Determinants of physical activity
In this questionnaire, the enquired determinants of physical activity refer to three categories of activity: sport, transportation and sedentary activities. To minimize time spent filling in the questionnaire for each determinant of physical activity a selection of two or three relevant questions is made. The following determinants of physical activity are assessed in both age groups:
Behavioural intention is the cognitive representation of a person's readiness to perform physically active behaviour, and it is considered to be the immediate antecedent of behaviour. Behavioural intention was measured with one item (i.e. 'Do you intend to play sport more in the following three months?' with response categories ranging from 1 = 'no, certainly don't' to 5 = 'yes certainly do'). According to the Theory of Planned Behaviour , behaviour in general is determined primarily by behavioural intention and postulates that this intention is determined by three constructs: attitude, subjective norms and perceived behavioural control.
Attitude, the beliefs that are associated with physical activity behaviour and the evaluations of these beliefs. The questionnaire contains two questions about attitude (i.e. 'I like playing sports?' with response categories ranging from 1 = 'strongly disagree' to 5 = 'strongly agree').
Social influences  such as social norms, social expectations, modelling and imitating. Social norms are determined by the normative beliefs of significant others about physical activity behaviour and the individual motivation to comply with these persons. The questionnaire of the young adults contains five questions about social influences , the questionnaire of the adolescents contains four questions regarding social influences (i.e. 'My friends think I should play sports' with response categories ranging from 1 = 'no, certainly don't' to 5 = 'yes certainly do'). For the categories sport and transportation, two questions about modelling are added.
Self-efficacy expectations, which are beliefs of a person about his abilities to perform physical activity. Increased self-efficacy will result in improved performance of this behaviour. The questionnaire contains two questions about self-efficacy  (i.e. 'Do you think you can manage to play sports for at least half an hour a day?' with response categories ranging from 1 = 'no, certainly don't' to 5 = 'yes certainly do').
Knowledge of daily physical activity is asked by one item (i.e. 'How much physical activity do you have to spend per day to stay healthy'?) with the response categories: one hour per week, three hours per week, thirty minutes per day, one hour per day, two hours per day no and don't know).
For the categories sport and transportation two extra determinants are assessed:
Personal barriers, many personal variables, including physiological, behavioural, and psychological factors, may affect our plans to become more physically active. Three items about personal barriers are asked  (i.e. 'Do you think you can manage to play sports when it rains?' with response categories ranging from 1 = 'no, certainly don't' to 5 = 'yes certainly do').
Awareness of physical activity, one item is included in the questionnaire (i.e. 'Do you think you spend enough time playing sports?' with the response categories yes and no). Awareness of the actual level of activity will be assessed by comparing the outcomes of the awareness questions with the total minutes per week of the concerning activity (i.e. sport, walking/cycling) from the questionnaire.
Quality of life
The quality of life of the adolescents is assessed by the Dutch version of the KIDSCREEN-10 Index . The KIDSCREEN is a health related quality of life-measure (HRQOL) and applicable for healthy children aged from 8 to 18 years. The KIDSCREEN measures 10 HRQOL dimensions (Physical-, Psychological Well-being, Moods and Emotions, Self-Perception, Autonomy, Parent Relations and Home Life, Peers and Social Support, School Environment, Bullying, Financial Resources) by 11 items on a five-point scale, ranging from 1 "completely not" to 5 "completely". From the total score t-values and percentages will be calculated by age and gender.
The quality of life of the young adults is assessed by the SF-36 Health Survey (SF-36) [26, 27]. The SF-36 provides a health related quality of life measure and is applicable for adults. The SF-36 is composed of 36 questions and standardized response choices, organized into eight multi-item scales: physical functioning, role limitations due to physical health problems, bodily pain, general health perceptions, vitality, social functioning, role limitations due to emotional problems and general mental health. All raw scale scores will be linearly converted to a 0 to 100 scale, with higher scores indicating higher levels of functioning or well-being. Item score and subscale scores will be calculated.