The main findings from this study were (1) about one fifth of all the adolescents reported symptoms of mild to moderate depression while more than half of them reported low physical activity levels (2) there was a significant inverse relationship between the depression scores of the adolescents and their physical activity scores with moderate physical activity being linked with lower risk of depression (3) Both individual and school factors were associated with depression and low physical activity, with being an older adolescent, female and in the most senior secondary class having significant links with depression and low physical activity among the adolescents. It was also found that compared with the public schools, physical activity was significantly lower and depression was significantly higher in the adolescents attending the private schools.
Data on the precise prevalence and level of depression among adolescents in Nigeria appear to be quite scant, but the prevalence of students experiencing severe depressive symptoms in this study (5.7%) is quite similar to that reported by a few other studies. Adewuya et al  reported a prevalence of major depressive disorder of 6.9% among a group of Nigerian adolescents with females having significantly higher prevalence than males. In a study to examine the proportion of children with psychiatric disorders attending primary care in a Nigerian setting, Gureje et al  also reported that depressive disorders were present in 6.0%, anxiety-related disorders in 4.7%, and conduct disorders in 6.1% of the children.
The present study found that more than half of the participants actually presented with low physical activity levels indicating that the adolescents were not engaging in sufficient physical activity that could benefit their mental health status. According to the Australian Government Department of Health and Ageing , adolescents between 12 and 18 years old should engage in at least 60 minutes of moderate to vigorous physical activity every day to keep healthy. However, where children have been inactive, 30 minutes of moderate activity per day is recommended and should be built up gradually. A study by Nikapota  reported that developing countries are subject to rapid socio-cultural and political changes which affect the life-styles of children and their families and hence their physical and emotional well-being. The present study implies that the sampled Nigerian adolescents were not sufficiently active. The high prevalence of low physical activity as well as the prevalence of depression seen in the sample may be indicative of a link between depression and physical activity. A previous study  describing physical exercise as a means of being physically active, had documented the link between physical exercise and depression by reporting that exercise withdrawal actually resulted in increased depressive symptomatology in healthy, non-depressed individuals.
The female adolescents in this study had higher depression scores and lower physical activity scores compared to the males. There may be many reasons for this however, it may also be an indication of the link between low physical activity and depression since the female participants in this study had shown a lower level of physical activity. For instance, group-based physical exercise programmes, which can increase daily physical activity or social relationships, have been observed to improve not only physiological fitness levels but also the depressive state and psychophysical stress conditions of participants . Berlin et al  also found that depressive symptomatology was more prevalent among sedentary than physically active individuals. The fact that the female adolescents in our sample had a higher depression score is well-recognized and confirms the reports of previous studies. The female adolescents had close to three times higher risk of having depressive symptoms than the males. In the study by Adewuya et al  on Nigerian adolescents, the females were also observed to have higher prevalence of depression than the boys, but the authors claimed that there was no age-gender interaction in the findings. It was however reported in a previous study that more boys participated in physical activity than girls, and probably as a result of the link between physical activity and depression, more of the girls than boys reported feelings of sadness, including considering and planning suicide .
Higher scores of depression and lower scores of physical activity were seen in the older adolescents compared to the younger ones and among those in the higher classes of study compared to those in the lower classes. Expectedly, the students' ages increase as their class of study increases and a higher class of study implies heavier workload. A combination of heavier workload and anxiety trailing the anticipation of the forthcoming final examinations at the highest level of secondary education could have placed the participants on a tighter academic schedule making it difficult for them to engage in purposeful physical activities. The same situation could also have influenced their psychological state making them present with higher depression scores. It was also observed that older adolescents had higher risk of having depressive symptoms compared to the younger adolescents while being in the topmost class more than tripled the risk of having depressive symptoms and increased more than four folds the risk of having low physical activity. This may be because the highest class of study (SS3) was occupied mainly by the older adolescents who happened to have higher risk of depression and low physical activities.
About one third of the adolescents in both the private and public schools were found to be sedentary for most part of the day while suicidal ideation previously reported to be about 20% and above in studies by Omigbodun et al  and Daley et al  was about 9% in this study. It is however important to note that the difference in the prevalence of suicidal ideation in this report and that of Omigbodun et al  which also surveyed a group of Nigerian adolescents may be due to a number of reasons. First, Omigbodun et al  measured suicidal behaviour using the Diagnostic Interview Schedule for Children (DISC) (Predictive Scales 432 - items 23-25) while the observation in this present study was a response to one of the questions on the CDI. Second, they conducted their study on both urban and rural adolescents while this study was limited to urban adolescents. Third, they considered all grades in the school while only the senior grade was considered in this study. Multiple psychosocial factors such as sexual abuse, physical attack and involvement in physical fights were found to be the significant predictors of suicidal behaviour among Nigerian adolescents as reported by Omigbodun et al . Despite the lower prevalence of suicidal ideation noted in this study, this is an area worthy of further research investigation. Depression has been reported to be the most important predictor of suicide, and failure to address depression in adolescents can lead to an increase in cases of suicides .
The coefficient of determination shows that a substantial variation in depression in adolescents may be explained by physical activity. Because the relationship may not be causal, the remaining variation seen in the depression of the adolescents may be explained by other factors that were not considered in the study. These factors may include the socioeconomic status of the parents and the presence of co-morbidities. An earlier cross-sectional analysis however, reported an association between physical activity and depression even when adjustments were made for a relatively large number of potentially confounding variables . This association may be because of the likely link between physical activity and depression. According to Rothon et al , no clear mechanism for the association between physical activity and depression has been established, but biochemical, physiological and psychological mechanisms have been proposed. The authors claimed that one of the explanations relates to the indirect effect that physical activity has on mood through providing increased opportunities for social interaction. It will be appropriate to state here however, that the association existing between physical activity and depression may actually be bidirectional. As presented in this study that depression is linked with low physical activity, it is also plausible for low physical activity to be linked to increasing depression. In a comprehensive review of published studies on correlates of physical activity in children and adolescents, a high level of depression was consistently associated with low physical activity in adolescents . This study is not able to say however whether depression precedes physical inactivity or physical inactivity precedes depression.
This study showed that adolescents with moderate physical activity had a reduced risk of having depression after adjustment for age and sex. In a previous study, it was also indicated that low to moderate intensity physical activity was a protective factor against depression and psychotic symptoms in Chinese adolescents . However, it was found in the study that high-intensity physical activity was not a protective factor against psychological disorders, but rather a risk factor for general mental health problems and hostility. In this present study, high physical activity reduced odds of having more severe depressive symptoms with a trend level finding (non significant) that high physical activity was linked with reduced risk of depression.
This study further found that the adolescents in private schools had a higher risk of low physical activity than those in public schools. No previous reports were found on the variation of physical activity between the students of private and public schools, but it is assumed that this disparity may have a lot to do with the socioeconomic background of the students. It is possible that the students in the public schools, hypothetically from low socioeconomic backgrounds had to exert more physical effort in executing their daily routines. This may include trekking to school and the use of manual force for their daily chores contrary to the life of the adolescents from higher socioeconomic backgrounds that abound in the private schools. This may explain the link between low physical activity and higher depression as seen in the students of private schools.
This study should be interpreted within the confines of its scope, limitations and strengths. The fact that this study was cross-sectional prevents any inference of causality. The school-based nature of the study also means that findings cannot be generalised to adolescents who do not attend school. However, apart from providing an organised research site, the schools were also considered as an avenue where the issues of depression and physical activity among adolescents could be effectively championed. According to Grzywacz and Fuqua  schools are in a position to prevent public health concerns such as depression. Our mode of assessment using the questionnaires may not be the most precise method because of the possibility of substantial recall bias associated with self reports; however, questionnaires have produced reliable assessments of numerous constructs. According to Corder et al , self-report methods may still be the only feasible way to assess physical activity in many situations and are important for assessing aspects of physical activity not easily measured objectively, such as mode and domain.
The relationships found in this study are not necessarily causal because there may be other potential confounders of depression apart from age and sex that this study did not investigate. Such confounders include health status, self esteem and the socioeconomic status of parents. There is also the possibility of residual confounding in this study which may be due to possible flaws in our assessment or the fact that we did not measure some other confounders outrightly. Further research may need to look at how issues such as levels of school and family care, motivation and serious life events will interact with depression and physical activity of the adolescents. However, all our analyses pointed to the fact that those who had lower physical activities had higher levels of depressive symptoms. This study identifies that there is a need to further explore the complex link between physical activity and depression among Nigerian adolescents, just like their counterparts in other developing countries. Furthermore, this study will provide an additional basis for exploring physical activity as complementary therapy in the intervention for depression among adolescents given its low-cost and the increasing cost of mental health care. Also from the outcome of this study, there is an urgent implication for government policy decision reviews based on the fact that there is a large number of adolescents who are not physically active and due to the fact that close to 6% of the students were experiencing quite severe depressive symptomatology.
In conclusion, the results of this study showed a sizable burden of both mild to moderate and definite symptoms of depression in addition to a prevalent level of low physical activity among the adolescents. There was an inverse relationship between depression and physical activity and both individual and school factors were linked with depression and low physical activity. Being an older adolescent, a female and in the most senior secondary class were the main contributors to both depression and low physical activity among the adolescents. For future research, we suggest longitudinal studies to shed light on causal issues and studies that will examine the possible effects of physical activity among clinical samples of adolescents with depression.