Accelerometer cut-points and youth physical activity prevalence
Accelerometer cut-points and youth physical activity prevalence
Accelerometer cut-points and youth physical activity prevalence
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02 081795 Mota 17/7/07 5:20 pm Page 287<br />
EUROPEAN PHYSICAL EDUCATION REVIEW [DOI: 10.1177/1356336X07081795]<br />
Volume13(3):287–299:081795<br />
EPER<br />
<strong>Accelerometer</strong> <strong>cut</strong>-<strong>points</strong> <strong>and</strong> <strong>youth</strong> <strong>physical</strong><br />
<strong>activity</strong> <strong>prevalence</strong><br />
<br />
Jorge Mota, Mónica Valente, Luísa Aires, Pedro Silva, Maria<br />
Paula Santos <strong>and</strong> José Carlos Ribeiro Research Centre in Physical<br />
Activity Health <strong>and</strong> Leisure, Faculty of Sport Sciences, University of Porto<br />
Abstract<br />
The purpose of this study was twofold: first, to examine the effects of specific <strong>cut</strong>off<br />
scoring <strong>points</strong> (on the estimated <strong>prevalence</strong> of meeting health-related guidelines<br />
for <strong>physical</strong> <strong>activity</strong> in <strong>youth</strong>) <strong>and</strong>, second, to document the differences in gender<br />
<strong>physical</strong> <strong>activity</strong> patterns according to two different <strong>cut</strong>-off <strong>points</strong>. The sample<br />
comprised 62 children (boys n = 23; girls n = 39) aged 8–16 years old. Children<br />
wore accelerometers for three conse<strong>cut</strong>ive weekdays. The daily time spent in<br />
moderate-to-vigorous <strong>physical</strong> <strong>activity</strong> (MVPA) was calculated using an equation<br />
regression developed for <strong>youth</strong> according to different <strong>cut</strong>-<strong>points</strong>. The data analysis<br />
from Freedson’s <strong>cut</strong> <strong>points</strong> showed that both sexes engaged in significantly (p ≤ .01)<br />
more MVPA when compared with Puyau’s <strong>cut</strong> <strong>points</strong>. Boys engaged in significantly<br />
(p ≤ .01) more MVPA activities than girls regardless of the <strong>cut</strong>-off point used. Our<br />
data also showed that the percentage of students that reach the <strong>physical</strong><br />
<strong>activity</strong>/health related guidelines was significantly higher in both boys (77.3 vs 6.9<br />
percent; p < .001) <strong>and</strong> girls (60 vs 2.3 percent; p < .001) when Freedson’s <strong>cut</strong>-off<br />
point was used. Our data showed that for preventive strategies <strong>youth</strong> specific <strong>cut</strong>off<br />
<strong>points</strong> still need to be refined <strong>and</strong>, as a result, health-related criteria for young<br />
people need to be based on further research evidence.<br />
Key-words: accelerometer • health-related criteria • <strong>physical</strong> <strong>activity</strong> • <strong>youth</strong><br />
Introduction<br />
PROOF ONLY<br />
Physical in<strong>activity</strong> is associated with increased risk of several chronic diseases<br />
(Raitakari et al., 1997). In adults, it was observed that each 1-MET increase in exercise<br />
capacity conferred a 12 percent improvement in survival (Myers et al., 2002). It has<br />
been suggested that in<strong>activity</strong> during <strong>youth</strong> is linked to several health-related risks<br />
in adulthood (Powell <strong>and</strong> Dysinger, 1987; Twisk et al., 1997). As researchers begin<br />
to explore the <strong>physical</strong> <strong>activity</strong> (PA) dose-response relationship with health<br />
parameters (Bouchard, 2001), it is increasingly important to provide a more precise<br />
estimate of both the quantity <strong>and</strong> quality of <strong>physical</strong> <strong>activity</strong> (Sallis <strong>and</strong> Saelens,<br />
Copyright © 2007 North West Counties Physical Education Association <strong>and</strong> SAGE Publications (Los Angeles, London, New Delhi <strong>and</strong> Singapore)<br />
www.sagepublications.com
02 081795 Mota 17/7/07 5:20 pm Page 288<br />
288 EUROPEAN PHYSICAL EDUCATION REVIEW 13(3)<br />
2000). Precise measures of habitual <strong>physical</strong> <strong>activity</strong> are necessary in studies designed<br />
to: (1) document the frequency <strong>and</strong> distribution of <strong>physical</strong> <strong>activity</strong> in defined<br />
population groups, (2) determine the amount or dose of <strong>physical</strong> <strong>activity</strong> required to<br />
influence specific health parameters (Trost et al., 2000).<br />
However, the assessment of PA in childhood <strong>and</strong> adolescence is a complex task,<br />
hampered by methodological difficulties in which quality measures play a critical role<br />
(Anderson et al., 2005; Freedson et al., 1998). Wide ranges of methods have been<br />
used to quantify <strong>youth</strong> PA behaviour. These methods include subjective measures<br />
such as child <strong>and</strong> parent self-reports <strong>and</strong> objective measures such as direct observation,<br />
heart rate monitoring, motion sensors <strong>and</strong> doubly labelled water. The<br />
advantages <strong>and</strong> disadvantages of different measures have been extensively reviewed<br />
(Sallis <strong>and</strong> Owen, 1999).<br />
Objective measures with real time data storage capabilities offer a distinct advantage<br />
over self-report methods in that they provide reliable information on patterns of<br />
<strong>physical</strong> <strong>activity</strong> within a given day or over several days (Trost et al., 2000). Activity<br />
monitors like Tritrac <strong>and</strong> MTI Actigraph accelerometer seem to be one solution to<br />
this problem (Freedson <strong>and</strong> Miller, 2000). The MTI has been shown to be highly<br />
significantly correlated with energy expenditure, as assessed by indirect calorimetry,<br />
<strong>and</strong> has a high degree of inter-instrument reliability (Ekelund et al., 2001). The<br />
validity studies for the instruments have been conducted in laboratory <strong>and</strong> field<br />
settings (Brage et al., 2003; Coe <strong>and</strong> Pivarnick, 2001). Since children are likely to<br />
engage in activities that involve bending, jumping, running <strong>and</strong> throwing as part of<br />
their daily <strong>physical</strong> <strong>activity</strong>, measurement tools should be validated for use with such<br />
activities (Ott et al., 2000 ). Studies carried out with those activities found moderate<br />
<strong>and</strong> significant correlations, <strong>and</strong> the researchers concluded that accelerometers are an<br />
appropriate instrument for measuring children’s free-play <strong>physical</strong> <strong>activity</strong> (Puyau<br />
et al., 2002).<br />
General guidelines are widely used to describe the amount of <strong>physical</strong> <strong>activity</strong><br />
related to health benefits in <strong>youth</strong> <strong>and</strong> are described elsewhere (Cavill et al., 2001;<br />
Corbin <strong>and</strong> Pangrazi, 1998). Estimates of adherence to published recommendations<br />
for PA depend on estimates of time spent engaging in moderate-to-vigorous <strong>physical</strong><br />
<strong>activity</strong> (MVPA). Since policy <strong>and</strong> programme strategies are based on <strong>prevalence</strong><br />
estimates for meeting these guidelines, it is essential that <strong>prevalence</strong> estimates are<br />
accurate (Sarkin et al., 2000). Nonetheless, <strong>prevalence</strong> rates are dependent on the<br />
instrument used to assess PA (Sarkin et al., 2000). For example, it was reported that<br />
for children the extent of sex <strong>and</strong> ethnic differences in children’s PA depends on the<br />
measure used (Sallis et al., 1998). Riddoch <strong>and</strong> Boreham (Anderson et al., 2005)<br />
raised this point, commenting that different conclusions had been reached depending<br />
upon the criterion selected to distinguish <strong>activity</strong> from in<strong>activity</strong>. Indeed, several<br />
age-specific <strong>activity</strong> thresholds have been suggested for <strong>youth</strong> with different<br />
accelerometer <strong>cut</strong> <strong>points</strong> proposed (Freedson et al., 1998; Puyau et al., 2002). This<br />
problem raised questions about the difference in the <strong>prevalence</strong> estimates of PA.<br />
PROOF ONLY
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MOTA ET AL.: ACCELEROMETER CUT-POINTS AND YOUTH PHYSICAL ACTIVITY 289<br />
However so far, to the best of our knowledge, no studies have attempted to<br />
compare the <strong>prevalence</strong> rates of health-related <strong>physical</strong> <strong>activity</strong> criteria in <strong>youth</strong>.<br />
Therefore, the purpose of this study was twofold: (1) to analyse the magnitude of<br />
differences in the <strong>prevalence</strong> estimates of accomplishment of <strong>youth</strong> PA guidelines<br />
(≥ 60 min/day MVPA) using two age-adjusted MTI <strong>cut</strong>-<strong>points</strong> values; <strong>and</strong> (2) to<br />
document the gender differences in PA according to different two different <strong>cut</strong>-point<br />
systems.<br />
Methodology<br />
Sample<br />
The participants for this study were a subsample from a longitudinal study to assess<br />
the cardiovascular risk factors in a Portuguese paediatric population. More detailed<br />
information is described elsewhere (Duarte et al., 2000). Data were collected from 84<br />
volunteer students recruited from local schools (Porto region). Of the total sample of<br />
84 students, 22 were excluded from the analysis due to incomplete MTI data. The<br />
sample for the present study comprised of 62 children (boys n= 23 <strong>and</strong> girls n= 39),<br />
aged 8 to 15 years old. Study procedures were approved by the Portuguese Ministry<br />
for Science <strong>and</strong> Technology. Informed written consent was obtained from the<br />
children’s parents <strong>and</strong> individual school principals.<br />
Anthropometric measures <strong>and</strong> body composition<br />
Stature was measured using the Harpenden Portable Stadiometer (Holtain Ltd, UK).<br />
The values of stature were recorded in centimetres to the nearest mm. Body mass was<br />
measured to the nearest 0.1 kg with an electronic weighing scale (Tanita Inner Scan<br />
BC 532, UK), with the subjects in T-shirt <strong>and</strong> shorts. Body mass index (BMI) (kg/m 2 )<br />
was calculated from the ratio of weight /height 2 .<br />
Daily <strong>physical</strong> <strong>activity</strong><br />
The MTI model 7164 accelerometer (Manufacturing Technology Incorporated, Fort<br />
Walton Beach, FL) was used as an objective measure of daily <strong>physical</strong> <strong>activity</strong>. The<br />
MTI (formerly known as the CSA) uniaxial accelerometer is a small, lightweight<br />
single-channel accelerometer designed to measure <strong>and</strong> record acceleration ranging in<br />
magnitude from 0.05 to 2.00 G with frequency response from 0.25 to 2.50 Hz. The<br />
filtered acceleration signal is digitized <strong>and</strong> the magnitude is summed over a userspecified<br />
period of time (an epoch interval). At the end of each epoch, the summed<br />
value is stored in memory <strong>and</strong> the numerical integrator is reset. This process can<br />
repeat itself for 22 conse<strong>cut</strong>ive days, if a one-minute epoch is used, before the memory<br />
is filled. With a reader interface unit (RIU) connected to a computer, it is possible to<br />
PROOF ONLY
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290 EUROPEAN PHYSICAL EDUCATION REVIEW 13(3)<br />
download the recorded data <strong>and</strong> use the software supplied with the RIU to<br />
analyse it.<br />
Protocol <strong>and</strong> MTI data reduction<br />
Each student in the present study was scheduled to wear the MTI for three conse<strong>cut</strong>ive<br />
school weekdays (Tuesday to Thursday) between April <strong>and</strong> May 2004. For<br />
the present study, the epoch duration or sampling period was set at 1 minute <strong>and</strong><br />
output was expressed as counts per minute (cnts.min –1 ). The accelerometer was placed<br />
in a small nylon pouch <strong>and</strong> firmly adjusted at the child’s right hip by an elastic waist<br />
belt. A data sheet was given to each child who was instructed to record the time when<br />
the monitor was attached in the morning <strong>and</strong> detached in the evening <strong>and</strong> every time<br />
he/she performed any restricted activities like showering <strong>and</strong> swimming. The<br />
programme excludes from analysis MTI output equal to zero for more than 20<br />
continuous minutes, assuming that the device is not being worn during these periods.<br />
Subjects were included in the analysis only if they had three complete days of<br />
MTI data which included at least 12 hours of recorded time for each day.<br />
Statistical analysis<br />
Analysis was carried-out with SPSS (Windows version 11.0). Descriptive statistics<br />
were used in order to characterize <strong>and</strong> describe the sample. The age-specific count<br />
ranges developed by Freedson et al. (1997) [F<strong>cut</strong>-point] <strong>and</strong> Puyau et al. (2002)<br />
[P<strong>cut</strong>-point] were used to determine the number of minutes of MVPA. For the<br />
analysis, only MVPA was considered since <strong>youth</strong> health-related <strong>physical</strong> <strong>activity</strong><br />
guidelines stressed the health-related participation in MVPA (Cavill et al., 2001).<br />
Thus the daily time spent in MVPA (≥ 3 MET) was calculated by summing the<br />
minutes of moderate, vigorous, <strong>and</strong> very vigorous minutes for each day.<br />
Intra-class correlations coefficients (ICC) were used to assess the stability of the<br />
accelerometer scores across the three days of assessment, which estimates the<br />
reliability of a single day’s measure. Spearman rank order correlations were used to<br />
compare the average number of MVPA minutes over three days obtained from the<br />
two <strong>cut</strong>-off point methods. Gender differences for total time spent in MVPA were<br />
analysed by independent t-test. For testing the significance of differences among boys<br />
<strong>and</strong> girls, the chi-squared was applied to determine differences between frequencies<br />
of participation in MVPA. The level of significance was set at p < 0.05.<br />
PROOF ONLY<br />
Results<br />
Subject descriptive characteristics <strong>and</strong> MTI raw data per day are summarized by<br />
gender in Table 1. No significant gender differences were found for age <strong>and</strong> <strong>physical</strong><br />
characteristics of the sample. The average daily hours of assessment were about 14.<br />
Results are consistent with the expected sleeping habits of children <strong>and</strong> adolescents
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MOTA ET AL.: ACCELEROMETER CUT-POINTS AND YOUTH PHYSICAL ACTIVITY 291<br />
Table 1<br />
Descriptive characteristics of the sample (mean ± SD)<br />
Boys (n = 23) Girls (n = 39) p<br />
X ±SD<br />
X ±SD<br />
Age 11.1 ± 2.82 12.4 ± 2.6 .092<br />
Body mass (kg) 42.5 ± 13.3 44.5 ± 12.6 .820<br />
Height (cm) 147.9 ± 15.1 149.9 ± 13.5 .571<br />
BMI (kg/m 2 ) 20.3 ± 3.5 19.4 ± 3.2 .281<br />
Time (h/day) 14.1 ± 1.1 14.2 ± 1.1 .930<br />
MTI (counts. min –1 ) 596.32 ± 220.20 498.96 ± 160.08 .01<br />
which have been described in a similar population (Mota et al., 2003). The total<br />
amount of PA expressed as total counts (cnts.min –1 ) by the MTI were significantly<br />
higher in boys than girls.<br />
The reliability of MTI scores across days (Table 2) provides an estimate of<br />
variability of PA. The intra-class correlations coefficients (ICC) estimated for a single<br />
day for MTI scores ranged between 0.30 <strong>and</strong> 0.60 for total counts. ICC for moderate<br />
intensities either in boys (0.70–0.78) or girls (0.60–073) was higher in F<strong>cut</strong>-<strong>points</strong><br />
than for P<strong>cut</strong>-<strong>points</strong> (0.31–0.49 in boys <strong>and</strong> 0.17–0.41 in girls). The ICC for MVPA<br />
was higher, regardless of gender, in F<strong>cut</strong>-<strong>points</strong> (0.69–0.72 in boys <strong>and</strong> 0.58–0.70 in<br />
girls) compared to P<strong>cut</strong>-<strong>points</strong> (0.35–0.53 in boys <strong>and</strong> 0.42–0.66 in girls).<br />
Spearman correlation coefficient <strong>and</strong> Intra-class correlation coefficient between<br />
MVPA from MTI accelerometer <strong>cut</strong>-<strong>points</strong> is shown in Table 3. Both, ICC (0.33;<br />
p < 0.05) <strong>and</strong> Spearman’s correlation (R = 0.57; p = .005) were statistically significant<br />
in boys but not in girls.<br />
Table 4 shows the time spent per day (min.) in MVPA according to the two <strong>cut</strong>off<br />
<strong>points</strong> by gender. As expected, the MVPA mean number in minutes is different<br />
according to the <strong>cut</strong>-point used. Total MVPA (min) was significantly higher when<br />
using the F<strong>cut</strong>-point (149 <strong>and</strong> 111 min.d –1 ) compared to the P<strong>cut</strong>-point (35 <strong>and</strong><br />
27 min.d –1 ). Boys engaged in significantly (p ≤ .01) more MVPA than girls if the<br />
F<strong>cut</strong>-point was used, while no statistical significance was found for the P<strong>cut</strong>-point.<br />
The <strong>prevalence</strong> estimates of accomplishment of <strong>youth</strong> PA guidelines (≥ 60<br />
min/day MVPA) using the two age-adjusted MTI <strong>cut</strong>-<strong>points</strong> methods are shown in<br />
Figure 1. The results showed that the percentage of students that reach the <strong>physical</strong><br />
<strong>activity</strong>-health related criteria was significantly higher in both boys (95.7% vs 17.4%,<br />
p < .000) <strong>and</strong> girls (87.2% vs 5.1%, p < .000) when the F<strong>cut</strong>-point was used instead<br />
the P<strong>cut</strong>-point.<br />
PROOF ONLY<br />
Discussion<br />
The aim of this study was to analyse the magnitude of differences in the <strong>prevalence</strong><br />
estimates of accomplishment of <strong>youth</strong> PA guidelines (≥ 60 min/day MVPA) using<br />
two age-adjusted MTI <strong>cut</strong>-<strong>points</strong> values. It might seem intuitive that using different
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292 EUROPEAN PHYSICAL EDUCATION REVIEW 13(3)<br />
Table 2 The reliability (intra-class correlations coefficients, ICC) of MTI scores across days<br />
Activity BOYS GIRLS<br />
Total Counts ICC (CI 95%) ICC (CI 95%)<br />
Day 1 vs Day 2 0.48 (0.09–074) *** 0.35 (0.04–0.59) ***<br />
Day 1 vs Day 3 0.60 (0.26–0.81) *** 0.40 (0.1–0.63) ***<br />
Day 2 vs Day 3 0.52 (0.14–0.76) *** 0.30 (0.02–0.55) ***<br />
F<strong>cut</strong>-point P<strong>cut</strong>-point F<strong>cut</strong>-point P<strong>cut</strong>-point<br />
Moderate ICC (CI 95%) ICC (CI 95%) ICC (CI 95%) ICC (CI 95%)<br />
Day 1 vs Day 2 0.78 (0.39–0.86) *** 0.36 (0.03–0.67)* 0.60 (0.35–0.77) *** 0.41 (0.11–0.64) **<br />
Day 1 vs Day 3 0.70 (0.41–0.86) *** 0.49 (0.10–0.74)** 0.73 (0.54–0.85) *** 0.26 (0.01–0.53)<br />
Day 2 vs Day 3 0.71 (0.43–0.87) *** 0.31 (0.03–0.63) 0.69 (0.47–0.82) *** 0.17 (–0.15–0.46)<br />
Vigorous<br />
Day 1 vs Day 2 0.48 (0.09–0.74) ** –0.01 (–0.41–0.40) 0.43 (0.14–0.66) *** 0.10 (–0.22–0.40)<br />
Day 1 vs Day 3 0.50 (0.13–0.76) ** 0.32 (–0.10–0.64) 0.48 (0.15–0.66) ** 0.46 (0.17–0.67) **<br />
Day 2 vs Day 3 0.57 (0.21–0.79) ** 0.37 (–0.04–0.67)* 0.42 (0.13–0.65) ** 0.10 (–0.22–0.40)<br />
MVPA<br />
Day 1 vs Day 2 0.72 (0.45–0.87) *** 0.35 (–0.07–0.66)* 0.18 (0.33–0.76) *** 0.42 (0.12–0.65) **<br />
Day 1 vs Day 3 0.70 (0.41–0.86)*** 0.72 (0.45–0.87)*** 0.70 (0.50–0.83) *** 0.66 (0.44–0.81) ***<br />
Day 2 vs Day 3 0.69 (0.40–0.86)*** 0.53 (0.15–0.77) * 0.64 (0.41–0.80) *** 0.66 (0.43–0.80) ***<br />
PROOF ONLY<br />
* p < .05; ** p < .01; *** p < .001.
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MOTA ET AL.: ACCELEROMETER CUT-POINTS AND YOUTH PHYSICAL ACTIVITY 293<br />
Table 3 Spearman correlation (Rho) <strong>and</strong> Intra-class correlation coefficient (ICC)<br />
between MVPA from MTI accelerometer <strong>cut</strong>-<strong>points</strong><br />
MTI DATA<br />
ICC (CI-95%) p Rho p<br />
Boys 0.33 (0.08–0.65) .05 0.57 .005<br />
Girls 0.16 (–0.15–0.45) ns 0.30 ns<br />
Table 4 Time spent per day (min.) in moderate, vigorous <strong>and</strong> MVPA according to the<br />
two <strong>cut</strong>-off <strong>points</strong> by gender<br />
Boys Girls p<br />
X ±SD CV X ±SD CV<br />
Freedson et al. (1997)<br />
MVPA (min. d –1 ) 149.0 ± 72.6 48.6 111.4 ± 56.9 51.1 .01<br />
Moderate (min. d –1 ) 129.8 ± 60.9 46.9 100.1 ± 49.2 49.2 .01<br />
Vigorous (min. d –1 ) 19.1 ± 14.3 74.8 11.3 ± 9.4 83.1 .01<br />
Puyau et al. (2002)<br />
MVPA (min. d –1 35.2 ± 24.5 69.6 27.8 ± 17.1 61.5 ns<br />
Moderate (min. d –1 ) 34.4 ± 23.7 68.9 27.2 ± 16.3 59.9 ns<br />
Vigorous (min. d –1 ) 0.8 ± 1.3 162.5 0.6 ± 1.2 200 ns<br />
(%) 100<br />
80<br />
60<br />
40<br />
20<br />
*<br />
PROOF ONLY<br />
*<br />
*<br />
0<br />
Boys<br />
Girls<br />
Physical Activity<br />
**<br />
Note.* p < 0.001 – differences between gender.<br />
Figure 1 Percentage of children that accomplished the <strong>youth</strong> <strong>physical</strong> <strong>activity</strong><br />
guidelines (60 minutes/day of MVPA) according to different <strong>cut</strong>-off <strong>points</strong>
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294 EUROPEAN PHYSICAL EDUCATION REVIEW 13(3)<br />
<strong>cut</strong>-<strong>points</strong> would lead to different estimates of PA <strong>prevalence</strong>, but to the best of our<br />
knowledge this is one of the first studies showing differences in PA level between two<br />
well-known <strong>cut</strong>-<strong>points</strong> used in young people MTI analysis.<br />
Published accelerometer <strong>youth</strong>-related <strong>cut</strong>-off levels corresponding to MVPA<br />
intensity levels were used in this study (Freedson et al., 1997; Puyau et al., 2002).<br />
Both proposed <strong>cut</strong> <strong>points</strong> are age-specific, which is needed since the correspondence<br />
between counts <strong>and</strong> energy expenditure at different workloads is likely to differ<br />
between age-groups (Schofield, 1985). Nevertheless, the critical point is that different<br />
<strong>cut</strong>-<strong>points</strong> influence the <strong>prevalence</strong> of MVPA among young people. To what<br />
extent <strong>youth</strong> meet the recommended guidelines is a timely issue with a public health<br />
interest, because there is a risk of misinterpretation depending on <strong>cut</strong>-<strong>points</strong> used.<br />
The analysis showed that using F<strong>cut</strong>-<strong>points</strong> both sexes engaged in significantly<br />
(p ≤ .01) more MVPA compared with P<strong>cut</strong>-<strong>points</strong>. Further, boys engaged in significantly<br />
more MVPA than girls (p ≤ 0.01) when F<strong>cut</strong>-point is considered. Comparisons<br />
of minutes of <strong>activity</strong> per day between the two <strong>cut</strong>-off <strong>points</strong> clearly showed that the<br />
P<strong>cut</strong>-point estimate significantly less minutes of MVPA <strong>activity</strong> level (p ≤ .001) than<br />
the F<strong>cut</strong>-point. Total MVPA was 149.0 vs 111.4 min.d –1 for F<strong>cut</strong>-point, <strong>and</strong> 35.2 vs<br />
27.8 min.d –1 for P<strong>cut</strong>-point, respectively for boys <strong>and</strong> girls. The findings of the<br />
present study agree with another survey using the two methods to validate an<br />
adolescent diary. In that study the sample was less active than in the present study.<br />
Total MVPA was 86.5 vs 46.9 min.d –1 for MTI F<strong>cut</strong>-point, <strong>and</strong> 27.1 vs 9.2 min.d –1<br />
for P<strong>cut</strong>-point, respectively for boys <strong>and</strong> girls. The mean values of MVPA within<br />
F<strong>cut</strong>-point are in accordance with other studies that have used objective measures of<br />
<strong>physical</strong> <strong>activity</strong>, which reported significantly higher levels of <strong>physical</strong> <strong>activity</strong> in boys<br />
than girls (Pate et al., 2002; Santos et al., 2003; Trost et al., 2002).<br />
Our data also showed that the percentage of students that reach the <strong>physical</strong><br />
<strong>activity</strong>-health related criteria (at least 60 min of MVPA/day) was significantly higher<br />
in both boys (95.7% vs 17.4%; p < .001) <strong>and</strong> girls (87.2% vs 5.1%, p < .001) when<br />
the F<strong>cut</strong>-point was used compared to P<strong>cut</strong>-point. Thus, when data were reported to<br />
F<strong>cut</strong>-point it appears that children meet minimum <strong>physical</strong> <strong>activity</strong> recommendations<br />
for health, which agrees with other studies (Hussey et al., 2001; Sleap <strong>and</strong><br />
Tolfrey, 2001). The differences in estimates of the mean number of minutes engaged<br />
in MVPA are important from a public health perspective when defining estimates <strong>and</strong> <strong>prevalence</strong><br />
of levels of health-related PA achievements. However those differences are difficult to<br />
explain. One can argue that a possible explanation lies in the fact that F<strong>cut</strong>-point<br />
tends to give higher <strong>prevalence</strong> estimates, probably because <strong>cut</strong>-offs are derived from<br />
a laboratory setting, while P<strong>cut</strong>-point was originated in daily routine activities with<br />
target on sedentary activities rather than MVPA, which may lead to a under-estimation<br />
of moderate activities. As reported for adults, the high <strong>cut</strong>-off <strong>points</strong> lead to a misclassification<br />
of some moderate <strong>physical</strong> activities (Ainsworth et al., 2000). In fact, a<br />
study carried out with the aim to evaluate the validity of a questionnaire against the<br />
two <strong>cut</strong>-<strong>points</strong> methods, showed that the best agreement was with F<strong>cut</strong>-point for<br />
students whose average total MVPA < 60 min. (Anderson et al., 2005). Additionally,<br />
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MOTA ET AL.: ACCELEROMETER CUT-POINTS AND YOUTH PHYSICAL ACTIVITY 295<br />
it might be considered that some differences in estimation of vigorous activities may<br />
relate to the epoch time (sampling interval) procedures. The 1 minute epoch was used<br />
in this study. Moreover the 5 seconds epoch has been shown as the best approach to<br />
detect vigorous PA in <strong>youth</strong> (Nilsson et al., 2002). Nevertheless, in the present study<br />
the reliability values of the MTI scores across the day were similar to other findings<br />
reported in the literature (Mota et al., 2002; Sallis et al., 1993; Trost et al., 2000). In<br />
children from grades 1 to 6, the single-day reliability of MVPA assessed by MTI<br />
ranged from 0.46 to 0.49 while the values decreased as children got older, ranging<br />
from 0.32 to 0.31 (Trost et al., 2000). The scores obtained may also be representative<br />
of subjects’ day-to-day variability in MVPA participation.<br />
Our data showed high CV values for boys <strong>and</strong> girls in vigorous activities with<br />
those related to P<strong>cut</strong>-point over 100 percent (Table1). This issue should also be<br />
considered along with gender. Indeed, our results showed statistically significant<br />
differences between boys <strong>and</strong> girls for F<strong>cut</strong>-point but not for P<strong>cut</strong>-point, which might<br />
suggest different accuracy <strong>and</strong> sensitivity of <strong>cut</strong>-<strong>points</strong> according to gender. Further,<br />
the correlations (ICC <strong>and</strong> Spearman’s correlation) between MVPA MTI <strong>cut</strong>-<strong>points</strong><br />
were statistically significant in boys but not in girls, which may be a reflection of the<br />
restricted range of MVPA among girls.<br />
Limitations to the present study should be recognized. The sample is small <strong>and</strong><br />
it is believed that a larger group of participants is needed to clarify to what extent<br />
the differences are significant for public health strategies. Further studies should be<br />
designed to improve such estimations since the validity of <strong>activity</strong> monitors vary by<br />
setting (laboratory or field) <strong>and</strong> type of <strong>activity</strong> (Welk et al., 2000).<br />
Conclusion<br />
The data clearly show that compliance with a specific PA guideline will depend on<br />
the <strong>cut</strong>-<strong>points</strong> used to interpret the data collected. Therefore, the <strong>cut</strong>-<strong>points</strong> still need<br />
to be refined, <strong>and</strong> as a result health-related criteria for young people need to be based<br />
on further research evidence.<br />
Acknowledgment<br />
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This study was supported in part by grant FCT: PSAU/122/96 <strong>and</strong> PAFID 284/2005.<br />
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Résumé<br />
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Seuil d’activation et fréquence d’Activité Physique chez les<br />
jeunes<br />
Le but de cette étude était double : tout d’abord, l’examen des effets de seuils spécifiques,<br />
puis, dans un second temps, la documentation des différences des modèles d’activité<br />
physiques liées au genre selon deux seuils différents. L’échantillon était composé de 62 enfants<br />
(garçons n = 23; filles n = 39) âgés de 8 à 16 ans. Les enfants ont porté des<br />
cardiofréquencemètres pendant trois jours consé<strong>cut</strong>ifs. Le temps quotidien d’activité<br />
physique modérée à vigoureuse a été calculé en utilisant une régression d’équation<br />
développée spécifiquement pour un jeune public au regard de seuils différents. L’analyse des<br />
données à partir des seuils de Freedson a montré que les deux sexes se sont engagés
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298 EUROPEAN PHYSICAL EDUCATION REVIEW 13(3)<br />
significativement plus (p ≤ 0.01) dans une activité physique modérée à vigoureuse qu’en<br />
comparaison avec les seuils de Puyau. Les garçons se sont engagés significativement plus<br />
(p ≤ 0.01) dans une activité physique modérée à vigoureuse que les filles indépendamment<br />
du seuil utilisé. Nos données ont également montré que le pourcentage d’élèves qui atteint<br />
le seuil d’une activité de santé en référence aux directives était significativement plus élevé<br />
tant chez les garçons (77.3 % contre 6.9 %; p < 0.001) et que chez les filles (60 % contre<br />
2.3 %; p < 0.001) en recourant au seuil de Freedson. Enfin, sur la base de nos résultats, il<br />
apparaît que, pour une stratégie préventive, les seuils spécifiques à un jeune public devraient<br />
être ré-affinés, et en conséquence les critères concernant la santé pour les jeunes devraient<br />
être basés sur de futurs résultats de recherche.<br />
Resumen<br />
Puntos de corte del acelerómetro y prevalencia de la actividad<br />
física juvenil<br />
El propósito de este estudio es doble. Por un lado se ha pretendido estudiar los efectos de<br />
la puntuación límite específica (sobre la prevalencia estimada de cumplir las directrices<br />
sanitarias sobre la actividad física en la juventud), y, por otro, se pretendía documentar las<br />
diferencias existentes en los modelos de actividad física según el sexo de acuerdo con dos<br />
puntos de corte diferentes. La muestra se realizó sobre 62 menores (23 niños y 39 niñas)<br />
de entre 8 y 16 años de edad. Los menores llevaron puesto un acelerómetro durante tres<br />
días laborables conse<strong>cut</strong>ivos. El tiempo que diariamente dedicaron a una actividad física entre<br />
moderada y vigorosa (AFMV) se calculó utiliz<strong>and</strong>o una ecuación de regresión desarrollada<br />
para los jóvenes según diferentes puntos de corte. El análisis de los datos a partir de los<br />
puntos de corte establecidos por Freedson ha demostrado que ambos sexos realizan<br />
bastante más AFMV (p ≤ 0,01) en comparación con los puntos de corte de Puyau. Los niños<br />
realizan bastante más AFMV (p ≤ 0,01) que las niñas, independientemente de los puntos de<br />
corte utilizados. Los datos que obtuvimos también indican que el porcentaje de estudiantes<br />
que cumplen las directrices sanitarias sobre actividad física fue bastante mayor tanto en el<br />
caso de los niños (77,3% frente al 6,9%; p < 0,001) como en el de las niñas (60% frente al<br />
2,3%; p < 0,001) cu<strong>and</strong>o se utilizaban los puntos de corte de Freedson. Nuestros resultados<br />
demuestran que los puntos de corte específicos para la juventud, en lo que se refiere a las<br />
estrategias preventivas, necesitan una reformulación, y, en consecuencia, los criterios sanitarios<br />
para los jóvenes deben basarse en los resultados de futuras investigaciones.<br />
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Zusammenfassung<br />
Messpunktvorgaben von Beschleunigungsmessern und<br />
Häufigkeit körperlicher Aktivität bei Jugendlichen<br />
Der Zweck dieser Studie war ein zweifacher. Zuerst galt es die Auswirkung spezifischer<br />
Grenzwertvorgaben (bezüglich der Einschätzung, mit gesundheitsbezogenen Richtlinien für
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MOTA ET AL.: ACCELEROMETER CUT-POINTS AND YOUTH PHYSICAL ACTIVITY 299<br />
körperliche Aktivitäten im Jugendalter übereinzustimmen) zu untersuchen, und zweitens<br />
sollten die Unterschiede in geschlechterspezifischen Mustern körperlicher Aktivität<br />
entsprechend zwei verschiedenen Grenzwertvorgaben dokumentiert werden.. Die<br />
Stichprobe umfasste 62 Kinder (Jungen n = 23; Mädchen n = 39) im Alter von 8–16 Jahren.<br />
Die Kinder trugen an drei aufein<strong>and</strong>er folgenden Wochentagen Beschleunigungsmesser.<br />
Die Zeit, die sie täglich für moderate bis starke körperliche Aktivität (moderate-to-vigorous<br />
<strong>physical</strong> <strong>activity</strong> (MVPA) aufwendeten wurde mittels einer Regressionsgleichung berechnet,<br />
die für Jugendliche entsprechend den unterschiedlichen Messpunktvorgaben entwickelt<br />
worden war. Die Datenauswertung nach Freedsons Schwellenwerten zeigten für beide<br />
Geschlechter signifikant mehr MVPA (p ≤ 0.01), mehr MVPA im Vergleich mit Puyaus<br />
Schwellenangaben. Unabhängig von den verwendeten Schwellenwertvorgaben beteiligten<br />
sich Jungen signifikant (p ≤ 0.01) mehr an MVPA Aktivitäten (Sic!) als Mädchen. Unsere Daten<br />
zeigten auch, dass der Prozentsatz der Schüler, die die Vorgaben der gesundheitsbezogenen<br />
Richtlinien für körperliche Aktivität erfüllten, sowohl bei Jungen (77.3% vs 6.9%; p < 0.001)<br />
als auch Mädchen (60% vs. 2.3%; p < 0.001) signifikant höher war, wenn Freedsons<br />
Schwellenwert benutzt wurde. Unsere Daten zeigten auch, dass es nötig ist, für<br />
Präventivmassnahmen jugendspezifische Messwertvorgaben noch feiner einzustellen. Weiters<br />
müssen gesundheitsbezogene Kriterien für Jugendliche auf weiteren wissenschaftlichen<br />
Erkenntnissen aufgebaut werden.<br />
Jorge Mota, Mónica Valente, Luísa Aires, Pedro Silva, Maria Paula Santos <strong>and</strong><br />
José Carlos Ribeiro are based at the Research Centre in Physical Activity Health <strong>and</strong><br />
Leisure, Faculty of Sport Sciences, University of Porto.<br />
Address for correspondence: Jorge Mota, FCDEF-UP, R.Plácido Costa, 91 4200 450,<br />
Porto, Portugal. [email: jmota@fcdef.up.pt]<br />
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