The main findings of the present study indicate that (1) some sedentary behaviours, such as the use of internet for non-study in boys and the time spent studying in girls are negatively associated with whole body and femoral neck BMC, respectively, and (2) active girls (≥3 h/week of osteogenic sports) present a lower prevalence for having low femoral neck BMC (at least 1SD below the mean), suggesting that the negative effect on bone mass of a sedentary behaviour such as time for study, could be counteracted by the practice of extra-curricular osteogenic sports.
It is well known that bone mass is around 60-80% genetically determined , therefore it is of importance to know which factors contribute to the development of bone mass. In this regard, weight bearing and high-impact intense PA, mainly through participation in sports, have shown osteogenic effects . It has been shown that boys doing ≥ 3 h/week of extra-curricular sport increase total lean and bone mass to a greater extent than those not doing extra-curricular sports , which has been observed also in young  and adolescent girls . In contrast, physical inactivity has been proposed as a determinant factor for low bone mass in young women .
Little is known about the association among sedentary behaviours and bone mass in a key period such as adolescence. In adolescents from the AVENA study, Vicente-Rodríguez et al. concluded that ≥3 h/day watching TV was associated with reduced total bone mass in boys, and their results suggested that extra-curricular PA could counteract this deleterious effect . The present study do so taking into account a large list of sedentary behaviours (i.e. time spent on TV viewing, computer games, console games, internet for non-study, internet for study and study), additional DXA measurements in regions of clinical relevance regarding osteoporosis (i.e. lumbar spine and femoral neck) and controlling for relevant confounders that have been shown to be associated with bone mass during adolescence, such as lean mass  and objectively measured PA .
Our results showed that among the sedentary behaviours studied, total sedentary time in boys and time spent studying in girls were negatively associated with whole body BMC, respectively. However, these associations disappeared after controlling for lean mass, which has been strongly and positively associated with bone mass in this sample of adolescents . Interestingly, the use of internet for non-study in boys and the time spent studying in girls were negatively associated with whole body and femoral neck BMC, respectively, even after controlling for lean mass and MVPA. In addition, it is important to notice that there was a negative borderline association between the total sedentary time and femoral neck BMC after controlling for lean mass and MVPA (models 2 and 3), supporting the main findings of this study.
The femoral neck, one of the regions with clinical relevance regarding osteoporosis, was significant and negatively associated with time spent studying in all models of adjustment in girls. Therefore, complementary analyses were performed at this site. The prevalence of girls with low femoral neck BMC was calculated considering both the time spent studying and the participation in extra-curricular sports. Our results showed that the prevalence of girls with low femoral neck BMC was smaller in those girls doing ≥3 h/week of extra-curricular osteogenic sports than in the rest of the sample (girls doing < 3 h/week of extra-curricular sports + girls doing non-osteogenic sports + girls not doing extra-curricular sport). Moreover, the cut-off for time of study (low: <3 h/day and high: ≥ 3 h/day) was also included in our analyses as in previous studies . Similar results were obtained independently of the chosen cut-off. The analyses of other regions, such as whole body and lumbar spine showed also similar outcomes. These results suggest the importance of extra-curricular and osteogenic sports in the development of healthy bones, especially when sedentary behaviours, such as to stay seated for studying are evident. Similarly, it has been shown that sedentary occupations in adulthood are associated with an increased risk of hip fracture in elderly people .
The mechanisms by which sedentary behaviours lead to poor bone health are not well understood. According to a recent literature review , sedentary behaviour leads to a rapid increase in bone resorption without concomitant changes in bone formation, resulting in reduced BMC. The present results indicate that some sedentary behaviours can be more detrimental for bone health than others. Both the amount and the pattern of sedentary time may have influence on bone metabolism. In a recent study, we observed that the time studying was a good surrogate marker of objectively measured sedentary time (the higher the time studying the higher the time spent sedentary) in a large sample of European adolescents . Moreover, sitting is common when a subject is studying or surfing internet. Compared with other sedentary activities, these activities are characterized by spending a lot of minutes in the same position. As a result of this, an excessive time without mechanical loading could be detrimental for bone health. We encourage adolescents to practice extra-curricular osteogenic sports at least 3 h/week in order to break the sedentary time caused by some sedentary behaviours such as study.
Limitations and strengths
Although we controlled for several potential confounders we cannot be certain that other unmeasured confounders have not influenced our observations. Our study focus on adolescents from Zaragoza, Spain, since bone mass by DXA was only assessed in this sub-sample of the HELENA-CSS, so the conclusions cannot be generalized to whatever population. Cross-sectional studies only can provide suggestive evidence concerning causal relationships. However, in this specific case, it seems reasonable to think that time spent on sedentary behaviours could influence BMC, whereas it is not so clear the mechanisms by which bone mass could determine the time spent on sedentary behaviours.
The use of sophisticated methods, such as DXA to assess body composition, and the use of accelerometers to assess PA are strengths of the study. In addition, this study includes a rather complete set of confounders, i.e. height, sexual maturation, lean mass and MVPA, which is crucial to examine the current research question.