CARDIOVASCULAR JOURNAL OF AFRICA • Vol 23, No 2, March 2012
AFRICA
67
Heart rate variability in physically active individuals:
reliability and gender characteristics
TAKSHITA SOOKAN, ANDREW J MCKUNE
Abstract
Purpose:
To evaluate the reliability of short-term recordings
(five minutes) of heart rate variability (HRV) and the asso-
ciation between HRV and gender.
Methods:
HRV time- and frequency-domain parameters
were calculated in 44 physically active students (21 males
and 23 females) over four consecutive days.A Suunto t6 heart
rate monitor was used to obtain inter-beat intervals (IBIs)
that were then transferred to Kubios HRV analysis software.
The relative reliability [intra-class correlation (ICC)] and
absolute reliability, [typical error of measurement (TEM)
and typical error of measurement as a percentage (TEM%)]
of the HRV parameters were then calculated for day 2 versus
day 3 and day 3 versus day 4, with day 1 being a familiarisa-
tion day. The following HRV parameters were calculated: (1)
time domain: resting heart rate (RHR), R–R intervals (IBI),
standard deviation of normal-to-normal intervals (SDNN),
root mean square differences of the standard deviation
(RMSSD), percentage of beats that changed more than 50 ms
from the previous beat (pNN50); and (2) frequency domain:
low-frequency normalised units (LFnu), high-frequency
normalised units (HFnu), low-frequency to high-frequency
ratio in normalised units (LF/HFnu). An analysis of vari-
ance (ANOVA) with Tukey
post-hoc
testing was performed to
compare HRV parameters in males and females. Significance
was set at
p
≤
0.05.
Results:
The ICCs for both relationship 1 and 2 indicated
primarily good to excellent (
>
0.8) relative reliability. The
lowest value was found in the LF/HFnu ratio (ICC
=
0.36)
for males. Absolute reliability was low with TEM% greater
than 10% for all HRV parameters, except IBIs. Females
demonstrated better relative (higher ICCs) and absolute reli-
ability (lower TEM and TEM%) compared to males for the
frequency domain. The relative and absolute reliability for
the time domains were similar except for SDNN where the
absolute reliability was higher in males. ANOVA illustrated
significant gender differences for the LF/HFnu ratio (41%
higher in males,
p
=
0.003), HFnu (12% higher in females,
p
=
0.02) and IBI (21% higher in females,
p
<
0.0001).
Conclusions:
Short-term recordings of HRV over three
consecutive days demonstrated a high relative reliability.
However, a low absolute reliability indicated large day-to-day
random variation in HRV, which would make the detection of
intervention effects using HRV difficult in individual partici-
pants. Females were shown to have a higher parasympa-
thetic modulation of HRV, which may indicate an underlying
cardioprotective mechanism in females compared to males.
Keywords:
heart rate variability, sympathovagal balance, reli-
ability
Submitted 10/12/10, accepted 9/5/11
Cardiovasc J Afr
2012;
23
: 67–72
DOI: CVJ-21.108
Heart rate variability (HRV) is recognised as a versatile and prom-
ising non-invasive marker of autonomic nervous system (ANS)
modulation.
1
Research into the use of HRV has increased in both
clinical and research environments and over a broad spectrum of
disciplines.
2,3
However, in the disciplines of sport and exercise
science, there is limited information available on the reliability
of HRV measures, in particular related to new, commercially
available equipment that coaches and athletes have access to.
This fact, together with the extensive number of variables that
alter HRV measures, make it difficult to compare HRV studies
and develop a universal standard.
2
While most studies agree that
age is inversely associated with HRV,
4
research is less consistent
on the impact of gender, with studies demonstrating that HRV
measures are either the same or differ considerably between
the genders and may also be HRV parameter dependant.
5-8
Heart rate variability reflects the changes in the interval
between heart beat (R waves) over time. The time between one
R wave and the next, in milliseconds, is termed the R–R interval
or the interbeat interval (IBI).
9
The ANS governs the IBIs via the
sympathetic and parasympathetic pathways.
10
The relative domi-
nance of either pathway over the other represents an alteration in
the sympathovagal balance which is reflected in IBI changes.
11
Under normal resting conditions in healthy individuals, it has
been suggested that the parasympathetic pathway is dominant,
resulting in a high HRV,
9
while lower HRV and poor health has
been linked to increased sympathetic activity at rest.
5,12
However,
research has demonstrated that the age, physical activity status,
gender and the HRV parameter examined are important factors
to consider when examining HRV.
5-8
Research has identified the potential use of HRV for iden-
tifying healthy and diseased states. In particular, a significant
relationship between the ANS, low HRV and cardiovascular
mortality, including sudden cardiac death, has been reported.
4,13
In addition, studies have shown that trained athletes have higher
HRV compared to sedentary individuals, suggesting that exercise
training can increase HRV in normal populations.
14
The over-
training syndrome is assumed to be the consequence of an imbal-
ance between long-term, inappropriate, high training volume and
too little time for regeneration.
15,16
Alterations in the ANS have
been presented as a mechanism underlying the signs and symp-
toms of the over-training syndrome.
17
A study that examined
ANS activity in several middle-distance runners suggested that
heavy training shifted the cardiac autonomic balance toward a
Department of Biokinetics, Exercise and Leisure Sciences,
School of Health Sciences, University of KwaZulu-Natal,
Durban, South Africa
TAKSHITA SOOKAN, BSpSc (Hons Biokinetics)
ANDREW J MCKUNE, DTech,
