Study setting and participants
The OUTDOOR ACTIVE intervention trial will be taking place in randomly selected subdistricts of the city of Bremen. The city municipality of Bremen is located in north western Germany with around 560,000 inhabitants. Bremen is organised hierarchically into 5 boroughs, 23 urban districts and 88 subdistricts. The first randomization unit are the subdistricts, where less populated subdistricts (with less than 500 inhabitants aged 65–75 years) and the five subdistricts of the OUTDOOR ACTIVE pilot study are excluded, leaving 53 subdistricts eligible for the study (see Fig. 1). The subdistricts of the city of Bremen are highly heterogeneous e.g. with respect to SES indicators (e.g. proportion of residents with low school education ranging from 18.5 to 89.2%, medium taxable income ranging from 10,069 € to 35,995 €), life expectancy (ranging from 72.4 [78.2] years to 81.0 [85.3] years in males [females]) or land use mix (e.g. proportion of recreational area ranging from 0.1 to 66.9%; all data from [28]).
Persons eligible for the study are non-institutionalized adults, aged 65–75 years residing in the sampled subdistricts. Depending of the number, either a simple random sample or the full sample of eligible participants will be invited to the study. Persons not able to give consent will be excluded.
Interventions
The eight subdistricts are manually grouped into homogenous pairs with regard to community socioeconomic status and land mix use. From these pairs, one intervention subdistrict is randomly chosen, the remainder serves as control subdistrict.
A physical activity program tailored to the specific situation in the respective subdistrict will be developed in each of the intervention subdistricts with active involvement of the general public and other stakeholders. For tailoring, data from situational analysis, baseline surveys, and participatory actions (community forum, workshops, and excursions) is used. Each development step is communicated and discussed in the community and tested for its feasibility. The intervention material is being developed as part of an ecological model [16]. Implementation will be done with the help of local stakeholders and key actors of the population to ensure sustainability. A community round table with all stakeholders will take place throughout the intervention phase in the intervention subdistricts.
Outcomes
For the formal CRT evaluation, baseline and follow up surveys are carried out in the eight study subdistricts. These include 7-days measurements of physical activity (3D accelerometer, ActiGraph, Pensacola (FL)) and fitness (modified Senior Fitness Test [29]) a short physical examination (blood pressure, short anthropometry) and a self-administered questionnaire on intrapersonal, interpersonal, and environmental determinants of physical activity. Training sessions on fitness test and physical examination are held regularly for the field staff to ensure standardised measurements.
The primary outcome is amount of PA measured by accelerometer in average counts per minutes (CPM). The secondary outcome is physical fitness. The tests are handgrip strength, chair stand, 2-min step, back scratch, sit and reach, and flamingo balance test. Further secondary outcomes are time spent outdoors in minutes per day (measured by questionnaire) and inactivity in hours per day (measured by questionnaire). The impact evaluation will include identified key determinants. Evaluation of efficacy and possible adverse effects of the PA promotion will be done stratified by sex. Mixed models will be used to account for the clustered structure of the data introduced by the two-stage sampling design.
Participant timeline, blinding and sample size
Participant timeline is depicted in Fig. 2. The four subdistrict pairs will be included consecutively and undergo identical procedures and time schedules. This design will help to control for seasonal or weather effects. Each pair starts and ends with baseline and follow up surveys in the subdistrict. In the intervention subdistrict, development and implementation of the intervention will take place. The address data will be obtained by the registry office of the city of Bremen. Prospective participants will be recruited via written and telephone contact. A detailed written feedback will be sent to all participants after the follow up survey.
The sample size calculation is based on data gathered during the OUTDOOR ACTIVE pilot study. Accelerometer-based average counts per minute (CPM) will be used for evaluation. Mean CPM ranged from 1587.5 (SD 470.7) to 1697.0 (SD 440.5) in the five highly heterogeneous subdistricts (ICC = 0.0024). CPM were consistently higher in women than in men with only moderate differences for SES. In the OUTDOOR ACTIVE intervention trial, a mean difference of 150 CPM (equivalent to standardized effect size of 0.33) is targeted. Assuming a fixed number of clusters (four intervention, four control), 204 participants will be needed in each study arm (significance level 5% two-sided, power 90%) summing up to 808 for sex-stratified trial evaluation. Assuming a fixed proportion of 30% for dropouts, 144 participants (72 female, 72 male) are needed in each of the subdistricts at baseline.
Since no intervention is developed in the control subdistricts, and active involvement of the study participants is required in the intervention subdistricts, blinding was not a feasible option for the intervention trial. Instead, for communication with the public, the study is separated into two parts: one part containing the surveys (“BUTEN AKTIV Gesundheitsuntersuchung” OUTDOOR ACTIVE health surveys), which take place in all eight subdistricts and focuses on longitudinal aspects of the ageing process; the second part containing the intervention development and implementation, takes only place in the four intervention subdistricts (“BUTEN AKTIV vor Ort” OUTDOOR ACTIVE on-site). Thus, participants of the survey might or might not be aware of the intervention development.