Regular physical activity (PA) prevents major chronic diseases such as diabetes, cardiovascular disease, mental illness, obesity, and various types of cancer [1, 2]. Although the health benefits of regular exercise and a physically active lifestyle are well known, many people are still not active. In the Dutch population, over 40% does not meet the national recommendation of being moderately active for at least half an hour on at least five days a week [3, 4]. In the US, the percentage of people not reaching the recommended level of PA is over 50% . Therefore, increasing PA comprises a large potential public health gain [1, 6].
Previously, the promotion of PA has focused mainly on changing individual cognitions towards PA, such as attitude and self-efficacy [7, 8]. Over the past decade, the focus of research has shifted more to environmental determinants of health and health behavior . In addition, ecological models suggest that health behavior is determined by individual as well as environmental factors and that they are interrelated [10, 11]. So far, little is known about these individual-environment interactions.
Sports participation is an important element of PA. Persons who participate in sports have a lower mortality than those who do not participate in sports . In Europe, only 40% of the adult population participates in sports with some regularity, ranging from 72% in Finland, to only 13% in Bulgaria . In the US, 24% of the population is regularly vigorously physically active . An environmental factor that has been suggested to be related to PA and sports participation is neighborhood safety [14, 15]. In the US, higher levels of perceived neighborhood safety were associated with lower levels of physical inactivity . A study by McGinn and colleagues reported that both perceived as objectively measured crime were related to physical activity .
Why does neighborhood safety influence physical activity? Macintyre suggests that the importance of environmental factors related to health are roughly following the order of human needs as defined by Maslow [18, 19]. In this order of human needs, safety is one of the main needs, just after air, water, food, and shelter . When a basic need like safety is unfulfilled, higher ranked needs, like sport participation, are less relevant.
Another explanation for the association between neighborhood safety and physical activity is that people most often have to leave their house when they want to exercise. An unsafe environment might act as a barrier for sports participation. Especially since, in the Netherlands, adults are most involved in sports activities in the evenings and weekends due to other responsibilities during the day. For types of sports that start from the doorstep (like running and cycling), this association is rather obvious, as these sports completely or partly take place in the neighborhood. For sports that are played at a sports club outside the own neighborhood, neighborhood safety may also act as an important perceived barrier, as one has to travel through his or her own neighborhood to get there.
A large pan-European study showed that perception of safety was associated with an increase in the likelihood to engage in occasional exercise of 22% in women and 39% in men . Sallis and colleagues  showed that women who reported low levels of crime in their neighborhood reported about an hour more moderate and vigorous physical activity compared to women who reported high levels of crime in their neighborhood. In a previous study by Kamphuis et al , it was demonstrated that people who perceived their neighborhood as safe were almost twice as likely to participate in sports than those who perceived their neighborhood as unsafe.
However, not all studies find a positive association between perceived safety and PA [23, 24]. Whether perceived neighborhood safety is a barrier for sports participation is likely to depend on individual cognitions. It seems plausible that positive cognitions towards PA might help people to deal with environmental barriers. The exact nature of this interdependency is largely unknown. Although previous studies have focused on the association between perceived neighborhood safety or individual cognitions and sports participation, very few investigated their interaction. For example, Deforche and colleagues  found that feelings of unsafety were only associated with the likelihood of active transportation in youth who had low self-efficacy and not in youth who had a strong self-efficacy. Thus, the aim of this study is to investigate whether perceived neighborhood safety modifies the associations between individual cognitions and sports participation.