Differences in heart rate variability between Asian and Caucasian children living in the same
Katharine E. Reed, Darren E.R. Warburton, Crystal L. Whitney, and
Heather A. McKay
Abstract: Heart rate variability (HRV) is an umbrella term for a variety of measures that assess autonomic influence on the heart. Reduced beat-to-beat variability is found in individuals with a variety of cardiac abnormalities. A reduced HRV positively correlates with obesity, poor aerobic fitness, and increasing age. Racial (black–white) differences are apparent in adults and adolescents. We aimed to evaluate (i) Asian–Caucasian differences in HRV and (ii) differences in HRV between girls and boys. Sixty-two children (30 male (15 Caucasian, 15 Asian) and 32 female (15 Caucasian,
17 Asians)) with a mean age of
10.3 ± 0.6 y underwent 5 min resting HRV recording, fitness testing (Leger’s
shuttle), and self-assessed maturity. Outcome HRV measures were a ratio of low to high frequency power (LF:HF), standard deviation of R–R intervals (SDRR) and root mean square of successive R–R intervals (RMSSD). Data were compared between groups using analysis of covariance (ANCOVA). There were no race or sex differences for time domain variables, mean R–R, body mass index, or blood pressure. Compared with Caucasian children, Asian children displayed a higher adjusted (fitness, R–R interval) LF:HF ratio (72.9 ± 59.4 vs. 120.6 ± 85.3, p < 0.05). Girls demonstrated
a higher adjusted LF:HF power than boys (117.2 ± 85.1 vs. 76.6 ± 62.4, p = < 0.05). In conclusion, Asian and
Caucasian children display different frequency domain components of heart rate variability.
Key words: autonomic nervous system, sympathetic, vagal, race, aerobic fitness, sex.
Résumé : La variabilité de la fréquence cardiaque (HRV) est un terme général qui englobe une série de mesures afin
d’évaluer l’influence autonome du coeur. Chez les individus présentant une anomalie cardiaque, on observe une moins
grande variation de battement à battement. Une HRV moindre est corrélée positivement avec l’obésité, une piètre
condition physique et le vieillissement. On observe des différences raciales (Noirs–Blancs) chez des adultes et des adolescents.
voulons analyser (i) les différences de
filles (15 Caucasiennes et 17 Asiatiques), âge : 10,3 ± 0,6 ans, participent à
une évaluation de
5 min au
repos, au test de Léger (course-navette sur
retenues sont le ratio de la puissance de basse fréquence sur la puissance de haute fréquence (LF:HF), l’écart type des
intervalles R–R (SDNN) et la valeur quadratique moyenne d’intervalles R–R successifs (RMSSD). On analyse les différences
au moyen d’une analyse de covariance. On n’observe aucune différence entre les races et les genres dans les résultats
suivants : variables à réponse temporelle, R–R moyen, IMC et pression sanguine. Comparativement aux enfants
caucasiens, les Asiatiques présentent un meilleur ratio LF:HF ajusté (condition physique, intervalle R–R) : 72,9 ± 59,4
vs. 120,6 ± 85,3, p < 0,05. Les filles présentent une plus grande puissance LF:HF ajustée : 117,2 ± 85,1 vs. 76,6 ±
62,4, p = < 0,05. En conclusion, les composantes de la variabilité de la fréquence cardiaque des enfants caucasiens et
asiatiques n’ont pas la même réponse temporelle.
Mots clés : système nerveux autonome, sympathique, vagal, race, puissance aérobie, sexe.
Appl. Physiol. Nutr. Metab. 31: 1–6 (2006) doi:10.1139/H05-015 © 2006 NRC Canada
Pagination not final/Pagination non finale
Received 8 January 2005. Accepted 18 July 2005. Published on the NRC Research Press Web site at http://apnm.nrc.ca on 23 March
D.E.R. Warburton, and C.L. Whitney. School of Human Kinetics,
H.A. McKay.1 Department of Orthopaedics / Family Practice, University of British Columbia, 5th Floor, Research Pavilion, 828
1Corresponding author (e-mail: firstname.lastname@example.org).
Heart rate variability (HRV) measured by power spectral
analysis provides a quantitative marker of autonomic nervous
system influence on heart rate and has been shown to
reflect cardiovascular health. In adults, impaired variability
has been reported following myocardial infarction
(Sosnowski et al. 2002), in chronic heart failure, and in left
ventricular dysfunction (Nolan et al. 1992). In young children,
reduced HRV values are associated with atrial septal
defects (Finley et al. 1989) and increased likelihood of sudden
infant death syndrome (Edner et al. 2002). An unfavourable
autonomic profile balance (manifesting as reduced beatto-
beat variability) reflects a predominately sympathetic influence
on control of heart rate and is positively correlated
with general obesity (Martini et al. 2001; Nagai et al. 2004;
Gutin et al. 2000), high visceral fat deposition (Gao et al.
1996), lower aerobic fitness (Gregoire et al. 1996; Aubert et
al. 2001), male gender (Sinnreich et al. 1998), and increasing
age (Umetani et al. 1998); (Reardon and Malik 1996).
Racial (black–white) differences in HRV have been previously
studied in adults, with blacks having a lower sympathetic
drive than age-matched whites (Guzzetti et al. 2000)
(Liao et al. 1995). There are only limited data in young children
and none that compare Asians and Caucasians. Previous
data concerning race differences in youth have been
equivocal. Whilst one investigation found that black adolescents
display less favourable HRV measures (i.e., a greater
sympathetic contribution to total power) than age-matched
whites (Faulkner et al. 2003), another found reduced sympathetic
activity in blacks (Urbina et al. 1998). Heart rate variability
has been explored exclusively in Asian populations in
adults and children, but has not been compared with other
races. Results from independent studies of Asian (Kikuchi et
al. 2003; Kazuma et al. 2002; Nagai et al. 2004) or Caucasian
children (Faulkner et al. 2003; Mandigout et al. 2002)
suggest there may be racial differences in time and frequency
domains of HRV, with Asian children
displaying a lower HRV than Caucasian children living in
than 2 million Canadians reporting an Asian origin (Statistics
Canada 2004). There have, however, been no comparisons
between Asian and Caucasian children living in the
same North American community.
The effect of male or female gender on HRV is well documented
in adults, with the majority of researchers reporting
that women, at least until late middle age, demonstrated a
higher vagal influence on heart rate control than men
(Gregoire et al. 1996; Liao et al. 1995; Antelmi et al. 2004).
However, the influence of sex appears to be modulated by
age (Umetani et al. 1998) and studies that have examined
sex differences in children have found that girls have lower
variability than boys (Umetani et al. 1998; Faulkner et al.
2003). These studies, which involved 24 h HRV monitoring,
showed lower time domain values in girls aged 14–16 y and
1–20 y, respectively. To our knowledge, no studies have used
5 min recording to examine sex differences.
Thus, our primary objective was to determine whether
differences in HRV existed between Asian–Canadian (AC)
and Caucasian–Canadian (CC) children living in the same
community. Our second objective was to explore differences
in HRV between prepubertal girls and boys using a 5 min
We hypothesize that compared with CC children, AC children
will have lower levels of the time domain variables
(standard deviation of R–R intervals (SDRR) and root mean
square of successive R–R intervals, (RMSSD)) accompanied
by greater sympathetic predominance, evidenced by a higher
LF:HF in the frequency domain variables. We also hypothesize
that girls will demonstrate altered HRV profiles, specifically
lower variability, compared with age-matched boys using
5 min recordings similar to those derived from 24 h recordings.
Materials and methods
Sixty-six Asian and Caucasian children from grades 4 and
5 were randomly selected from a larger cohort of participants
in a school-based exercise intervention (Action
Schools!, B.C., N = 514). Parents of the children completed
a health history questionnaire on the child’s behalf. Children
with cardiovascular disease were deemed ineligible to participate
in the intervention; thus, no further children were excluded
from the present study. Children were classified as
Caucasian, Asian, East Indian, or Other based on the birthplace
of both parents. Children were classified as “Asian” if
both parents, or all 4
grandparents, were born in
parents or all 4 grandparents were born
(i.e., Africa or
Board approved the investigation and all participants and
their legal guardians provided written consent.
Short-term, 5 min resting HRV was taken using Polar
Rate Monitors (Polar Electro,
measurement error attenuates the correlation observed between
variables, we attempted to control potentially confounding
variables by (i) instructing children not to consume
a caffeinated beverage for at least 2 h before HRV measurement,
(ii) taking all measurements before lunch break activity,
and (iii) making all recordings on school premises.
Children lay supine on a padded mat in a quiet, softly lit
room. Recording began immediately and lasted for 6 min.
Digitally coded R–R interval length was input into Polar
software (Polar Electro) using an infrared transmitter to display
a tachogram on screen. After the first minute of data
was discarded, R–R intervals were automatically filtered
using median- and moving-average-based filtered methods.
Acquisition and filtering of R–R data using Polar software
has been previously described (Jurca et al. 2004). Data (R–R
intervals) were then exported as text to HRV Analysis Software
R–R series were transformed to the frequency domain via
fast Fourier transformation. Spectral power was determined,
in accordance with the Task Force of the European Society
of Cardiology and the North American Society of Pacing
Electrophysiology (Task Force 1996), as very low frequency
(VLF; 0.01–0.04 Hz), low frequency (LF; 0.04–0.15 Hz), or
high frequency (HF; 0.15–0.5 Hz). LF:HF was chosen as the
© 2006 NRC Canada
2 Appl. Physiol. Nutr. Metab. Vol. 31, 2006
primary measure of interest, as it provides information regarding
relative vagal or sympathetic predominance (Pagani
et al. 1986). Values for HF and LF are given as normalized
units (nu). For HF, normalized units are calculated as
HF / (total power – VLF) × 100
where HF is measured in ms2. For LF, normalized units are
LF / (total power – VLF) × 100
where LF is measured in ms2. Secondary variables of interest
were the global time domain measure, the standard deviation
of normalized R–R intervals (measured in milliseconds
(ms); SDRR), and the root mean squared of successive R–R
intervals (also measured in ms; RMSSD), which is thought
to represent vagal activity.
Height in bare feet was
measured to the nearest
Weight, in light indoor clothing, was measured using an
electronic scale (SECA,
made. The average of 2 values or the median of 3 values was
used for analysis. Body mass index (BMI) was calculated as
kg/m2. Aerobic fitness was
determined via Leger’s
shuttle run (Leger et al. 1988). Children begin running
minute. Children continued running until they were unable
to maintain the required pace at a given level. This maximal
test was developed for children and estimates aerobic capacity
from running speed and duration. Blood pressure was
measured in duplicate on the left arm after 5–10 min quiet
rest using an automated sphygmomanometer (VSM
were within 5 mmHg for systolic blood pressure, the
lowest value was recorded. If the difference exceeded 5 mmHg
then a third measurement was taken. Children self-assessed
their physical maturity using line drawings and descriptions
of pubic hair (boys and girls) and breast stage (girls) based
on Tanner staging (Tanner 1955). Stage 1 represents prepuberty,
stage 2 represents early puberty, stage 3 represents
middle puberty, stage 4 late represents puberty, and stage 5
is considered post pubertal.
HRV measures that were skewed (Kolmogorov–Smirnov
Z, p < 0.05) were transformed (natural logarithm (ln)) to
normalize the distribution. In these cases, statistical comparisons
were based on the ln scales. Analysis of covariance
(ANCOVA) was used to evaluate race and sex differences in
HRV. Owing to their established relationship with HRV, we
controlled for aerobic fitness (Leger’s
mean resting heart rate by including them as covariates in
the analysis. Statistical significance was set at alpha level p <
0.05. All statistical analyses were performed using SPSS
version 12.0 for Windows (SPSS Inc.,
Sixty-two children (
and 17 girls; 30 CC children comprising 15 boys and 15
girls) aged 9–11 years were included in the analysis. Data
from 4 children were excluded owing to excessive movement
during recording. Descriptive characteristics (means ±
SD) of the participants by sex and race are provided
(Table 1). All children reported to be in Tanner stage 1 or 2.
There were no racial differences for age, BMI, heart rate,
or blood pressure. CC children ran a significantly greater
11.3 and 19.9 ± 5.7, respectively, p = 0.006).
There were no sex differences for age, BMI, aerobic fitness,
heart rate, or blood pressure.
Heart rate variability measurements
Group means (non-adjusted) for raw and log-transformed
data by race and sex are provided (Table 2). There were no
race or sex differences for total power, SDRR intervals, or
There was a difference in LF:HF between AC and CC
children (F = 5.8, p = 0.01), with AC children having a
higher LF:HF than CC children (Table 3). Analysis by sex
showed a higher LF:HF ratio (F = 4.3, p = 0.04) in girls than
in boys (Table 3). Within-sex differences showed that AC
children had higher (NS) LF:HF than CC children (93.9 ±
16.6 and 59.2 ± 16.5, F = 2.1, p = 0.17 for AC and CC boys,
respectively; 144.5 ± 20.7 and 86.2 ± 22.1, F = 3.5, p = 0.71
for AC and CC girls, respectively).
Comparison between Asian and Caucasian children
This was the first investigation to examine differences in
HRV between Caucasian and Asian children living in the
same Canadian community. AC children had higher LF:HF
when compared with CC children of the same age. A lower
© 2006 NRC Canada
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Asians Caucasians Boys Girls
n 32 30 30 32
Age (y) 10.3 (0.6) 10.5 (0.6) 10.2 (0.6) 10.5 (0.6)
BMI (kg/m2) 18.6 (3.5) 18.6 (2.6) 18.8 (2.9) 18.4 (3.2)
SBP (mmHg) 102.2 (9.1) 105.7 (10.5) 103.9 (8.1) 103.6 (11.2)
DBP (mmHg) 65.1 (7.5) 65.5 (8.2) 66.6 (6.8) 64.1 (8.5)
Heart rate (b/min) 81.1 (12.1) 76.9 (10.1) 77.9 (12.1) 80.0 (10.1)
Note: Values are means (± SD). BMI, body mass index; DBP, diastolic blood pressure; SBP, systolic blood
pressure; b/min, beats per minute).
*Race difference at a significance level of p < 0.05.
Table 1. Descriptive characteristics of participants by sex and race.
LF:HF is indicative, although not definitive, of a higher
sympathetic and (or) lower vagal influence on heart rate.
However, this finding of a differential influence on heart
rate according to race is further supported by the higher
(NS) values for RMSSD seen in CC children. This time domain
measure is strongly influenced by vagal modulation of
the SA node. Additionally, when measured in the supine position,
as in the present study, global measure of HRV such
as SDNN are modulated primarily by the activity of the
vagus. Although the differences in SDNN and RMSSD between
AC and CC children failed to reach statistical significance
here, both measures are clearly higher in the latter
Our findings support previous work that highlights a racial
difference in adults and adolescents in HRV (Liao et al.
1995; Faulkner et al. 2003); (Urbina et al. 1998). Investigations
that compared African–American with Caucasian–
American adolescents have been equivocal. While one study
revealed that 15-year-old African–American males generally
had less favourable HRV outcome measures (greater sympathetic
modulation of heart rate) (Faulkner et al. 2003), a similar
study (Urbina et al. 1998), reported that 13- to 17-yearold
African–American male youth had less sympathetic tone
and greater short-term variability. Racial differences in HRV
were evident at rest and during cardiovascular stress tests
such as the Valsalva manoeuvre (Urbina et al. 1998). Similar
comparisons have not been previously made in younger age
groups or, until now, between Asian and Caucasians.
Comparison between girls and boys
In this cohort, boys had a greater contribution of highfrequency
power to total power than girls. Boys and girls
scored similarly on aerobic fitness tests, but girls had an elevated
LF:HF power ratio compared with boys. These results
support previously reported sex differences (Faulkner et al.
2003) (Silvetti et al. 2001) wherein girls, aged 15 y and 1–
20 y, respectively, demonstrated a lower HRV than boys.
Both of these investigations found that time domain variables
(namely SDRR and the standard deviation of normal R–R
intervals for 5 min segments, SDANN) were higher in boys.
In the same investigation, however, Silvetti and colleagues
(Silvetti et al. 2001) reported no sex differences for
RMSSD. Conversely, the present study found that differences
in HRV measures were significant when measured in
the frequency domain (LF:HF). Differences were also evident
in the time domain variable measures (SDRR and
RMSSD), but these failed to reach statistical significance.
Previously, it has not been shown that sex differences in
HRV were observed with short-term (5 min) recordings. The
discrepancies between the findings of this study, and those
of other studies i.e., no difference in time domain, only in
frequency domain variables, is likely because of the duration
of the recording. Frequency domain methods should be preferred
to the time domain methods when short-term recordings
are investigated (Task Force 1996).
The relationships between physical activity and indices of
spectral power parallel those reported in adults (i.e., more
active individuals typically demonstrate greater vagal predominance)
(Gregoire et al. 1996). Aerobic training is believed
to improve the electrical stability of the myocardium,
with regular exercise improving the cardiac autonomic profile
(Melanson and Freedson 2001). Nagai and colleagues
(Nagai et al. 2004) conducted a cross-sectional investigation
and separated 96 girls and boys into 4 groups; lean physically
active, lean sedentary, obese physically active, and
obese sedentary. Lean active children had significantly better
HRV parameters compared with the other group. However,
physical activity also appeared to contribute to enhanced autonomic
nervous system activity in both lean and obese children.
In the present investigation, differences in heart rate
variability between girls and boys, and between AC and CC
children persisted after adjusting for physical fitness.
Although a substantial proportion of the variance in HRV
can be accounted for by factors such as age, sex, BMI, physical
activity, and fitness, the Framingham Heart Study and
the Kibbutzim Family study found that up to 34% of the
variance was accounted for by genetic factors (Singh et al.
1999; Sinnreich et al. 1999). Although sex differences in
© 2006 NRC Canada
4 Appl. Physiol. Nutr. Metab. Vol. 31, 2006
Asian Caucasian Boys Girls
LF 48.78 (16.62) 37.57 (14.81) 38.71 (14.81) 47.62 (17.41)
Ln 3.82 (0.38)* 3.54 (0.40)* 3.57 (0.41)† 3.79 (0.39)†
HF 51.22 (16.62) 62.53 (14.83) 61.25 (14.81) 52.33 (17.39)
Ln 3.87 (0.35)* 4.10 (0.28)* 4.07 (0.28)† 3.89 (0.36)†
LF:HF 120.57 (85.31) 72.92 (59.42) 76.57 (62.4) 117.19 (85.12)
Ln 4.54 (0.73)* 4.05 (0.67)* 4.11 (0.68)† 4.51 (0.74)†
RMSSD 55.11 (34.45) 75.50 (54.77) 65.18 (38.21) 64.12 (53.17)
Ln 3.83 (0.59) 4.06 (0.72) 4.03 (0.56) 3.87 (0.76)
SDRR 56.41 (27.37) 64.92 (38.02) 60.17 (30.01) 60.81 (35.91)
Ln 3.89 (0.59) 4.01 (0.62) 3.94 (0.63) 3.94 (0.59)
TP 559 (660) 626 (824) 624 (855) 558 (735)
Ln 5.66 (1.27) 5.58 (1.41) 5.64 (1.41) 5.61 (1.27)
Note: Statistical significance refers to log transformed and raw units data in each case. HF, high frequency
power; LF, low frequency power; RMSSD, root mean squared of successive R–R intervals; SDRR, standard
deviation of R–R intervals; TP, total power; Ln, log-transformed value).
*Race difference at p < 0.05.
†Sex difference at p < 0.05.
Table 2. Group (race and sex) mean (± SD) of raw and log-transformed heart rate variability
adult HRV measures are well documented, race differences
and sex differences in children have not been well investigated.
We acknowledge some limitations in the study. Although
we controlled for aerobic fitness, we were unable to match
children on physical activity. Second, we performed a crosssectional
comparison of sex and race. Analysis of heart rate
variability in children is a complex issue, and the evolving
nature of the autonomic nervous system as maturation occurs
adds further difficulty. Longitudinal investigations of
HRV are necessary to determine whether time and frequency
domain measures show similar age-, race-, and sex-related
patterns over time.
There is a paucity of literature describing HRV in young
children and none that compared Asian to Caucasian children.
We demonstrated that AC children aged 9–11 y had
significantly elevated LF:HF power compared with CC children
living in the same community. These findings support
previous investigations suggesting black–white differences
in heart rate variability in children, but introduce new findings
regarding altered variability profiles between Asian and
Caucasian children. Although this difference is unlikely to
result in adverse health implications during childhood, racial
norms for HRV measures should be determined and considered
during clinical examinations and experimental investigation.
Further studies to establish racial norms may be
We acknowledge support from the Ministry of Health
Planning, B.C., the
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Group Adjusted log LF:HF
Asians 4.52 (0.12)*
Caucasians 4.07 (0.13)*
Males 4.11 (0.13)†
Females 4.48 (0.12)†
*Race difference at p < 0.05.
†Sex difference at p < 0.05.
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