JSAMS-274; No. of Pages 4
ARTICLE IN PRESS
Journal of Science and Medicine in Sport (2007) xxx, xxx—xxx
SHORT REPORT
Validation of the RT3 in the measurement
of physical activity in children
Juliette Hussey a,∗, Kathleen Bennett b, Jamie O. Dwyer a,
Sinead Langford a, Christopher Bell c, John Gormley a
a
Department of Physiotherapy, Trinity Centre for Health Sciences,
St James’s Hospital, Ireland
b Department of Pharmacology and Therapeutics, Trinity Centre for Health Sciences,
St James’s Hospital, Ireland
c Department of Physiology, School of Medicine, Trinity College, Ireland
Received 13 February 2007 ; received in revised form 5 September 2007; accepted 25 September 2007
KEYWORDS
Accelerometer;
Physical activity;
Children;
Validation
Summary The aim of this study was to assess the validity of the RT3 accelerometer,
and its inbuilt algorithm, in measuring inactivity, walking and running in children.
Twenty children, aged 7—12 years, participated in the study. The RT3 was compared
to physiological energy expenditure obtained via a wireless portable ergospirometric
system. Data analysis was performed using Bland and Altman plots and Pearson product moment correlation coefficients. There were no significant differences between
the methods for each activity. Bland and Altman 95% limits of agreement between
the two measures in kcal min−1 for each of the activities were as follows: inactivity
(−0.058, 0.47), walking at 3 km h−1 (−1.22, 0.83) brisk walking at 6 km h−1 (−2.74,
0.54), brisk walking at 6 km h−1 on an incline of a 10% gradient (−1.69, 2.11), and
jogging at 9 km h−1 (−3.67, 1.24). Energy expenditure via the RT3 correlated significantly with that obtained by indirect calorimetry for each activity independently
(r = 0.56—0.84, all P < 0.01). The RT3 provided a valid estimate of inactivity, walking
and running and would thus appear appropriate for the objective measurement of
physical activity levels.
© 2007 Sports Medicine Australia. Published by Elsevier Ltd. All rights reserved.
The RT3 (Stayhealthy Inc.)* is a triaxial accelerometer that integrates acceleration from three planes
to yield a vector magnitude. Its predecessor; the
TriTrac has been validated in adults and children1,2
in both laboratory and field settings.3,4 The Tritrac
∗
Corresponding author.
E-mail address: jmhussey@tcd.ie (J. Hussey).
can distinguish between light, moderate and vigorous activity,1,3 and has been found to be a valid
measure of energy expenditure in both boys and
men in a number of activities.5
The aim of this study was to assess the absolute validity of the RT3 accelerometer in children in
the estimation of energy expenditure in inactivity,
light, moderate and vigorous activity in a laboratory
1440-2440/$ — see front matter © 2007 Sports Medicine Australia. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.jsams.2007.09.010
Please cite this article in press as: Hussey J, et al., Validation of the RT3 in the measurement of physical activity
in children, J Sci Med Sport (2007), doi:10.1016/j.jsams.2007.09.010
JSAMS-274; No. of Pages 4
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J. Hussey et al.
setting. Validity was examined in terms of limits of
agreement.
Method
Twenty children (8 girls and 12 boys) aged between
7 and 12 years, with no contraindications to physical activity, were included in the study. Exclusion
criteria were any neurological or musculoskeletal
conditions. Parents and subjects were briefed on
the protocol and safety, and written consent was
obtained. Approval for this study was granted by the
Human Ethics Committee of Trinity College Dublin.
Subjects were recruited through staff in the School
of Medicine.
Subjects were requested not to eat for 3 h before
commencement of the test to avoid the influence of
the thermal effects of food on energy expenditure.
Height was measured to the nearest 0.1 cm using a
stadiometer (Seca 220) and weight was measured
to the nearest 0.1 kg using a Seca electronic scales.
Participants were familiarized with the treadmill
(Viasys LE 300 CE), fitted with the RT3 accelerometer (Stayhealthy Inc.) and a face mask attached to
the Oxycon mobile system.
At the start of testing the subject had an
initial 10 min period of inactivity to collect baseline/inactivity data. Subjects watched a DVD during
rest periods and were requested to reduce extraneous movements so as to minimize expired air
changes. Each exercise was performed for 5 min
followed by a 5 min rest period where the subject
was seated in a chair placed on the treadmill. The
exercises were: walking at 3 km h−1 , brisk walking/running at 6 km h−1 , brisk walking/running at
6 km h−1 on an incline of a 10% gradient, and running at 9 km h−1 . For safety reasons a research
assistant stood beside the treadmill at all times and
the child could lightly place his/her hands on the
cross bar if required. It was decided a priori (based
on pilot work in this laboratory) that the final exercise of running on the treadmill at 9 km h−1 , would
not performed by subjects under 8 years of age (six
subjects). Throughout the testing process the child
was verbally encouraged to maintain the pace.
Expired respiratory gases were collected and
oxygen consumption was measured on a breath
by breath basis using the Oxycon mobile system
(Viasys Healthcare) which has been validated.6 The
light-weight portable system records data breathby-breath collected through a facemask which is
sent to the host computer via telemetry. The system was calibrated daily with standard gases. The
sensorbox and data exchange unit were attached to
the cross arm of the treadmill.
The age, gender, height and weight were entered
into the RT3 to set a profile from which activity and
total energy expenditure is calculated. The RT3 unit
was attached to the waistband on the right side.
The same accelerometer was used by all subjects.
Following each exercise session, the data from
the RT3 was downloaded. The metabolic measures
were collected continuously from 5 min prior to
the beginning of the first 5 min resting period to
the completion of the final stage of the running at
9 km h−1 . The data used for comparison between
the methods was the average of the last 2 min of
each activity.
The data were expressed graphically in means
and confidence intervals for each activity with
both measures (repeated measures). Limits of
agreement and 95% confidence intervals between
the measures were calculated, according to the
method described by Bland and Altman.7 Pearson product-moment correlation coefficients were
also calculated for the energy expenditure data
with both methods for each activity. SPSS (version
12) was used for data analysis and Prism software
was used for creating the graphs. This study had
80% power to detect a smallest average difference
between pairs of 0.33 with a significance level of
0.05.
Results
Twenty children (8 girls and 12 boys) completed the
study. The mean age and 95% CI was 10.2 (9.1,
11.3) years in the girls and 8.2 (7.5, 8.9) years
in the boys. The heights and weights were 144
(134.5, 153.5) cm and 37.7 (30.6, 44.8) kg in the
girls and 136 (129.9, 142.1) cm and 30.8 (26.5,
35.1) kg in the boys.
Fig. 1 presents the means and 95% confidence
intervals of energy expenditure in kcal min−1 with
both methods for each activity. There was no significant difference between the two measures for most
activities (other than 6 km h−1 ) as can be seen by
the confidence intervals on the repeated measures.
Pearson correlations between the two methods
for each activity measured ranged from r = 0.56
for walking at 3 km h−1 to r = 0.84 for running at
9 km h−1 (all P < 0.01). Fig. 2 presents the limits
of agreement of energy expenditure measured by
the RT3 and physiological energy expenditure for
baseline, 3 and 6 km h−1 .
The limits of agreement between the two measures were found to be very close for inactivity
and walking at 3 km h−1 . Walking at 6 km h−1 on an
incline of ten degrees and on the level and running
at 9 km h−1 showed slightly wider limits of agree-
Please cite this article in press as: Hussey J, et al., Validation of the RT3 in the measurement of physical activity
in children, J Sci Med Sport (2007), doi:10.1016/j.jsams.2007.09.010
JSAMS-274; No. of Pages 4
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Validity of the RT3 in children
Figure 1 Means and 95% confidence intervals of energy
expenditure measured by gas analysis (indirect calorimetry) and estimated by the RT3 accelerometer.
3
time spent inactive may also be a focus of research.
Rowlands et al.5 investigated correlations between
the RT3 activity counts for unregulated activities
(which included hopscotch, kicking a ball and
sitting). It is difficult to interpret the relationship
between activity counts and VO2 for sitting as the
analysis was performed for all regulated activities
combined. The validity of the RT3 in measuring
other typical activities across bio-mechanically
diverse activities warrants investigation and
could be done with the portable gas analysis
system.
The ability to detect periods of inactivity is of
particular importance when studying activity levels
as so much of the day is spent sedentary in both
adults and children. The ability of the RT3 to measure inactivity accurately does provide a means of
determining periods of inactivity in those with and
at risk of diseases associated with inactivity. The
monitor can be worn during most activities of daily
life and as it is unobtrusive does not interfere with
regular activity.
In summary the RT3 was found to be a valid measure of walking and running at speeds of 3, 6 and
9 km h−1 in this cohort of children aged 7—12 years.
In addition the RT3 permits classification of specific
activity levels. On the basis of these findings the use
of the RT3 accelerometer was deemed appropriate
for the objective measurement of physical activity
in children.
Acknowledgements
Figure 2 The limits of agreement of energy expenditure
for baseline, 3 and 6 km h−1 .
ment. In both of the latter the bias was below zero
indicating the RT3 overestimated energy expenditure.
The authors would like to thank the children for
their participation in the study.
There was no external financial support for this
study.
References
Discussion
This study demonstrates that the RT3 appears
to provide a valid measure of activity in girls
and boys of 7—12 years. There were no significant differences between the measures of energy
expenditure for activities other than walking at
6 km h−1 .
There is limited data on the absolute energy
cost of activities in children of this age but walking
at 2 mph for a 36 kg person would be 1.9 kcal min−1
8 so the limits of agreement would appear acceptable in terms of classifying physical activity. The
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Please cite this article in press as: Hussey J, et al., Validation of the RT3 in the measurement of physical activity
in children, J Sci Med Sport (2007), doi:10.1016/j.jsams.2007.09.010
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Please cite this article in press as: Hussey J, et al., Validation of the RT3 in the measurement of physical activity
in children, J Sci Med Sport (2007), doi:10.1016/j.jsams.2007.09.010