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^-1.

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 = 10. ^‡ n = 11. ^§ 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.