A Comparison of Time in Health Enhancing Physical Activity Intensities During a School Day Assessed Using Two Methods in 11 to 16 Year Old Children

Evidence suggests that physical inactivity in adolescence is a strong indicator that can predict
total abdominal obesity in young adulthood (Pietiläinen et al., 2008: Obesity, 16, 409-414).
Recommendations now state that children should accumulate at least 60 minutes of MVPA
daily (WHO, 2010: Global Recommendations on Physical Activity for Health. Geneva:
Switzerland). There are many methods that are used to measure the amount of physical
activity in a field setting. Accelerometers offer an accurate and feasible method of gathering
physical activity in free-living individuals and are becoming easier to use and buy, thus
prompting an increased use in research. Due to advances in technology, measuring heart rate
has become easier and is therefore also becoming a popular method to gain insight into
physical activity intensity in a free-living environment (Achten & Jeukendrup, 2003: Sports
Medicine, 33, 517-538). The purpose therefore was to compare time spent in health enhancing
physical activity intensities and achievement of method-specific physical activity guidelines
assessed using heart rate monitoring and accelerometry in 11 to 16 year old school children.
Following ethical approval, 42 children (Male=20, Female= 22; Age 13.4±1.7yrs; height
156.9±8.7cm; weight 59.7±15.4kg) completed the study. The accelerometers and heart rate
monitors were fitted before school worn throughout the school day. Time spent with
accelerometer counts above the health enhancing physical activity threshold of 58 count per 5 seconds (Puyau et al, 2002: Obesity Research, 10, 150-157) and time spent above the heart
rate thresholds formulated by 50% Karvonen formula which is suggested by Scruggs et al.
(Scruggs et al, 2005; Measurement in Physical Activity Education & Exercise Science, 9, 203-
218) and 60% of (208- 0.7*age) suggested by Laurson et al. (Laurson et al, 2008: Research
Quarterly for Exercise and Sport, 79, 85-91) were calculated in minutes (ACC, HR1, HR2
respectively). Pearson correlation identified that there were relationships between the three
methods (ACC-HR1, r=0.39 p<.05; ACC-HR2, r=0.58, p<0.001; HR1-HR2, r=0.74,
p<0.001). A mixed factorial ANOVA was performed using the different methods as within
subject factors and gender, age and BMI category as the between subject factors. Sphericity
was assumed (Mauchly's χ2=4.60, p=0.1) and a significant within-subjects main effect for
method was identified (F(2,32)=89.04, p<.001, partial eta2 = 0.85) with no interaction effects.
Homogeneity of variances of the between-subject effects were confirmed (Levene's tests all
p>0.05) and no main effects were identified for gender (p=0.32), age group (p=0.07) or BMI
category (p=0.38). Post hoc analyses of the within-subjects main effect for method with
Bonferroni correction identified that ACC (49.59±17.41mins) was greater than both HR1 and
HR2 (11.98 ±13.21mins & 29.48±16.66mins respectively, p<0.001) with HR2 also greater
than HR1 (p<.001). Thirteen children achieved the ACC guideline, two achieved the HR2
guideline and none achieved the HR1 guideline levels.
Relationships existed between the minutes spent in health enhancing physical activity
measured using the three different methods of quantification. There was, however,
considerable disagreement with systematic bias evident with ACC recording almost double
the amount of time spent above its threshold compared to HR2 and nearly five times more
than HR1. Combined, the findings identify that there are considerable differences between the
methods in relation to both the quantification of minutes spent in health enhancing physical
activity and the subsequent achievement of the method specific guideline thresholds. As such
comparison of achievement of physical activity guidelines when quantified using different
monitoring methods is not possible. Further cross-sectional research is required to identify
which method specific guideline attainment (accelerometer, heart rate) is most closely related
to higher health status in children and adolescents. Longitudinal research is then required to
identify if this relationship is predictive of future health status and to determine if the relation
between guideline satisfaction and health status is transient throughout the life course.