The first major aim of the current study was to investigate the SDT tenets in an obese adolescent population. First, we executed an exploratory factor analysis of principal components to examine the factor structure of the BREQ-2. Results revealed that two items failed to load on their intended original factor. The low loadings of item 17 (i.e., 'I get restless if I don't do physical activities regularly') with its original factor 'identified regulation' has already been found in previous studies [10, 30, 31]. Item 2 (i.e., 'I feel guilty when I don't do physical activities), which taps into feelings of guilt, also failed to load on its intended introjected factor. Instead, the retained introjected factor primarily yielded a reference to the avoidance of feelings of shame and failure. Because these are prominent among obese adolescents who feel ashamed of their figure and weight , these items seem to cluster apart from items tapping into feelings of guilt. These findings are consistent with other authors' claim that feelings of shame and guilt need to be distinguished given their different antecedents and consequences [33, 34]. In general, it is notable that items that are crossing the distinction between controlled and autonomous motivation were found to yield wrong loadings or cross-loadings. Indeed, the difference between introjected and identified regulation does not represent a sharp line, but rather represents a gradual change away from inner pressures to personal convictions. Along similar lines, Mullan et al.  reported that introjected regulation correlated more strongly with the more self-determined identified subscale than it did with the less self-determined external subscale.
The low validation scores of item 2 and 17 and the various cross-validation scores could be due to the fact that 177 adolescents is a relatively small sample to investigate the factor structure of a questionnaire with 19 items as it is suggested to have ten participants per questionnaire item or to have at least 200 participants [36, 37]. A possible strategy to deal with the low validation scores or cross-validation scores is to exclude those specific items from the subscale calculation. However, since the BREQ-2 is strongly validated in other populations [10, 17, 30, 31] and since the BREQ-2 has been used for the first time in an obese adolescent population, it was preferred to use the current classification. Moreover, internal consistency was rather similar using factor structure suggested by exploratory factor analysis or the current classification.
The second part of the first study aim showed that the association between autonomous types of motivation and PA was present in obese adolescents. Results showed that higher levels of the composite score of relative autonomy, identified and intrinsic regulation were related to higher amounts of total PA, sport participation and active transportation. Introjected regulation was also positively related to total PA and sport participation. These results in severely obese adolescents are similar to results of previous studies in normal-weight adolescents and in normal-weight and obese adults [9–16]. Despite the positive association between introjected regulation and PA among the obese adolescents, it should be noted that introjected regulation is a more controlled form of motivation. Previous studies have shown that introjected regulation appears to be associated with PA on the short-term, but not on the long-term [38, 39]. This implies the need for a persistent emphasis on the pleasure and personal benefits associated with PA to prevent a dominant internal obligation to be physically active . Amotivation was negatively associated with sport participation among the obese adolescents, which is comparable to the study of Markland and Ingledew  in normal-weight adolescents. Overall, we can conclude that higher levels of autonomous motivation are related to higher amounts of PA in obese adolescents.
Recommendations to increase autonomous types of motivation could therefore be used in obesity treatment programs with the intention to increase PA levels of obese adolescents. According to SDT, an environment which fosters the psychological needs for autonomy, competence and relatedness is a prerequisite to increase autonomous motivation [8, 41]. In practice, more autonomy can be obtained by providing choices, supporting the patients' initiatives, avoiding the use of external rewards, offering relevant information for changing behaviour and using autonomy supportive language (e.g. "may" and "could" rather than "should" and "must") [41–43]. A feeling of competence is attained when the youngsters experience success while participating in activities. Activities need to be tailored to the capabilities of the obese adolescent and sufficient instructions, practice and positive feedback are needed to obtain a sense of competence [9, 41, 43]. Finally, relatedness with the supervisor or therapist and the other peers is important. Supervisors and therapists need to show enjoyment, enthusiasm and interest in the obese adolescents [43–45]. Group sessions and group activities could increase the feeling of relatedness and decrease the feeling of being isolated . Former recommendations should be taken into account during an obesity treatment program to enhance autonomous motivation towards PA in obese adolescents.
The second aim was to investigate differences in the composite score of relative autonomy and the motivation types in low versus high educated obese adolescents. Results revealed that lower educated youngsters had a lower score on the composite score of relative autonomy and showed less introjected, identified and intrinsic regulation at the start of the obesity treatment program. A possible explanation for the difference in motivation could be situated in the environment of the lower educated adolescents. For example, lower educated people have lower perceived competence to produce desired outcomes such as PA behaviour , probably because they are provided with less relevant information about how to change their behaviour. Further, lower educated adolescents mostly have restricted access to resources and sports facilities , thereby missing opportunities to be physically active. These findings do not contribute to the fostering of the need for autonomy and competence. The need for relatedness is less satisfied either, since lower educated adolescents get less support for being physically active from their social network . In conclusion, the physical and social environment of lower educated adolescents is less likely to support the need for autonomy, competence and relatedness which could have negative consequences for the autonomous motivation towards PA. Consequently, lower educated obese adolescents could be at major risk of not being sufficiently physically active to maintain weight loss after treatment because of their lower autonomous motivation. Therefore, special attention concerning satisfaction of the need for autonomy, competence and relatedness is required for this group during the treatment in order to increase their autonomous motivation towards PA.
The third study aim investigated whether attending a residential obesity treatment program focusing on the three psychological needs could lead to an increase in autonomous motivation towards PA. Results showed that obese adolescents had a significant increase in the composite score of relative autonomy and in identified and intrinsic regulation after treatment. No change over time was found for amotivation. Evidence is provided for the effectiveness of a residential obesity treatment program, characterized by a well-structured environment with continuous supervision of a professional team, in increasing more autonomous types of motivation towards PA, provided that attention is paid to autonomy, competence and relatedness. To our knowledge, no studies previously investigated the change in autonomous motivation among obese adolescents following a residential obesity treatment program. However, similar research was conducted in obese adults following an ambulant obesity treatment program. Silva et al. [41, 47] investigated the impact of a 1-year weight management intervention with 30 group sessions for obese women. The intervention was based on SDT with a special focus on increasing autonomous regulation towards exercise and weight control in an autonomy-supportive environment. Results of that study revealed a significant increase in exercise intrinsic motivation and autonomous motives to exercise at the end of the treatment. Conversely, in a study of Edmunds et al. , obese female adults taking part in regular exercise classes had no significant change in intrinsic motivation and even a decrease in identified regulation, possibly due to unrealistic weight loss expectations. These findings suggest that an obesity treatment program should specifically focus on satisfying the need for autonomy, competence and relatedness to increase the autonomous motivation towards PA.
Despite the positive results for the more autonomous types of motivation in the present study, it should be noted that there was a significant increase in introjected regulation and even a borderline significant increase in external regulation as well. Thus, the residential treatment program might have put pressure on the adolescents to become physically active, which has contributed to the increase in external and introjected regulation. The increases could also be partly explained by the increases in autonomous forms of motivation since these forms are interrelated. For example, introjected regulation was shown to relate positively to both identified and intrinsic regulation, which has been found by previous studies as well [10, 13, 28]. As a result, the adolescents of the present study did not only have an increase in autonomous forms of motivation, but also in controlled forms of motivation towards PA. Thus, adolescents' overall motivation increased. This suggests that the residential program may contain a mix of controlling and need-thwarting components and more need-supportive features, although future research may want to directly tap into the experience of the social environment. In a recent study of Haerens et al. , normal-weight college students with high scores on both autonomous and controlled motivation towards PA (i.e. high quantity motivation), engaged less in PA than their contemporaries with high scores on autonomous motivation and low scores on controlled motivation towards PA (i.e. high quality motivation). From this study, it can be concluded that the quality of motivation is more important than the quantity. If these findings can be generalized to obese adolescents, it is important that a residential obesity treatment program focuses primarily on increasing the autonomous forms of motivation and minimizes control to enhance PA behaviour.
Additionally, we wanted to investigate if the treatment effect on motivation was different in low versus high educated youth. Results revealed that the change in the composite score of relative autonomy and the motivation types was not significantly different for lower and higher educated youngsters, except for introjected regulation. Lower educated youngsters had a significant increase in introjected regulation during the course of the treatment, whereas no change in introjected regulation was found among the higher educated adolescents. As mentioned before, introjected regulation seems to be positively related to PA only on the short-term [38, 39], which highlights again the special attention required for the lower educated adolescents during the treatment program. However, concerning the other motivation types, mean values showed that the lower educated adolescents kept pace with the higher educated adolescents after treatment. Thus, although lower educated adolescents had lower autonomous motivation at the start of the treatment, this difference in autonomous motivation according to educational level was no longer present at the end of the treatment. Consequently, the treatment program cleared away the differences in motivation between lower and higher educated adolescents in the course of the program, thereby decreasing socio-economic inequalities. Further research should investigate possible changes in autonomous motivation towards PA among lower educated adolescents when they end the treatment and return to their home environment.
There are some limitations in the present study that need to be acknowledged. A first limitation is the cross-sectional observational design through which we cannot rule out the possibility that the association between autonomous types of motivation and PA represented reverse causality and that a higher PA level could have led to more autonomous motivation towards PA. Notwithstanding the previously demonstrated reliability and validity of the measures, the use of self-report measures can be seen as a second limitation. Particularly the self-report of adolescents' PA level could involve overestimation: the completion of the FPAQ took place in the local centre under supervision of the physiotherapist which could have led to social desirable answers. However, the presence of the physiotherapist can also be considered positively, since he or she could clarify vague questions as well as check if all questions were completed. Nevertheless, it can be concluded that accelerometers or other objective motion sensors would have been more appropriate and accurate PA measurements. Moreover, using objective motion sensors would have had the advantage to detect differences in PA at the end of treatment, which was now useless to measure by means of the FPAQ because of the standard activity program for all adolescents at the treatment centre. It should also be notified that using the BREQ-2 in younger obese samples could require adjustments as regards calculations of the five motivation types according to the exploratory factor analysis, despite a similar internal consistency when using the current classification. It can be argued that a confirmatory factor analysis might be a preferred method to examine to factor structure of the BREQ-2 given its ability to test a priori theory. However, we were unable to conduct such analysis because of the relatively low study sample. Further, the present study has not investigated the cause of the increase in autonomous motivation during obesity treatment. Future research should therefore examine which specific factors mediate the increase in autonomous types of motivation during treatment (e.g. increase in psychological need satisfaction, increase in fitness, loss of body mass, etc.). A final limitation could be the relatively small sample size and the very specific population of extreme obese adolescents in a residential setting, thereby limiting the extent to which findings can be generalized to all obese adolescents. However, the specificity of the study population can also be seen as a strength, since the significant results of this study demonstrated the universality of the application of SDT. Further, to our knowledge, no other study has previously investigated the application of SDT in lower versus higher educated individuals, which can be seen as a valuable strength of this study.