Background
Physical activity (PA) is essential for the promotion of general health and the prevention of chronic health conditions in all ages, including adolescents and young adults [
1]. Physical inactivity is well documented as a determinant of cardiovascular and metabolic health [
2], type 2 diabetes, several forms of cancer, [
3,
4] as well as the rising obesity epidemic among youth in the US [
5]. The 2008 Physical Activity Guidelines (Guidelines) for Youth recommend that children and adolescents (17 years and younger) engage in 60 min of daily PA, most of which should be either moderate (M) or vigorous (V), aerobic PA and should include VPA at least 3 days a week [
1]. The Guidelines for Adults recommend that adults (18 years and older) should engage in at least 150 min a week of MVA (alternatively 75 min/week of VPA) for health benefits and/or at least 300 min a week of MVA (alternatively 150 min/week of VPA) for additional and more extensive health benefits. However, less than 20 % of U.S. adolescents (17 years and younger) meet the recommended level of aerobic PA [
6]. For mix-age US youth (9
th to 12
th grade) less than 30 % engaged in MPA at least 60 min/day on all seven days per week and less than 50 % on five or more days per week [
7]. The developmental period from adolescence to young adulthood is characterized by a surprisingly high risk of obesity (prevalence of obesity = 22.1 % with body mass index ≥ 30) [
8]. Moreover, it is a critical period marked by sharp declines in PA [
9‐
11]. However, to our knowledge, no studies have investigated the transitional change of youth PA engagement and potential determinants of the change at each stage controlling for the behavior and other covariates in the prior stage (i.e., the PA engagement and other covariates in the previous wave).
Some evidence suggests that certain psychosocial and social-contextual variables, such as peer influence, family support, and action planning [
12‐
14] are positively associated with levels of PA in high school. Only a few studies have examined longitudinal changes [
15] and trajectories [
11] of PA and their temporal and prospective predictors with limited causal conclusions, but no transitional associations were tested. In addition, no studies have investigated differences according to environmental status after high school.
Generally, a transition model estimates the probability of a categorical response (e.g., meeting PA recommendations) given the past responses, and explores the covariates’ effect on the transition probability [
16]. More specifically, it estimates the average of most proximal past measurement of covariates on the outcome of interest (e. g., PA at time 1 on PA at time 2, time 2 on time 3, …, time n on time n + 1), and captures the factors affecting behavioral change over time.
Overall, the objective of this study is to identify the determinants of PA during the transition from adolescence to adulthood among emerging adults, specifically the year after high school. The specific aims were to examine across the four annual waves from 10th grade to one year after high school: (1) changes in self-reported PA (including MPA to VPA [MVPA] and VPA); and (2) predictors of these changes, including perceived peer PA, family support for PA, VPA planning, and post-high school environmental status (school status, work status, and residence).
Discussion
This study used a nationally representative sample to examine the longitudinal change of MVPA and VPA among youth during their transition from adolescence into early adulthood. To our knowledge, this is the first-time application of a transition model approach to test how change in PA can be explained by psychosocial, social and environmental variables. We found that engagement in MVPA, but not VPA, continuously declined from 10
th grade to the first year post-high school, which is consistent with previous findings [
11]. Similar to previous findings for adolescents [
12‐
14], psychosocial variables including perceived parent support of PA, perceived peer PA engagement, and VPA planning strongly, were associated with increased likelihood of engaging in MVPA and VPA. Findings also indicate that environmental variables were associated with levels of MVPA one year after high school. Specifically, those who were working, were not attending college, and were not living on campus, were more likely to engage in MVPA at W4.
While previous studies have observed a decline in levels of PA during adolescence, few studies have explored trends in PA during the transition from adolescence to young adulthood [
9,
15]. Our findings are consistent with two Canadian cohort studies [
9,
11], which similarly observed declines in PA during this transition. Given that MVPA declined in both high-school and post high-school contexts, more research is needed to explore the mechanism resulting in, as well as protecting against, the steady declined MVPA during the transition.
Consistent with a previous international cohort study [
33], our findings confirm the importance of encouraging the maintenance of consistent levels of PA throughout this developmental period. The joint findings display the need for establishing and promoting sustainable strategies to initiate and maintain youth PA engagement over the developmental period.
A number of cross-sectional studies have documented that parent [
34,
35] and peer [
36] social influences are positively associated with increased levels of PA. A limited number of longitudinal data also demonstrate that parental support [
37] and peer/friend PA behavior [
4] positively influence PA engagement among adolescents. However, there is a paucity of longitudinal evidence during the transition period from high school to the first year after high school. A review study [
38] examined parent influence on drinking of the first-year college students and found that high parental monitoring and disapproval of alcohol use were negatively associated and parental permissiveness was positively associated with alcohol use. In addition, parentally imposed stringent drinking limits attenuated the powerful facilitating effects of peers on drinking. The current study extended the findings from alcohol drinking to a healthful behavior, i.e., PA, and upheld the continued parent and peer influence on emerging adults. We conducted additional analyses to test the potential moderation effect (including interaction term of parent support × peer PA in the model) of parent support and peer influence on PA, however the data did not show a significant moderation effect. Given the methodological strengths of the current study, our findings provide compelling evidence of the continued importance of parent and peer support in levels of PA.
A body of evidence suggests that action planning (the act of consciously scheduling and/or arranging to engage in a behavior) provides a bridge between intention to engage in PA and actual engagement with PA [
18,
39]. A recent cross-sectional study found that those who planned for PA were more likely to engage in PA [
14]. Moreover, we also observed prospective associations between planning and both MVPA and VPA, in the current study. These findings are consistent with previous studies suggesting the importance of planning [
40]. Given the demands, stresses and increased levels of independence associated with beginning college, action planning may be an important strategy, particularly important for first-year college students, to implement.
In addition to psychological and social determinants, environmental context may provide variable opportunities and barriers to youth engagement in PA [
41]. There are a limited number of studies that have explored the extent to which a recent change in environmental status, such as residence, school status, and work status, influence levels of PA during the transition from adolescence to early adulthood. A cross-sectional study among Australian college students examining the association between environmental status and levels of PA found that females who worked were more likely to engage in sufficient levels of PA compared to female students who did not work [
42]. The findings from our longitudinal study, consistent with previous findings, document the importance of environment and raise intriguing questions about its role.
Curiously, in the transition models, student part-time and full-time work status at W4, relative to not working, were significantly associated with MVPA. Yet, the mechanism for employment being related to more engagement of MVPA is still unclear. In the Australian study, the authors proposed that a job commitment may lead to better organization and time management, increasing the likelihood of PA participation [
42]. Alternatively, there may be something unique about the motivation of youth who work that also contributes to motivation to engage in PA. However, our data did not support that hypothesis, at least regarding PA planning. We conducted an additional analysis to examine the interaction of VPA planning with work status in the MVPA transition model and the association between VPA planning and work status in W4 data, but no significant results were observed in either test (data not shown). The relationship between work status and MVPA was not replicated in models assessing VPA. The item used to capture MVPA did not specify the activity type, i.e., organized PA, recreational PA or occupational PA; it is plausible that occupational PA may play a role in increasing levels of MVPA in this population of emerging adults. That is, the types of jobs likely to be obtained among this age group, including service sector and jobs requiring physical labor, contribute to the amount of physical activity acquired. However, future studies are needed to confirm the current findings and further explicate the relationship between work and PA.
Our findings indicate that those attending traditional 4-year colleges were less likely to engage in MVPA than those not attending colleges, technical schools, or community colleges. No significant associations were found between school status and VPA. Given the similar pattern with the association of work status with MVPA and VPA, we posit that the association between school status and MVPA may be work-related, in that more participants not attending school reported working full or part time (64.7 % of participants not attending college, 53.8 % attending technical/community school, and 38.8 % attending college). Additional research investigating why those attending 4-year colleges are less likely to engage in adequate MVPA is warranted.
Previous studies suggest that access and quality of PA resources may influence engagement with MVPA and VPA in the general population [
43]. Many college campuses in the US enable MVPA and VPA through structure and design (spacious campuses conducive walking and bicycling), facilities (recreational centers, outdoor and indoor courts), and extracurricular resources (intramural sports and fitness clubs) [
44]. Notably, studies have found that accessibility and proximity of exercise facilities were positively associated with students’ PA engagement [
45,
46], but PA was higher among those at on-campus and off-campus settings [
45]. According to the results of the current study, residence may have different effects on MVPA and VPA. In the multivariate logistic regression model living on campus is positively associated with VPA and not associated with MVPA. In the transition models, the association of campus residence with MVPA was significant, and the association with VPA was not. It is possible that participants who lived on-campus during their first-year of college take advantage of campus-related amenities for PA; however the residence effect may be suppressed by their individual determinants such as social support and planning or others (e.g., academic and other school-related or competing commitments) not measured in this study. Those who lived off-campus during their first-year of college may take advantage of non-campus related amenities they had prior familiarity with, such as parks and local fitness centers, to engage in MVPA. Further research should explore more specific residence-related factors determining the engagement of MVPA and VPA for first-year high-school graduates.
There are several limitations of this study. First, our measures of MVPA did not differentiate between different types of activities (such as competitive or recreational exercise), and everyday activities (such as active transportation and job-based activities). Second, regardless of its wide use, the two self-reported questions derived from YRBS may not estimate the proportion of recommended moderate PA and vigorous PA accurately among youth, with one previous study reporting that study participants overestimate how vigorous their activities were compared with objectively measured PA) [
47]. Third, the single-item measure of parental support on participant PA as well as peer physical activity may not capture all dimensions of the constructs. Fourth, we did not have a measure of access to PA facilities, which may have helped with understanding of the environmental variables.
The main strengths of this study include prospective longitudinal design encompassing the transition from adolescence to young adulthood, a nationally-representative sample which increases generalizability of our findings, and multiple social and environmental variables, providing a more comprehensive understanding of potential intervention targets.
A large body of literature has documented that 60 min or more of MVPA is developmentally appropriate and enjoyable to school-age youth [
48], and school-based [
49] and/or family and community combined [
50] interventions can increase regular participation in PA among high-school students. However, few studies have examined the dynamic change in the transitional period from adolescence to early adulthood, and identified the intervention targets appropriate to this group specifically in this period of time. The main findings of this study are that MVPA engagement decreased from high school to one year after high school and was associated with previous MVPA engagement and social contextual factors during this transitional period. Based on the findings, health professionals and administrators in both high schools and post-high school organizations (e.g., universities, worksites) recognize the need for interventions that would foster sustained PA engagement. PA promotion in high schools may be particularly important to the extent that it may be easier to foster maintenance than initiation of PA. Comprehensive PA intervention programs including social level factors (i.e., schools/universities, parents, peers, and environments) and individual level factors (i.e., planning skills) are needed to promote and sustain youth PA more effectively.
In addition, the findings that the environmental variables were associated with MVPA only may suggest the measurement of MVPA may account for “incidental” unplanned PA compared to VPA. In other words, while planning was important for both PA types, environmental variables were significant only for MVPA, possibly because it is more sensitive to environmental influences during the transitional first year after high school. These findings also raised questions, “why did environment impact first-year college students’ VPA poorly despite generally great facilities and flexible schedules? Is this about not having time, competition from other activities, or some strange norm among college students?” The answers to those questions may help university health administrators and educators to design and implement tailored program to freshmen particularly.