Relationships between socioeconomic position and objectively measured sedentary behaviour in older adults in three prospective cohorts

Objectives To investigate whether sedentary behaviour in older adults is associated with a systematic and comprehensive range of socioeconomic position (SEP) measures across the life course. SEP measures included prospective measures of social class, income, educational qualifications and parental social class and contemporaneous measures of area deprivation. Setting Glasgow and the surrounding (West of Scotland) combined with Edinburgh and the surrounding area (the Lothians). Participants Community-dwelling adults aged around 79, 83, and 64 years from, respectively, the Lothian Birth Cohort 1936 (LBC1936) (n=271) and the 1930s (n=119) and 1950s (n=310) cohorts of the West of Scotland Twenty-07 study. Primary outcome measure Sedentary behaviour was measured objectively using an activPAL activity monitor worn continuously for 7 days and used to calculate percentage of waking time spent sedentary. Results Among retired participants, for most cohort and SEP combinations, greater social disadvantage was associated with increased sedentary time. For example, in the Twenty-07 1930s cohort, those most deprived on the Carstairs measure spent 6.5% (95% CI 0.3 to 12.7) more of their waking time sedentary than the least deprived. However, for employed people, the relationship between SEP and sedentary behaviour was much weaker. For example, in terms of social class differences, among the retired, the most disadvantaged spent 5.7% more waking time sedentary (95% CI 2.6% to 87%), whereas among the employed, there was effectively no difference (−0.5%; 95% CI −9.0 to 8.0). Conclusions Diverse SEP measures were associated with increased sedentary behaviour among retired people. There was little evidence for a relationship between SEP measures and sedentary behaviour among employed older adults. Prior to retirement, the constraints of the workplace may be masking effects that are only apparent at weekends.

unsurprising that trends were not picked up in working adults because higher SEP participants will likely have office based jobs and a much smaller proportion of their sitting hours are discretionary, thus vastly reducing individual variability. 6. There are no real explanations offered in the discussion to interpret the results. why do more socially disadvantaged people sit more?? Is it simply being explained by confounding?

GENERAL COMMENTS
Thank you for the opportunity to review this very interesting study. Please see below some suggestions that could improve the clarity of your manuscript.
Abstract "For example, the social class difference in percentage waking time sedentary was -0.5% (95% CI -9.0 to 8.0) for employed people in the Twenty-07 1950s cohort as opposed to 5.7% (95% CI 2.6% to 8.7%) for the retired people in all cohorts combined." The % difference is not clear. Who had 0.5% less sedentary time the least socially deprived or the most socially deprived? Please clarify.
"To be eligible, participants had to …. complete sleep diaries." Are these the eligibility criteria for your study or for the cohort studies included?
In the methods section please comment whether the sample was representative of the general population of the area or whether it was a convenient sample.
Were the data about SEP and sedentary behavior collected similarly among the three cohorts or were there any difference in how questions were asked and response options?
Please provide more details about the ActivPAL device. How often data points were collected? How were data processed and analyzed?
Provide more information about the missing data (9% of your sample)? How many participants were excluded because of missing ActivPAL data? Did any participant remove the device? Did any device stop working during data collection? Was this 9% of the sample different from the rest of the group regarding demographics and SEP measures?
Sedentary time can be different between weekdays and weekends especially for employed participants? As a sensitivity analysis it could be interesting to repeat your analysis separate for weekend and weekdays and examine whether this may have an impact on the findings.
Provide more details about how "employed" versus "retired" was defined. How was this determined? Did people have to work full time? What about volunteer positions? Were the questions similar for all cohorts?

VERSION 1 -AUTHOR RESPONSE
Thank you for the opportunity to review this very interesting study. Please see below some suggestions that could improve the clarity of your manuscript.
Abstract "For example, the social class difference in percentage waking time sedentary was -0.5% (95% CI -9.0 to 8.0) for employed people in the Twenty-07 1950s cohort as opposed to 5.7% (95% CI 2.6% to 8.7%) for the retired people in all cohorts combined." The % difference is not clear. Who had 0.5% less sedentary time the least socially deprived or the most socially deprived? Please clarify.
"To be eligible, participants had to …. complete sleep diaries." Are these the eligibility criteria for your study or for the cohort studies included?
In the methods section please comment whether the sample was representative of the general population of the area or whether it was a convenient sample.
Were the data about SEP and sedentary behavior collected similarly among the three cohorts or were there any difference in how questions were asked and response options?
Please provide more details about the ActivPAL device. How often data points were collected? How were data processed and analyzed?
Provide more information about the missing data (9% of your sample)? How many participants were excluded because of missing ActivPAL data? Did any participant remove the device? Did any device stop working during data collection? Was this 9% of the sample different from the rest of the group regarding demographics and SEP measures?
Sedentary time can be different between weekdays and weekends especially for employed participants? As a sensitivity analysis it could be interesting to repeat your analysis separate for weekend and weekdays and examine whether this may have an impact on the findings.
Provide more details about how "employed" versus "retired" was defined. How was this determined? Did people have to work full time? What about volunteer positions? Were the questions similar for all cohorts?

GENERAL COMMENTS
To provide additional information about the feasibility of using accelerometers/inclinometers, please add the following information from your response to the manuscript "two participants were excluded because they did not return activPAL devices. Seven were excluded because there was incomplete sleep diary data. Five were excluded due to poor activPAL quality, assessed using a graphical inspection of the data and 59 due to having sufficient days of data. The reasons for insufficient days of data were varied and not always reported. Eight removed devices due to skin irritation, in other cases the device had fallen off, become wet, or removed for a variety of other reasons. "