The present study was conducted in Ghent (250,000 inhabitants, 156.18 square km (60.3 mile2), 1601 inhabitants/square km), Flanders, Belgium. Baseline data were collected in two waves, a first wave in December 2012 and a second wave in May 2013. Follow-up data were similarly collected in two waves, 2 years after baseline data collection (December 2014, May 2015).
Procedures and participants
At baseline, individuals who retired recently (>6 months, <5 years of retirement) and individuals who planned to retire within the next 18 months were targeted. In Flanders, the formal retirement age of the current workforce over 50 years of age varies between 58 and 65 years , but official records with information on retirement status are not publicly available. Consequently, in order to recruit a sufficient number of recently retired adults and of adults who were planning to retire within the next 18 months, the Public Service of Ghent selected a random sample of 7500 58–65 year old adults from the municipal register. At baseline, all these adults received an invitation letter with information on the study (2500 adults in December 2012, 5000 adults in May 2013). After 2 weeks, a reminder was sent. Only adults who planned to retire within the next 18 months, and those who had been retired for more than 6 months but less than 5 years could participate in the study. Recently retired adults needed to be full-time retired from their main occupation, but engaging in voluntary work was allowed. Furthermore, as PA was one of the outcome variables in this study, participants had to be able to walk 100 m without assistance in order to be eligible for the study. Adults who were willing to participate in the study and met the inclusion criteria, were asked to confirm their participation by phone or email. These adults received a postal questionnaire (with a pre-stamped envelope to return the questionnaire) including questions on socio-demographic characteristics, PA and sedentary time, and physical and mental health. In total, 597 adults (455 recently retired, 142 planning to retire) returned a complete questionnaire. Because it is unknown how many of the 7500 addressed adults were truly eligible to participate in the study, it is not possible to calculate the response rate.
After 2 years (December 2014 and May 2015) these 597 adults received the same postal questionnaire again (follow-up measurements). In total, 463 adults (77.6 %) returned a complete questionnaire at follow-up. Of these 463 participants, five were not yet retired, three did not report the month/year of retirement, and nine participants had not been working before they officially retired (seven housewives and two disabled persons). Consequently, the final sample used in the analyses consisted of 446 participants (341 recently retired adults (i.e. already retired at baseline) and 105 adults who retired between baseline and follow-up). The study protocol was approved by the ethics committee of the Ghent University Hospital (B670201215326). Written informed consent was obtained from all participants.
Physical activity and sedentary time
Self-reported PA was measured with the International Physical Activity Questionnaire (IPAQ; long past 7 days version). PA assessed by the IPAQ showed good reliability (intra-class correlations range from 0.46 to 0.96) and fair-to-moderate criterion validity compared against accelerometers (median ρ 0.30) in a 12-country study . Frequency (number of days) and duration (minutes/day) of PA in different domains were queried. Based on this information, separate estimates of weekly minutes of cycling for transport, walking for transport, household-related moderate PA, moderate-to-vigorous PA (MVPA) during gardening, (voluntary) work-related walking and MVPA, leisure-time cycling, leisure-time walking and leisure-time MVPA were calculated.
Self-reported minutes/week of passive transport, TV viewing, computer use, sitting during hobbies, household chores and sitting during meals were assessed using a translated (Flemish) version of the leisure-time sedentary behavior questionnaire developed by Salmon and colleagues . The English-language version of the questionnaire has fair to excellent reliability (intra-class range from 0.56 to 0.82). Concurrent validity, assessed against a 3-day behavioral log was fair-to-moderate, with rho’s ranging from 0.20 to 0.60 .
Socio-demographic information and physical functioning
Self-reported socio-demographics included gender, age, weight, height, current marital status (alone, married, living together, widowed, divorced), and educational level (primary, secondary, tertiary education). BMI was calculated by dividing the weight (kg) by the height (m) squared. For the analyses, educational level was dichotomized into high education (i.e. tertiary education) versus low education (i.e. primary and secondary education). Self-reported physical functioning was assessed with the physical functioning subscale of the Short Form 36 item Survey (SF-36) . This subscale comprises ten activities (e.g. moderate activities, climb several flights, bend or kneel) and participants were asked to report on a three-point scale whether or not they were restricted by their physical health to perform these activities (severely limited; somewhat limited; not limited). Based on these answers, an overall score of physical functioning was calculated, ranging from 0 to 100 with a higher score representing better physical functioning. This calculation was based on the standard protocol to process the SF-36 .
All data were analyzed using SPSS 22.0. Baseline demographic characteristics were compared between recently retired and non-retired participants using independent samples t-tests and chi-square tests. To examine whether longitudinal changes in PA and sedentary behaviors differed between retiring and recently retired adults, and to examine whether this effect was dependent on (1) educational level and (2) gender, repeated measures MANOVAs with time (baseline/follow-up) as within-subjects factor and retirement status (recently retired/retiring) as between-subjects factor were conducted. Educational level and gender were alternately entered as an additional between-subjects factor. In total, four MANOVA models were constructed: two models for self-reported PA (nine measures: walking and cycling for transport, household-related moderate PA, MVPA during gardening, (voluntary) work-related walking and MVPA, leisure-time walking, cycling and MVPA), of which one model with educational level and one with gender as additional between-subjects factor; and two (again one with educational level and one with gender as additional between-subjects factor) for self-reported sedentary time (six measures: passive transport, TV viewing, computer use, sitting during hobbies, during household chores and sitting during meals). Because all PA and sedentary time variables were positively skewed, logarithmic transformations (log10) were applied to improve normality . The repeated measures MANOVAs were conducted using these transformed variables, but for ease of interpretation, raw descriptive statistics are reported in Table 1. Time by retirement status interactions were used to test if the changes in PA and sedentary behaviors differed between retiring and recently retired adults. Significant interactions are presented graphically in Fig. 1. Time by retirement by educational level (respectively gender) interactions were used to test if the effect of retirement status differed according to educational level/gender. In order to report complete results, also time effects are reported in Table 2, but these effects were not interpreted in further detail. Statistical significance was set at p < 0.05 but because of the relatively small sample size, marginally significant results (p < 0.10) were also reported.