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J Appl Physiol 99: 535-541, 2005. First published April 7, 2005; doi:10.1152/japplphysiol.00107.2005
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Cardiac structure and function in monozygotic twin pairs discordant for physical fitness

Jarna C. Hannukainen,1 Urho M. Kujala,4 Jyri Toikka,5 Olli J. Heinonen,2 Jukka Kapanen,2 Tero Vahlberg,3 Jaakko Kaprio,6,7 and Kari K. Kalliokoski1

1Turku PET Centre, 2Paavo Nurmi Centre, Department of Physiology, and 3Department of Biostatics, University of Turku, Turku, Finland; 4Department of Health Sciences, University of Jyväskylä, Jyväskylä, Finland; 5Department of Clinical Physiology, Tampere University Hospital, Tampere, Finland; 6Department of Public Health, University of Helsinki, Helsinki, Finland; and 7Department of Mental Health and Alcohol Research, National Public Health Institute, Helsinki, Finland

Submitted 28 January 2005 ; accepted in final form 5 April 2005


   ABSTRACT
TOP
 ABSTRACT
METHODS
 RESULTS
 DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
Cross-sectional studies in athletes and untrained subjects suggest that exercise training induces adaptations in cardiac structure and function. However, the role of genetic variation on the results has largely been ignored in these studies. The purpose of this study was to investigate the effects of long-term volitionally increased physical activity on electrocardiographic and echocardiographic parameters in male monozygotic twin pairs discordant for physical activity and fitness. On the basis of the mailed questionnaires, a telephone interview, and the inclusion criteria, 12 pairs of young adult male monozygotic twins were recruited from a Finnish twin cohort. All subjects completed a maximal oxygen uptake (O2 max) test and electrocardiography and echocardiography studies. Nine pairs had at least 9% difference in O2 max and were selected for further analysis and for a second echocardiography study. Twins were divided into the more (MAG) and less active group (LAG), according to their O2 max. On average, MAG had 18% higher O2 max compared with LAG. In electrocardiography, MAG had 29% (P = 0.02) higher Cornell voltage and 37% (P = 0.01) higher right-side hypertrophy index. In echocardiography, no significant differences were observed between the groups, and left ventricular mass index was only 7% (P = 0.16) higher in MAG. These results show that the volitionally increased physical activity that has led to an 18% increase in cardiorespiratory fitness induces greater changes in electro- than echocardiographic parameters. Electrocardiographic changes were suggestive of left ventricular hypertrophy, and echocardiography showed a similar but statistically nonsignificant trend.

electrocardiography; echocardiography

EXERCISE TRAINING INDUCES significant structural and functional changes in the heart. Aerobic training induces volume overload on the heart, mostly increasing the cavity dimensions and less the thickness of the left ventricular (LV) walls (24). On the other hand, resistance training mostly increases the thickness of the LV walls but only slightly increases the cavity dimensions (27).

The combined effect of both types of training is increased LV mass. Several cross-sectional electro- (ECG) and echocardiographic (Echo) studies have shown evidence that highly trained athletes have increased LV mass compared with healthy sedentary subjects (19, 23, 24, 30, 37). In addition to training, heredity also has a large impact on LV mass (15, 41, 43), and the independent role of exercise training or increased physical activity on LV mass cannot be determined from the cross-sectional studies in independent groups. The training effect can be studied with interventions; however, long-term interventions may cause problems in relation to a subject's motivation to exercise, and this may be one reason for inconsistent findings regarding the amount and duration of training needed to induce changes in the intervention studies (1, 14, 20, 39). In addition, results from the HERITAGE family study clearly show that trainability differs largely between the subjects (6, 33), and therefore a part of the inconsistency is explained by the use of two independent groups that are training or completely sedentary.

One possible way to minimize the aforementioned problems is to study monozygotic (MZ) twin pairs who are discordant for physical activity habits or long-term training. MZ twins have the same inherited genes (5), and if they differ in a particular trait, the difference can be considered to be due to environmental factors. In the present study, we studied young adult male MZ twin pairs who were discordant for physical activity habits and fitness to investigate the heredity-independent effects of increased physical activity and fitness on ECG- and Echo parameters.


   METHODS
TOP
 ABSTRACT
 METHODS
RESULTS
 DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
Subjects and study design.   Study subjects were recruited from five consecutive twin-birth cohorts (born 1975 to 1979), as ascertained from the Central Population Register of Finland. They were participating in the ongoing FinnTwin16 study, and there were a total of 3,065 twin pairs with both cotwins alive and residing in Finland at the time of their 16th birthday in 1991–1995, when the baseline assessment of the FinnTwin16 study was performed. Their health habits, including numerous questions on physical activity, have been studied by mailed questionnaires four times, and currently the last follow-up was completed in 2002 (18).

The subjects were initially selected among the MZ male twins on the basis of the results of this fourth follow-up. A pair was included in the study if the healthy brothers had a marked difference in leisure-time physical activity habits. The criteria for the marked difference were that one brother was inactive and the other exercised at least two to three times per week or that, if both brothers exercised, the more active brother exercised twice as much as the less active brother. Other inclusion criteria were that subjects had to be healthy and that the brothers had to have a similar work-related physical activity load. Exclusion criteria were physical restrictions; a history of anorexia nervosa or bulimia; obesity [body mass index (BMI) >30 kg/m2]; any chronic disease, rest-, or exercise-induced asthma; regular use medication influencing the investigated organs; previous use of anabolic steroids, additives, or any other substances; and use of marijuana or other illicit drugs. Tobacco smoking or taking of snuff were allowed if both brothers did so equally. On the basis of these inclusion and exclusion criteria, a letter of invitation was sent to 26 MZ twin pairs. After this, a more detailed telephone interview was performed, and as a result 12 consenting twin pairs were selected and invited to participate in the study. Before the subjects started the study, their written, informed consent was obtained after the purpose, nature, and potential risks were carefully explained to them. The Joint Commission of Ethics of the University of Turku and Turku University Central Hospital approved the study protocol.

Both brothers were studied on the same day. They were instructed to fast overnight, and physical exercise was prohibited for the preceding 48 h. They arrived at our center in the morning of the study day and were allowed to rest for 0.5 h before starting the measurements. First, the subjects completed the physical activity questionnaire of Baecke et al. (2). Thereafter, a thorough physical examination was performed, and blood samples were taken from an antecubital vein. Then the brothers had a similar breakfast. This was followed by 30 min of rest and followed by a standard 12-lead ECG and Echo. Finally, maximal oxygen uptake (O2 max) was measured using cycle ergometer, and muscle strength tests were performed. On the basis of the O2 max results, nine twin pairs with at least 9% differences between the brothers were selected for future study, and the resting Echo was repeated for them after 2–8 wk.

Health habits, exercise history, and confirmation of zygosity.   No limitations on physical exercise were observed during physical examination or in the ECG and Echo studies. One subject used medication regularly for depression, but that was not assumed to influence the measured parameters. In one pair both brothers smoked, and in another pair both brothers took snuff regularly.

The physical activity of the study subjects consisted of different sports: strength training, ice hockey, cross-country skiing, soccer, running, fencing, floor ball, and tennis. Usually, the twins had had the same sporting history while they were teenagers; however, the situation started to change on average at the age of 21 yr, and there were differences in the amount of physical activity at least during the preceding 3 yr. The amount of physical activity of the preceding year was studied by a questionnaire. The physical activity was divided into conditioning exercise (e.g., running, cross-country skiing, strength training, and intensive ball games) and other physical activity (e.g., light walking, gardening, removal of snow, and field sports). Twins were divided into the more (MAG) and less active group (LAG). On average, MAG had 4.0 ± 2.9 and LAG 1.7 ± 1.5 (P = 0.003) conditioning exercise workouts per week. The average time per week spent for conditioning exercise was 229 ± 156 min in MAG and 98 ± 71 min in LAG (P = 0.013). The times per week spent for other physical activities was on average 4.3 ± 2.1 in MAG and 4.4 ± 4.3 in LAG (P = 0.96). The average time per week spent for other physical activities was 144 ± 110 min in MAG and 157 ± 162 min in LAG (P = 0.77).

Zygosity was determined in the paternity testing laboratory of the National Public Health Institute, Helsinki, Finland. The DNA was extracted from venous blood samples, and 10 highly polymorphic genetic markers were determined. The monozygosity of a twin pair was inferred if no intrapair differences in the markers were observed.

ECG.   A standard 12-lead resting ECG was recorded at 50 mm/s and 1 mV/cm standardization with a three-channel MAC 5000 recorder (GE) while the subjects were in a supine position. The equipment met the recommendations of the American Heart Association concerning frequency response characteristics (0.01–250 Hz in our equipment). The analyzed parameters were collected from the automatic listings of the ECG recorder. They were heart rate, PQ interval, QRS duration, QT time, corrected QT time, ST elevation, and R-, S-, and T-wave amplitudes. From these measures, further parameters were derived: Cornell voltage (RaVL + SV3) (7), right side hypertrophy index (RSHI) (RV1 + SV5), and Sokolow-Lyon voltage (SV1 + RV5) (35).

Echo.   All measurements were made using Acuson Sequoia C512 ultrasound equipment (Siemens). A comprehensive Doppler Echo study was performed. The same experienced investigator (J. Toikka) performed all the Echo measurements blinded to the physical activity and the fitness status of the study subjects. M-mode measurements of the LV were obtained using guidance by two-dimensional Echo at end systole and at end diastole, as recommended by the American Society of Echocardiography (36). LV mass was calculated as previously described (11). LV mass index was calculated as LV mass in grams divided by body surface area in square meters. In addition, LV mass/height, LV mass/height2.7, and LV mass/lean body mass were calculated. LV end-systolic and end-diastolic volumes were measured using the modified biplane Simpson method (method of disks) using the apical four-chamber and two-chamber views (38). The LV ejection fraction (LVEF) was calculated as the ratio of stroke volume (SV) to end-diastolic volume. From the Doppler scan, peak early (E) and peak atrial flow velocity (A) were measured, and the E/A ratio was calculated by dividing E by A. In addition, the following parameters were measured: left atrium dimension, right ventricle dimension, and aortic root dimension. The results of Echo are expressed as the mean of the two independent measurements.

Body composition.   Subjects' height and weight were measured by standard procedures. The waist circumference was measured at the level of the umbilicus in the late-exhalation phase and the hip circumference at the level of the greater trochanter in standing position. Four skinfolds (subscapular, triceps, biceps, and suprailiac) were measured on the left side of the body with a caliper (model HSK-BI, skinfold caliper Harpenden), and the percentage of body fat content was estimated according to Durnin and Womersly (13). The same investigator (J. Kapanen) performed all the measurements.

Fitness tests.   All fitness tests were performed at the Paavo Nurmi Centre, University of Turku, by an experienced exercise physiologist (J. Kapanen). O2 max was determined using an electrically braked cycle ergometer (model 800 S, Ergoline, Mijnhardt, The Netherlands) with a continuous incremental protocol. Initial exercise intensity was 50 W, and it was increased by 30 W every 2 min until exhaustion. Direct respiratory gas measurements were made using an automated system (model 202; Medikro, Kuopio, Finland). A fingertip blood sample was taken immediately after and 1 min after the exercise to analyze blood lactate (YSI 2300 STAT, YSI, Yellow Springs, OH). The highest value of oxygen uptake during the test (1-min collection) represent the O2 max. All subjects reached their O2 max on the basis of the criteria that all exercised until subjective exhaustion and reached a respiratory exchange ratio >1.1 as well as a blood lactate concentration >10 mmol/l.

Muscle strength tests were performed after the O2 max test. Maximal vertical jump height was determined with the countermovement jump test (44). Maximal isometric strength of handgrip was measured using dynamometer (In Good Shape, Metitur Oy, Jyväskylä, Finland). Finally, traditional sit-up and back tests were performed, and repetitions during 30 s were counted (44).

Statistical analysis.   Statistical analyses were performed with SAS/STAT statistical analysis program package, version 8.02 (SAS Institute, Cary, NC). Normality of variables was assessed by Shapiro-Wilks test. A two-tailed Student's paired t-test was used for normally distributed variables and the Wilcoxon signed rank test for nonnormally distributed parameters to determine whether there were differences between more and less fit twins, according to different parameters. Associations between continuous parameters and O2 max in the whole group (i.e., all individuals) were evaluated using a linear mixed model where twin pair membership was used as a random effect. Values of P < 0.05 were considered statistically significant. All results are expressed as means ± SD. The 95% confidence interval was calculated for the absolute mean difference between groups.


   RESULTS
TOP
 ABSTRACT
 METHODS
 RESULTS
DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
Demographic findings and physical fitness.   Members of each twin pair were included, and they were divided into the MAG and the LAG, according to consistent discordance in both their earlier questionnaire reported physical activity and the O2 max measured in the present study. According to Baecke's questionnaire (2), recent sport-related physical activity differed significantly between the groups, but the work-related and other leisure-time activity levels did not differ (Table 1). In addition, the estimated energy expenditure per week during exercise was twice as high in MAG as in LAG (Table 1). Percentage of body fat was lower in the MAG, but BMI and waist-to-hip ratio were similar between the groups (Table 1).


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  		Table 1. Differences in anthropometry and physical activity according to Baecke's questionnaire between the more and less active twins

 
Inherent to the study design, MAG had 18% higher O2 max compared with LAG (Table 2). Also, the maximal power during the O2 max test was significantly higher in MAG. In addition, MAG also achieved significantly better results in countermovement jump, sit-up, and back tests compared with LAG (Table 2). O2 max (ml·min–1·kg–1) associated inversely with the amount of body fat (kg) in the whole study group ({beta} = –0.917, SE = 0.255, P = 0.007). In addition, absolute O2 max (l/min) associated with lean body mass in the whole study group ({beta} = –0.055, SE = 0.020, P = 0.026).


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  		Table 2. O2max and strength test results in the more and less active groups of twins

 
ECG findings.   All the ECG findings are shown in Table 3. Neither heart rate nor the different time durations in ECG (PQ, QRS, and QT) differed between groups. In contrast, many of the ECG amplitudes were greater in the MAG than in the LAG. Because of higher amplitudes, the indexes of hypertrophy, Cornell voltage, and RSHI were also significantly higher in MAG than in LAG. Cornell voltage associated with absolute O2 max (l/min) in the whole study group ({beta} = 0.59, SE = 0.245, P = 0.049; Fig. 1A).


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  		Table 3. ECG measurements in the more and less active twins

 


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  		Fig. 1. Association between maximal oxygen uptake (O2 max) and Cornell voltage (A) and between O2 max and left ventricular (LV) mass (B) in all twins. MAG, more active group; LAG, less active group. Each twin pair is connected with a line. Dashed lines, association between O2max and Cornell voltage (A) and between O2max and LV mass (B) in all twins.

 
Echo findings.   There were no significant differences in Echo parameters between the groups (Table 4). A trend toward higher values in MAG was observed in LV mass index and LV mass-to-lean body mass ratio. Interestingly, LV mass associated with absolute O2 max (l/min) in the whole study group ({beta} = 0.011, SE = 0.0.004, P = 0.025; Fig. 1B).


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  		Table 4. Echocardiographical left ventricular measurements in the more and less active twins

 

   DISCUSSION
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 
The purpose of the present study was to investigate to what degree the effects of volitionally increased physical activity and fitness on cardiac structure and function, determined by ECG and Echo, are independent of genetic effects. The results showed that, despite an 18% higher O2 max in MAG compared with LAG, LV mass index determined by Echo differed only by 7% between the groups, which was statistically nonsignificant. In contrast, widely used ECG indexes of cardiac hypertrophy, Cornell voltage, and RSHI were significantly increased by 20–30% in the MAG compared with the LAG. Thus somewhat unexpectedly, the present study suggests that improvement in cardiorespiratory fitness induces ECG changes more easily than Echo changes.

Discordance between the groups and general findings.   It is a challenge to find MZ twin pairs who are volitionally discordant for physical activity and fitness to an extent that any different adaptation in the function of human body could be observed. To the best of our knowledge, there are no other studies on the effects of several years of volitionally increased physical activity and fitness on ECG and Echo parameters. None of the subjects in the present study were elite athletes. Furthermore, the variation in sports in which the subjects were participating was very broad, ranging from gym training and mixture of ball games to endurance events. This may be one reason why we were unable to find even larger differences, especially in LV mass, between the groups. On the other hand, great variability in the sports and, thereby, in O2 max (43–57 ml·min–1·kg–1 in MAG and 31–52 ml·min–1·kg–1in LAG) enabled us to determine associations between different parameters, and some interesting associations were found.

From the public health perspective, one interesting finding was the significantly (10%) decreased percentage of fat in the MAG. Previous training studies have shown comparable decrease of ~10% in percentage of fat with a slightly larger increase of 20–30% in O2 max (22, 34). If the decrease in fat percent is expressed as kilograms, it means that on average subjects in MAG had 2.4 kg less fat than their less active brothers (13.4 vs. 15.8 kg). Because the reduction in body fat is known to be associated with an improved lipid profile (9, 46), our result, controlled for genetic effects, strengthens the evidence of the health benefits of even small increases in physical activity.

ECG findings.   We observed typical exercise training-induced voltage differences between groups in S-wave amplitude in the V3, V4, and V5 leads and in R-wave amplitude in the V5 lead, which were all greater in MAG. Because of greater S-wave amplitude in the V3 lead, Cornell voltage, which has recently been used as a marker of LV hypertrophy (7, 28, 40), was significantly higher in MAG than in LAG. In addition, because of increased S-wave amplitude in the V3 lead, RSHI was also significantly higher in MAG than in LAG. The results of higher Cornell voltage and RSHI are in line with previous findings in cross-sectional studies comparing ECG differences between trained and untrained subjects with larger differences in O2 max between the groups (4, 12, 40). Both Cornell voltage and RSHI associated with O2 max (ml·min–1·kg–1) in all twins. Longitudinal studies of exercise training-induced ECG changes are sparse, but in one study, 3 mo of intensive aerobic training induced a 16% increase in O2 max and an increase in R-wave amplitudes in the V5 and V6 leads but not in S-wave amplitudes (1).

One parameter that has received attention in many previous studies investigating exercise-induced changes in ECG is Sokolow-Lyon voltage, which has been widely used as ECG index reflecting physiological LV hypertrophy (4, 10, 12, 39). In our study, Sokolow-Lyon voltage was almost statistically significantly higher in MAG (3.2 ± 0.7 vs. 2.8 ± 0.4 mV; P = 0.06), but the mean group difference was smaller than in the Cornell voltage. Thus, according to our findings, Cornell voltage could be a more sensitive parameter than Sokolow-Lyon voltage.

Echo findings.   We did not observe significant differences in Echo parameters between the groups. LV mass index was 7% greater (P = 0.16) in MAG than in LAG, and this was due to slightly increased posterior and septal wall thickness, whereas end-diastolic LV diameter was similar between the groups. LV mass and LV mass index in the both groups in the present study were near to the values observed in healthy controls and not to those observed in endurance athletes in previous cross-sectional studies (16, 17, 26, 29, 42, 45). A part of the greater difference between the trained and untrained groups in numerous previous studies (4, 23, 24, 26, 27, 29, 30, 37, 45) or detraining (14, 25, 31) may be explained by the genetic differences between the groups. Those who have had genetically increased LV diameter-to-mass ratio have inherently been prone to become endurance athletes, and this may have caused genetic selection bias for group comparisons and observations of larger than the true training-induced differences in LV mass in cross-sectional studies. This is further supported by the fact that short-term training studies have shown inconsistent results regarding the effects of exercise training on LV mass (1, 3, 21, 25, 39).

The Echo results show also that the shape of LV in MAG was more spherical (tendency to group difference in LV diameter-to-LV length ratio; P < 0.07) compared with LAG, which tended to have greater LV length (Table 4). We determined LV mass using the cube-function technique (11), which ignores LV length. In contrast to the cube-function technique, LV length is used when LV mass is calculated with the area-length formula (32), which is used in two-dimensional echocardiographic measurements. However, the difference in LV mass between the groups would have been smaller if the area-length method had been used in the present study.

In the present study, LV mass index associated significantly with absolute but not with body weight-adjusted O2 max in the whole study group. This latter finding matches with the previous studies showing poor correlation between relative O2 max and different LV mass parameters in different athlete groups (8, 45). O2 max is a complex trait dependent on a combination of inherited and training-induced structural and functional differences, one of the important determinants being the properties of skeletal muscle.

Potential explanations for the different findings with ECG and Echo.   Echo has become almost a gold standard to detect physiological and pathophysiological structural changes in human heart. To minimize measurement-related errors, we used mean values from two different Echo measurements. In addition, the voltage increments in ECG have also been regarded as the markers of LV wall thickening. Yet, in the present study, we found discrepancies between the results with different methods: ECG findings suggested LV hypertrophy that could not be verified as clearly with Echo. These somewhat unexpected findings in the light of previous cross-sectional studies may be explained by several factors. Because ECG voltages are indirect measures of LV hypertrophy, they are of course influenced by many other factors than just LV wall thickness (for example, electrode positioning in relation to the heart position and orientation, and the fat content of the body). In the present study, the ECG electrodes were carefully attached over the skin in exactly the same positions within each pair, and thus the effect of electrode positioning on the results was minimized. Furthermore, no significant differences were found in electrical axis of the heart (data not shown), and most probably the ECG results were not affected by electrode positioning in relation to the heart orientation. The fat percentage of the cotwins in MAG was slightly and significantly lower than in the cotwins in LAG. This might have had a small influence on the results, increasing the difference in voltage values between the groups. However, neither of the groups was even near overweight, and thus no correction was made for the fat percent. One, and perhaps the most likely, explanation is that the ECG voltages may, in addition to the hypertrophy, be influenced by training-induced changes in electrophysiological features of the heart muscle and may thus be used as early markers of training-induced adaptations in heart.

In conclusion, these results show that volitionally increased physical activity that has led to an 18% increase in cardiorespiratory fitness has induced greater changes in ECG rather than Echo parameters. ECG changes were suggestive of LV hypertrophy, but statistically significant hypertrophy was not seen by Echo. These somewhat unexpected findings are perhaps best explained by the fact that the heart muscle hypertrophy is under rather strong genetic control and that ECG voltages may also be influenced by other training-induced changes in the electrophysiological features of heart muscle.


   GRANTS
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 GRANTS
ACKNOWLEDGMENTS
 REFERENCES
 
This study was financially supported by the Academy of Finland (Grants 206970 and 204240), the Ministry of Education (Grants 143/722/2002, 51/722/2003, and 40/627/2005), the Juho Vainio Foundation, the Turku University Foundation, the South Western Finland Cultural Foundation, and the Finnish Sports Institute Foundation. The FinnTwin16 study has been supported by the National Institute on Alcohol Abuse and Alcoholism (Grants AA-08315 and AA-12502), the Academy of Finland (Grants 44069 and 100499), and the European Union Fifth Framework Program (contract no. QLG2-CT-2002-01254).


   ACKNOWLEDGMENTS
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
REFERENCES
 
The authors thank the personnel of the Turku PET Centre for help during the study.


   FOOTNOTES
 

Address for reprint requests and other correspondence: J. Hannukainen, Turku PET Centre, PO Box 52, FIN-20521 Turku, Finland (E-mail: jarna.hannukainen{at}tyks.fi)

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.


   REFERENCES
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 GRANTS
 ACKNOWLEDGMENTS
 REFERENCES
 

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