Insufficient physical activity (PA) is one of the major risk factors for chronic diseases such as cardiovascular diseases, cancer and obesity [1, 2]. The Dutch PA guidelines state that adolescents have to engage in moderate-intensity PA for at least one hour each day [3, 4], and to engage at least three times a week for at least 20 minutes in vigorous intensity activities such as sports ("fitness norm") . Only 27% of Dutch adolescents meet this guideline , and 34% meet the "fitness norm". Similar figures have been found in other Western countries [5, 7–10]. Increasing PA is therefore important for population health. Adolescents are a particularly important group to target, since sufficient PA can result in considerable health gains for this group. The health benefits of regular PA for adolescents include a lower risk of becoming overweight or obese [2, 11], higher bone density [2, 11], a lower risk of depression  and healthier cardiovascular risk profiles . These benefits may be experienced earlier as well as later in life . Furthermore, physically active adolescents are somewhat more likely to become physically active adults . To be able to increase PA levels, it is important to develop interventions that target the most important determinants of PA.
Although socio-ecological models of health behavior have suggested that the physical environment (such as the availability and accessibility of PA opportunities such as parks, sport facilities, bicycle lanes and sidewalks) may be a potent determining factor for PA [13–15], recent reviews of the literature among adolescents  as well as adults  show that the evidence is not consistent. This indicates that more research is warranted, and that special attention should be directed at studying these associations among adolescents, since their patterns of activity and use of facilities differ from those of adults. Moreover, different conceptualisations of the environment may apply to different population groups. Therefore care should be taken in translating finding from adult literature to adolescents.
An important issue related to the study of environmental influences on PA behavior is the measurement of the physical environment, such as availability and accessibility of physical activity opportunities and barriers in the relevant environment. In recent years more detailed objective measures of PA opportunities have become available, for example, those documented in geographic information systems (GIS). Objective measures are generally regarded as being superior to subjective self-reports. However, adolescents may perceive their environments differently even if they live in the same "objective" environment. For example, a person who is motivated to be physically active may be more likely to perceive more opportunities to be physically active than someone who is less motivated. Also, adolescents who are physically active may be more knowledgeable about the available opportunities than an inactive person.
Earlier research has indicated that there are indeed differences between perceived and objectively measured environments, and both may have different associations with health behaviors. Previous studies have found associations between objective availability of parks , recreational facilities, commercial PA-related facilities  with moderate-to-vigorous PA (MVPA) among adolescents. No associations were found between objective measures of accessibility and walkability and MVPA . Objective measures of walkability  and features of school routes  were associated with active commuting to school; no associations were found for intersection density  and land use . Studies that examined associations between perceived environmental factors and PA behavior have found that perceived availability of recreational facilities [23–25] and perceived access  were not associated with MVPA. Perceived availability of walking and bicycle facilities such as sidewalks and bicycle lanes (by parents) aesthetics  and street connectivity  were, and parental perceived traffic safety was not related to active commuting to school [21, 22].
Most of the above-mentioned studies did not explicitly explore differences between objective and perceived environment. A recent paper by Ball et al.  found a mismatch between the objective and perceived environment in adults. Another study  did compare the perceived environment and the objective environment in an adolescent population and found significant associations between objective and perceived environmental factors. However, for adolescent girls only the perceived environment was related to MVPA. These studies were conducted in an older population  or a female-only sample of adolescents  and not a general sample of adolescents in Europe. Better insight into these differences between the perceived and objective environment and their associations among adolescents could have important consequences for intervention development, because changing people's perceptions requires other strategies than modifying the actual environment. Perceptions of the environment may be changed by health education techniques by making adolescents aware of possibilities to be active, whereas modifying the actual environment may involve building new parks or sports facilities or enhancing accessibility.
In studying how physical environmental factors influence PA, it is important to study PA sub-domains instead of total PA, since environmental factors may be specific to particular sub-behaviors . Hence, the availability of sports facilities may be important for engaging in sports, but not important for engaging in active transportation. The present study focuses on engagement in sports activities and walking and cycling during leisure time, as these are both important contributors to adolescent PA . The environmental factors examined in relation to these PA sub-domains were objectively measured and perceived availability of facilities for being physically active - such as parks, sports facilities, bicycle lanes and sidewalks - in the neighborhoods where adolescents live.
The aims of the present study are to 1) examine associations of objective and perceived availability of PA facilities with engagement in two leisure time activities: sports and walking and cycling during leisure time, and 2) to explore the degree of agreement and associations between objective and perceived availability of PA facilities.