For the HSE 2012, a sample of 1,732 unique children were included in the present analysis, of which 50% were female. Table 1 presents means of total and domain-specific MET mins per week by individual factors for the HSE 2012. For the HSE 2015, 5,346 unique individuals were entered into the analysis, of which 49% were female. Table 2 presents means of total and domain-specific MET mins per week by individual factors for the HSE 2015. The data are presented in separate tables as, while the means and standard deviations broadly correspond across datasets, the inclusion of school physical activity within HSE 2015 renders the estimates not directly comparable. Comparisons between HSE 2012 and 2015 data reveal that although occasional differences are observed between isolated subgroups, no systematic differences occur between survey iterations.
Effect by Sex
For the HSE 2012, boys reported higher total levels of physical activity than girls (t1732 = 4.86, p < 0.001). There were only minor differences between boys and girls on totals for active travel and non-specific physical activity, with no specific differences emerging when stratified by age group. Boys accrued a higher percentage of physical activity from formal sports than girls (t1674 = 5.47, p < 0.001), while girls conversely recruited a higher percentage of physical activity from informal activities than boys (t1674 = -2.83, p = 0.005), although boys still reported higher absolute levels of informal physical activity (t1730 = 3.15, p = 0.005). When stratified by age group, informal activity only differed significantly between sexes within the 13- to 15-year-old group, but formal activity showed large differences between sexes in all school-age groups, with males consistently achieving more informal activity and total physical activity than females.
A similar pattern was observed within the HSE 2015 data, with boys reporting significantly higher physical activity levels than girls on total MET mins per week including (t5436 = 7.29, p < 0.001) and excluding (t5436 = 7.19, p < 0.001) school physical activity. These differences between boys and girls largely persisted when stratified by age. Boys again recruited a higher percentage of total physical activity than girls from formal activity (t5122 = 10.30, p < 0.001), with girls reporting a higher percentage from informal activity (t5122 = -1.61, p = 0.107), although once again boys reported higher absolute amounts within both domains. The remaining domains of active transport, non-specific, and school physical activity showed little variation by sex.
Effect by Age
Within the HSE 2012, age predicted outcomes on total MET minutes per week, and on all domain specific physical activity, on linear regressions controlling for sex. Overall, age was positively correlated with active travel, non-specific, and formal physical activity, but negatively with total and informal physical activity. Domain-specific contributions from each domain to total MET minutes per week are presented in Fig. 1, while summary results from regressions are presented in Additional file 1. Further regressions were run stratifying for sex, finding age a significant predictor of domain-specific outcomes. However, while age was a predictor of total MET minutes per week for females (B = -236.78, p < 0.001) it was not for males (B = -5.06, p = 0.949).
For the HSE 2015 data, age again predicted outcomes on total MET minutes per week, both including and excluding school physical activity, and predicted all domains of physical activity. Age was once more positively correlated with totals for active travel, non-specific physical activity, formal activity, and also for school-based activity, and was negatively correlated with informal activity and total MET minutes per week both including and excluding the contribution from school time. Domain-specific contribution to total MET minutes per week is presented in Fig. 2, with further summary results presented in Additional file 2. Age remained a predictor for all outcomes when stratifying by sex, with the exception of total MET minutes per week including school activity for boys (B = -7.71, p = 0.876).
The HSE 2015 incorporated a measure of physical activity within curriculum time, the first time this had been included in any HSE iteration. The relative contributions of school activity are shown in Fig. 2 stratified by age and sex, and more explicitly in Additional file 3. There were significant increases in school physical activity levels for both sexes, with age predicting an increase in school-based MET minutes per week for both boys (B = 87.69, p < 0.001) and girls (B = 74.67, p < 0.001).
Effect by Deprivation
Within the HSE 2012, QIMD was not a strong predictor of total MET minutes per week for males (B = -42.96, p = 0.583) and was only weakly associated for females (B = -124.41, p = 0.054). There were no significant effects for QIMD across domain-specific totals with the exception of formal physical activity, for which increasing QIMD was a highly significant predictor (B = -58.84, p < 0.001) of reduced activity, an effect which persists when stratified by sex. Details on domain-specific regression coefficients are displayed in Additional file 1, with relative contributions from each sex presented in Additional file 4.
For HSE 2015, QIMD did not predict total MET minutes per week including (B = 19.10, p = 0.554) or excluding (B = 26.67, p = 0.407) school activity. Increasing QIMD negatively associated non-specific, formal, and school activity levels, but positively predicted levels of informal activity (see Additional file 2). Stratification by sex revealed no differences between sexes on specific outcomes with the exception of school activity, which showed an association for females (B = -14.93, p = 0.001) but not for males (B = -1.00, p = 0.823). The relative contributions from specific domains to total MET minutes per week are presented in Additional file 5.
Effect by Weight Status
For the HSE 2012 data, weight status did not predict either total MET minutes per week (B = 122.26, p = 0.296) nor any domain-specific total, nor did any significant effects emerge when stratifying by sex (further detail in Additional file 6). On the HSE 2015, weight status significantly predicted total MET minutes per week both including (B = -222.65, p = 0.002) and excluding (B = -222.60, p = 0.002) school activity, and persisted when stratifying by sex. Increasing weight status also predicted reductions in informal activity (B = -174.95, p = 0.005). When stratifying by sex, boys retained a significant effect for formal (B = -81.04, p = 0.045) but not informal (B = -112.49, p = 0.202) activity, while girls retained a significant effect for informal (B = -249.21, p = 0.004) but not formal (B = -15.95, p = 0.626) activity (full effects by weight status and relative contributions for each domain stratified by weight and sex are presented in Additional file 7).
Effect by Ethnicity
Within the HSE 2012, participants from different identified ethnicities showed a significant variation on total MET minutes per week (F4 = 6.02, p < 0.001) when controlling for age and sex. At domain level, there were no large differences on active travel or non-specific activity levels but there were effects on formal (F4 = 5.83, p < 0.001) and informal (F4 = 4.54, p = 0.001) activity. The relative contribution from domains to total MET minutes per week by ethnicity and sex for the 2012 HSE shows clear disparities within sex groups between identified ethnic groups (see Additional file 8). All significant effects persisted when stratified by sex (see Additional file 9).
For the HSE 2015, different identified ethnicities showed a significant variation on total MET minutes per week both when including (F4 = 28.97, p < 0.001) and excluding (F4 = 28.12, p < 0.001) school activity, when controlling for age and sex (see Additional file 10). There were significant effects for all domain-specific totals, which persisted when stratified by sex with the exception of boy’s active travel (F4 = 0.79, p = 0.533) and school activity (F4 = 0.59, p = 0.667) (see Additional file 11).
Effect by Geographical Region
Within the HSE 2012, total MET minutes per week did not vary significantly between region of residence (F8 = 1.07, p = 0.380) when controlling for age and sex (see Additional file 12). With the exception of formal activity (F8 = 1.96, p = 0.049), no regional variations were noted on domain-specific totals and no significant variation was noted in any outcome when stratifying by sex (see Additional file 9). Within the HSE 2015, total MET minutes per week significantly varied across region of residence when including (F8 = 5.88, p < 0.001) and excluding (F8 = 6.41, p < 0.001) school activity, as well as for all domain-specific totals with the exception of active travel (see Additional file 13). When stratifying by sex, effects remained significant for girls but not for boys on total MET minutes per week both including and excluding school activity, a pattern that also held for non-specific activity (see Additional file 11). Effects remained significant for both boys and girls on formal and informal activity.
Stratified Analysis by Levels of Physical Activity
In addition to the above analyses of the whole survey sample, it is possible to stratify the cohorts according to levels of physical activity to provide a more nuanced description of the data. Stratifying the HSE 2012 cohort by levels of physical activity reveals the relative contributions from each domain of physical activity to total MET minutes per week for activity-based quintiles of the population. The results of linear regressions for each domain by physical activity quintile show highly significant effects for each domain. For the HSE 2015 data, all domain-specific contributions showed significant regression effects by physical activity quintile when controlling for age and sex, effects which persisted when stratifying by sex. There were no statistically significant differences between survey years on comparable domains, although considerable variation appears when represented graphically. The percentage contributions across both HSE 2012 and HSE 2015 from comparable domains are presented in Fig. 3, with further data specifically including school activity from HSE 2015 presented in Fig. 4, and further detail in Additional file 14.