Study design
Data from three observational studies with a similar methodology were combined to examine associations between neighborhood built environmental factors and LPA across the lifespan. All three studies were conducted in Ghent, Flanders. The Belgian Environmental Physical Activity Study (BEPAS) collected data from adults (20-65 years) between May 2007 and September 2008 (24), and the BEPAS Seniors collected data from older adults (≥65 years) between October 2010 and September 2012 (25). The Belgian International Physical activity and the Environment Network (IPEN) study in Adolescents collected data from adolescents (11-17 years) between September 2014 and December 2015 (26). All three studies were approved by the Ethics Committee of the Ghent University Hospital and all participants provided written informed consent.
Participants and procedure
Stratified cluster sampling based on walkability (low vs high) and neighborhood socio-economic status (SES) (low vs high) was used to select neighborhoods (i.e. 1 to 5 adjacent statistical sectors) in Ghent (i.e. city in Flanders, Belgium) for the three observational studies (24, 25). A total of 24 neighborhoods were selected from four neighborhood types (i.e. high walkable/high SES; high walkable/low SES; low walkable/high SES; low walkable/low SES) to recruit adult participants for BEPAS (24). Subsequently, 250 adults of each neighborhood were randomly sampled by the Public Service of Ghent. For BEPAS Seniors, 20 out of these 24 neighborhoods were selected to randomly sample 1750 older adults stratified by age and gender (25). For IPEN Adolescents, 442 adolescents were randomly sampled from the 24 neighborhoods that were initially selected for BEPAS. Next to the recruitment by residential address, adolescents were also recruited from schools located in the 24 neighborhoods (26). Selected adolescents, adults and older adults received an invitation letter with the announcement of a home or school visit of a trained researcher within the next days. Candidates were considered to be eligible for the study if they lived in a private dwelling, were able to walk a couple of hundred meters without assistance and were able to fill out a Dutch questionnaire. The recruitment process resulted in a sample of 373 adolescents, 1200 adults, and 508 older adults who were found at home/school, met the inclusion criteria, and willing to participate. All participants filled in a questionnaire on sociodemographic and psychosocial factors, and physical activity. Additionally, one of the parents of each adolescent participant also completed a brief socio-demographic questionnaire. By the end of the first home/school visit, participants received an Actigraph accelerometer, which they were instructed to wear for seven consecutive days. After seven days, a second home/school visit took place to collect the Actigraph accelerometers.
Measures
Outcome variable: LPA
LPA was objectively assessed with ActiGraph 7164, GT1M, GT3X and GT3X + accelerometers (ActiGraph, Fort Walton Beach, FL, USA), which are valid and reliable tools to measure PA levels in different age groups (27-30). Accelerometers were attached using an adjustable elastic waist belt above the right hip for seven consecutive days. Participants were asked to only remove the accelerometer while sleeping, and for water-based activities, such as swimming or bathing. Accelerometer counts were collected using 60-second epochs. Non-wear time, which was defined as ≥60 min of consecutive zeros, was removed (31), and participants with less than five valid days of data (i.e. at least 10 wearing hours) were excluded from the analysis (32). According to the recommended cut points of Freedson (33), and Evenson (34), 101 through 1,951 counts/minute were considered LPA in adults and older adults, and 101 through 2296 counts/minutes were considered LPA in adolescents. The complete accelerometer data processing was performed using Actilife software version 6.
Predictor variables: GIS-based neighborhood built environmental factors
GIS-based neighborhood built environmental factors were calculated using sausage buffers of 500 m and 1000 m (1 km) around the home addresses of all participants based on the International Physical Environmental Network (IPEN) guidelines (35). Sausage buffers are preferred over the more traditional Euclidian buffers, as sausage buffers are directly based on the road networks used to travel (36). Five GIS-based neighborhood built environmental factors were included in the current study: residential density, intersection density, park density, public transport density, and entropy. Residential density was defined as the ratio between the number of residences fully or partially in the buffer and the total buffer area. Intersection density was described as the ratio between the number of three- or more-way intersections and the total buffer area. Park density was the ratio between the number of parks fully or partially in the buffer and the total buffer area, and public transport density was calculated by dividing the number of public transport stops (i.e. bus, tram, train stops) by the total buffer area. Finally, the entropy index was a measure of land use mix which took into account the relative percentage of six land use types (i.e. residential, commercial, institutional, entertainment, food and private/public recreation parcels) within the total buffer area (37).
Potential confounding/moderating variables: socio-demographic factors, valid days, and wear time
Age group, gender, educational level (primary, secondary, or tertiary), neighborhood SES, number of valid days, and wear time were selected a priori as potential confounding/moderating variables. Socio-demographic confounding variables were self-reported by the participants (or their parents) during the first home/school visit. Since adolescents were still studying, highest achieved educational level of the parent who filled in the questionnaire was included in the analyses as a proxy for their SES. Neighborhood SES was based on Belgian census income data from the National Institute of Statistics. Number of valid days and wear time were extracted from the accelerometer data.
Statistical analyses
Descriptive statistics of participants’ characteristics were calculated for the total sample and the three age groups (adolescents, adults and older adults) separately. Means and standard deviations were provided for normally distributed continuous variables, medians and interquartile ranges for skewed continuous variables, and percentages for discrete variables. Linear mixed models were performed using the lmer() function of the lme4 package in R (v 4.1.0) to account for the nested structure of the data (i.e. individuals were nested within neighborhoods) while examining the associations between GIS-based neighborhood built environmental factors and accelerometer-derived LPA (38). Firstly, a random intercept null model was fitted to estimate the variance in LPA explained at the neighborhood level. The intraclass cluster coefficient (ICC) was calculated from this model to estimate the proportion of total variance in LPA that could be attributed to neighborhood factors. Secondly, single-predictor models were run with each potential confounding variable (i.e. age group, gender, educational level, neighborhood SES, number of valid days, and wear time), and each GIS-based neighborhood built environmental factor (i.e. residential density, intersection density, park density, public transport density, and entropy) separately. Thirdly, multiple-predictor models were fitted including the significant confounding variables from the previous step, and the GIS-based built environmental neighborhood factors. Finally, the multiple-predictor models were extended with an interaction term (i.e. age group*GIS-based neighborhood built environmental factor) to investigate the potential moderating role of age group. A likelihood ratio test was used to test the significance of the interaction terms by comparing models with and without interaction terms. All single- and multiple-predictor models were run separately for the environmental variables measured in a 500 m and 1 km sausage buffer. All analyses were performed in R (v 4.1.0) and the alpha level was set 0.05.