 to 12.6) .72 QT interval (ms) 404.9 (26.9) 420.3 (14.0) -15.4 (-48.1 to 17.3) .29 QTc (ms) 423.3 (22.2) 414.0 (13.0) 9.3 (-10.4 to 28.9) .29 Sokolow Lyon voltage (mV) 2.7 (.7) 2.9 (1.1) -.2 (-1.6 to 1.3) .87 Cornell voltage (mV) 1.3 (.5) 1.5 (.6) -.2 (-.6 to .4) .56 Cornell product (mV x ms) 116.6 (42.2) 133.6 (67.9) -17.0 (-69.7 to 35.7) .46 P axis (degree) 49.1 (24.4) 50.4 (17.4) -1.3 (-18.9 to 16.4) .86 QRS axis (degree) 30.0 (26.6) 43.3 (22.5) -13.3 (-55.7 to 29.1) .47 T axis (degree) 34.1 (11.9) 40.1 (19.1) -6.0 (-28.5 to 16.5) .54 T wave amplitude in lead V1 (mV) .07 (.16) .10 (.12) -.03 (-.27 to .20) .74 T wave amplitude in lead V5 (mV) .45 (.10) .52 (.22) -.07 (-.21 to .09) .35 T wave amplitude in lead II (mV) .27 (.05) .32 (.10) -.05 (-.13 to .04) .25 For the difference between the inactive and active co-twins, based on paired samples Student’s t-test for normally distributed variables and Wilcoxon signed ranks test for non-normally distributed variables. n = 11. n = 10. n = 5. n = 6. LVM = left ventricular mass (see the calculation in Methods section); BSA = body surface area (see the calculation in Methods section); LVEDD = left ventricular end-diastolic diameter; EF = ejection fraction.