In this study, nearly half of the participants reported meeting the recommended guideline of 60 minutes a day 5 days a week, although there was considerable variability. We examined associations with adolescent MVPA and internal and external motivation, MVPA planning, peer PA, and parental support. MVPA prevalence was higher among males than females, White than Hispanic or Black youth, and among moderate and higher family affluence and parent education compared to lower affluence and education, consistent with other research. Importantly, the findings are consistent with other literature and theory that suggest that MVPA planning is associated with MVPA [19, 27]. Our findings support part of the first hypothesis that more PA engagement is directly associated with positive intrinsic motivation, but only indirectly with extrinsic motivation. The findings partially support our second hypothesis that MVPA planning mediates the relationship between internal motivation and MVPA engagement. Motivation was associated with planning for MVPA, such that internal motivation completely mediated the relation between external motivation and planning. Partial support was also found for our third hypothesis that features of the social environment were associated with adolescent MVPA. Peer PA was directly associated with adolescent MVPA, adolescent MVPA motivation, and adolescent MVPA planning (and thereby also indirectly related to adolescent MVPA through MVPA planning). Parental support was indirectly associated with MVPA, through its direct association with internal and external motivation.
Ryan and colleagues  posit that external motives such as losing weight and feeling more attractive may be important in initiating MVPA, but motivation driven by internal factors (e.g., enjoyment and competence) are more important for long-term adherence to MVPA. Ntoumanis reported a SEM analysis that showed that internal motivation was related to MVPA intention (a key prerequisite to performing a behavior) among adolescents. In contrast, external regulation and amotivation were not related to MVPA intention and were predictors of boredom while engaging in MVPA . Our findings are consistent with previous research showing that internal motivation has a strong, positive, direct association with youth MVPA, and external motivation has a non-significant negative association. Given that our MVPA questions measured non-habitual MVPA behavior, the results suggest adolescents’ MVPA may be mainly the product of internal motivation.
Social influences can affect motivation positively or negatively, such that internal motivation for MVPA can be promoted through support and positive feedback from other people; but can also be undermined by undesired external pressure and control . Adolescents may be particularly susceptible to social influence, given their relatively insufficiently developed decision making capabilities  and heightened reward sensitivity in the presence of peers . Keegan and colleagues  found that peers and parents, among other social agents, can influence youth motivation and participation in sport. Specifically, parents fostered children’s motivation by support and facilitation, whereas peers influenced motivation and collaborative behaviors . Our findings indicate that perceived peer PA and parental support were associated with both internal and external motivation for PA. Interestingly, the association between peer PA and internal motivation is stronger than the association between peer PA and external motivation. Adolescents may be more likely to perceive peer PA as an echo of their enjoyment in MVPA (internal motivation) than external pressure (external motivation). Alternatively, participation in MVPA with peers may contribute to the enjoyment of MVPA, and also serve to enhance peer relations, providing or reinforcing internal motivation . This suggests that friends may play an important role in fostering adolescents’ adherence to a long-term MVPA regimen . The comparable associations of parental support with internal/external motivation indicate that parents’ support of adolescents’ MVPA engagement may be perceived by adolescents as either as external pressure or encouragement of their inherent interest in MVPA engagement. However, if parental support of adolescents’ MVPA is perceived as pressure, its influence may be minimal given that adolescents’ external motivation is not linked to MVPA directly. Consistent with a recent systematic review, in this study adolescent’s MVPA was significantly associated with peer PA but not with parental support . Parental encouragement of MVPA appears to decline between early and middle adolescence , possibly in relation to the increase in the salience of peer influence . Additionally, although the direct association between parents’ support and MVPA was not significant, the indirect association via internal motivation was significant. This suggests that parents’ influence on adolescent MVPA may be more likely to be exerted through adolescents’ internal motivation than by only showing their expectation for children’s MVPA. It may also suggest the possibility that an important function of parenting with respect to adolescent PA is facilitation of planning. Future studies are needed to look into the mechanisms by which parents influence children’s PA MVPA.
We examined the mediation effect of MVPA planning on internal motivation and MVPA engagement and found a direct path between MVPA planning and MVPA among adolescents. In the current study, we also found significantly indirect association between internal motivation and MVPA via MVPA planning, although it was weaker compared to the direct association. Therefore, we believe MVPA planning may be an important intermediary to MVPA. Planning reflects the details of how, when, and how much MVPA and therefore reflects skill and competence at planning, which are known to derive from as well as to enhance motivation . A previous study found self-regulation skills to be the most proximal predictor of MVPA behavior and mediator of the link between planning and MVPA in adults . A recent study found that adolescents with greater planning skills were more likely to successfully translate their intentions into MVPA plans . Therefore, it may be useful to include MVPA planning skills training in MVPA promotion programs.
The study has limitations. First, the lack of longitudinal data precludes examination of causal relationships between motivation, planning, and MVPA behavior . It may also limit our ability of to identify changes in the relations between planning and engaging in MVPA. Sniehotta and colleagues  found that the effect of coping planning increased over time in cardiac patients. In this regard, future longitudinal studies are needed to investigate the prospective association between MVPA planning, internal/external motivation for MVPA, and MVPA engagement. Second, the school-based recruitment might limit the generalization of the findings to adolescents not in school. Third, the internal consistency coefficient of our three items of external motivation is relatively low. Fourth, the single-item measure of parental support on adolescents’ MVPA may limit the dimensions of the construct. Finally, our measures of MVPA did not differentiate between sport and exercise, and may have included some everyday activities such as active transportation. Each of these sources of MVPA may have somewhat different motivational origins.