The main findings of this study were: 1) Compared to objective, accelerometer-derived measures, using the IPAQ to determine activity measures led to significant over-reporting of physical activity and under-reporting of sedentary behaviour. The concordance correlation coefficient for accelerometer-derived vs
IPAQ-reported activity measures was reasonably strong for sedentary behaviour (Pc
<0.0001), but much weaker for indices of physical activity (Pc
≤0.22 for all measures), indicating that the IPAQ quantified sedentary behaviour more accurately than it quantified physical activity. 2) For some metabolic and vascular disease risk factors, significant trends were observed between amount of sedentary behaviour or MVPA and the risk factor when activity was assessed by IPAQ. However, for other risk factors significant trends with amount of sedentary behaviour or MVPA were only apparent when activity was assessed objectively by accelerometer. In addition, significant interactions were observed for some risk factors (insulin, triglyceride and HOMAIR
) in the gradient of the activity vs
risk factor regression lines for accelerometer-derived compared to IPAQ-reported activity measures. These data suggest that, compared to the use of objective accelerometry, using the IPAQ to estimate physical activity and sedentary behaviour may result in failure to detect real relationships with metabolic and vascular disease risk factors or in underestimation of the strength of those relationships. 3) Irrespective of the activity measurement method, time spent engaging in sedentary behaviour was more robustly associated with the cardio-metabolic risk profile than time spent undertaking MVPA.
The IPAQ systematically overestimated vigorous activity by ~8 minutes per day (4-fold), and, when walking was included in the moderate activity domain, overestimated moderate activity by 55 minutes per day – a 2.6-fold difference. However, agreement between accelerometer- and IPAQ-derived indices of sedentary behaviour was somewhat better with a mean difference of 69 minutes per day (~13%), despite the measures not being directly equivalent (i.e. time spent sitting down vs
time at <100 accelerometer counts.min−1
). These data suggest that while reports of absolute amounts of physical activity determined by the IPAQ questionnaire should be viewed with caution – a minute of IPAQ-reported physical activity is not equivalent to a minute of accelerometer-derived activity – questionnaire-derived estimates of sedentary time agree reasonably well with the objective measure. The Pearson correlations observed between IPAQ-reported and accelerometer-derived indices of physical activity in the present study were in line with, or slightly higher than, previous reports in the literature 
, indicating that the IPAQ performed at least as well in our hands as for others. Thus, the relative differences in the magnitude of dose-response relationships with risk factors between IPAQ-reported and accelerometer-derived activity measures observed in the present study are likely to be broadly transferable to populations beyond that used in the present study.
In the analysis reported here, the relationships between activity variables and risk factor levels were determined in two different ways. Firstly, β-coefficients for the change in risk factor level per unit change in sedentary behaviour or MVPA were calculated for the IPAQ-reported and accelerometer-determined activity measures. This provides a measure of the unit change in risk factor per 100 minute change in sedentary behaviour or per 100 MET.min−1
change (equivalent to 25 minutes of moderate activity) in MVPA. This approach revealed significant relationships between sedentary behaviour and all of the measured risk factors, irrespective of whether sedentary behaviour was assessed by IPAQ or accelerometer. However, the β-coefficients for insulin and HOMAIR
were over 50% larger when sedentary behaviour was determined objectively by accelerometer, indicating that using the IPAQ for activity assessment leads to a significant underestimation of the steepness of the dose-response relationship between sedentary behaviour and metabolic risk factors related to insulin resistance. The IPAQ also performed less well for MVPA than it did for sitting time in revealing significant associations with risk factors that were evident when activity was objectively measured. The IPAQ missed significant trends for MVPA that were evident for the accelerometer for total, LDL and HDL cholesterol, triglyceride, BMI, waist circumference and body fat, with only significant trends for glucose, insulin and HOMAIR
being detected. Furthermore, the β-coefficients for insulin, triglyceride and HOMAIR
were significantly lower for the IPAQ-reported compared to accelerometer-derived MVPA measures. These findings are in broad agreement with the NHANES report which found that the relationships of MVPA with a large number of risk factors were less strong when activity was assessed by questionnaire rather than accelerometer 
. The present findings extend the NHANES data by presenting data for sedentary behaviour as well as MVPA.
In a separate analysis, the trends for activity measures with risk factors were presented in terms of change in risk factor per quartile change in MVPA or sedentary behaviour within the population, rather than change in risk factor by change in sedentary behaviour or MVPA in terms of absolute units (i.e. per min.day−1 or per MET.min−1.day−1). This approach, in effect, adjusts for any systematic bias in reporting from the questionnaires. For example, the mean reported MET.min.day−1 value for MVPA for IPAQ was 2.6 times the accelerometer-derived MVPA measure, but a consistent 2.6 fold over-reporting of MVPA with the IPAQ compared to the accelerometer measure would have no effect on the population distribution into quartiles of MVPA. Using this approach, the present data revealed that the trends for changes in MVPA/sedentary behaviour across the population distribution with a wide range of vascular and metabolic risk factors were broadly similar – there were no significant interactions with measurement method in the activity vs risk factor relationships for any risk factor – but not all of the significant trends identified when activity was objectively quantified were observed with the IPAQ. This suggests that imprecision of measurement of activity – i.e. regression dilution bias – when using the IPAQ reduced the ability to detect significant trends with some risk factors, but the overall pattern of the activity vs risk factor trends were similar for both measurement approaches. Thus, the relative imprecision of activity measurement by IPAQ could potentially be overcome by studying larger cohorts, but accelerometers offer the advantage of detecting significant trends with risk factors in smaller numbers of individuals, when activity variables are expressed in terms of position in the population distribution. However, while this analytical approach is useful in determining general trends between activity and risk factors, it does have the clear limitations of being unable of providing absolute activity values to quantify the dose-response relationships, as well as having less statistical power than when activity behaviour is considered as a continuous variable.
The observation that the dose-response relationship between activity measures and some risk factors is quantitatively different between self-reported and objectively measured activity is an important consideration when formulating guidelines for physical activity. Current physical activity guidelines, which were largely based on evidence from self-report activity measures, recommend that adults undertake 150 minutes of moderate intensity physical activity or 60–75 minutes of vigorous activity per week to maintain and improve health 
. The present findings suggest that the amounts of activity required for good health are likely to be lower for objectively measured (compared to reported) activity, an issue which has been alluded to by others 
. Indeed, accelerometer-derived measures of physical activity are consistently lower than values obtained from self-reported data 
. Thus, it is important that objective activity monitoring methods are used in the epidemiological studies to determine the optimal activity levels for guidelines as well as in surveillance of activity levels within the population. Using objective measures to assess compliance with guidelines that were based on evidence from self-reported activity, risks providing an inaccurate picture of the proportion of the population who are insufficiently active, which can have important implications with respect to formulation of physical activity policy.
It is of interest that sedentary behaviour was more consistently associated with vascular and metabolic risk factor levels than MVPA was. The difference was most evident for the IPAQ-reported activity measures (12 vs
3 significant β-coefficients with risk factors for sitting time vs
MVPA), so may reflect in part the fact that the IPAQ quantifies sedentary behaviour more accurately than MVPA. However, the effect was still evident, albeit to a lesser extent, for accelerometer-derived measures (12 vs
10 significant β-coefficients). Furthermore, adjusting the sedentary behaviour vs
risk factor trends for MVPA did not alter the strength of association. A number of significant trends for MVPA vs
risk factor were lost after adjustment for sedentary behaviour, even for accelerometer-derived activity measures, suggesting that this is a real biological effect and not simply a consequence of measurement error. This observation is in agreement with other published data revealing a larger effect of sedentary behaviour than physical activity on a number of vascular and metabolic risk factors 
, although this is not an unequivocal finding 
. Nevertheless, the present findings add to the growing literature highlighting the important influence of sedentary behaviour on indices of vascular and metabolic health.
A particular strength of this study is that the study population was diverse. Participants were men and women, with a wide range of educational and socioeconomic backgrounds and spanned a wide range of activity levels. Mean activity levels in the present cohort were higher than in NHANES, which represented a nationally representative sample for the US, and half the cohort lived in rural settings. Our findings were robust to adjustment for age, sex, ethnicity, environment and socio-economic status, and thus should be generalisable across a wide demographic range and particularly to populations outside the US where physical activity levels may be higher. This is also the first study to compare the effects of objective vs
subjective measurements of sedentary time on the dose-response relationship with vascular and metabolic risk factors. However, while we showed good agreement between the accelerometer-derived and IPAQ-reported sedentary time measures, it is important to recognise the <100 accelerometer counts per minute is not exactly the same as time spent sitting down, as the former measure can also include standing and some very low intensity upright activities (such as swaying). It has been suggested that contractions in postural muscles elicited by standing may confer some metabolic benefit compared to sitting 
, thus the potential inclusion of standing activities in our accelerometer-derived sedentary behaviour measure could conceivably have attenuated the apparent risk factor dose-response relationships. Further study, using inclinometers, comparing the risk factor dose-response relationships for objectively- and questionnaire-assessed sitting time is needed to address this issue.
In conclusion, the findings of this study indicate that using IPAQ to determine physical activity or sitting time reveals a number of significant trends with metabolic and vascular disease risk factors. However, the IPAQ missed some significant trends that were evident when activity was objectively assessed, particularly for MVPA, and led to underestimation of the strength of some relationships between activity and risk factors. Thus, a degree of caution is advised when using activity measurements obtained from the IPAQ to quantify dose-response relationships for activity and risk factors for metabolic and vascular disease.