Physical activity, bone mass and muscle strength in children

Osteoporosis is a disease characterised by reduced bone mass and deterioration of bone microarchitecture, resulting in increased risk of fragility fractures. Figures estimate osteoporosis to affect ~22 million women and ~5.5 million men between the ages of 50 and 84 years in Europe and is associated with high rates of morbidity and mortality 1. In addition, about 3.5 million European men and women suffer an osteoporotic fracture every year, resulting in an economic cost of ~€98 billion. The scientific literature seems to be consistent suggesting prevention as a key public health strategy to improve osteoporosis-related outcomes. This is particularly important in females due to their elevated risk of osteoporosis. It is well known that the acquisition of bone mass in the growing years is a strong determinant of osteoporosis risk later in life, as (up to) 50% of total body bone mass is achieved during early life stages, such as childhood and adolescence. Unfortunately, there is strong evidence suggesting that an important number of children and adolescents worldwide fail in attaining a high peak bone mass, which may have disastrous consequences and result in fragile bones and increased fracture risk not only during adolescence but also in adulthood 2. There is strong evidence suggesting that physical activity (PA), measured using objective devices, contributes to the development of bone mass in young people. In fact, around 78 minutes of moderate-to-vigorous PA (MVPA) or 32 minutes of vigorous PA (VPA) per day have been related to optimal bone mineral density (BMD) at clinical regions (i.e. femoral neck) 3. The effect of PA during childhood and adolescence depends on the ability of the skeleton to adapt to mechanical loading after exercise 4. However, it is not just the amount of PA that researchers have to consider, but the intensity, frequency and importantly, the type of PA, due to the ground reaction forces applied at different sites. In addition, lean mass (independently of fat mass) has been suggested as a strong predictor of bone adaptations during childhood and adolescence 5, with this being explained by the mechanostat theory ‘bigger muscles exert higher tensile forces on the bones they attach’ 6. Several studies have also positively related PA and lean mass in young populations, so both must be taken into account. In this issue of the journal, Fritz et al. 7 evaluated bone mass, bone structure, muscle mass and muscle strength in children after doing a seven-year controlled PA (moderate intensity) intervention study based on increasing time spent on school-based physical education. Fritz's study was initiated in the prepubertal period. So far, this is the longest reported school-based intervention with bone, lean and strength as outcomes. Girls in the intervention schools performed 200 minute/week over seven years of PE lessons and gained more areal BMD (aBMD) at the lumbar spine during the study period than those in the control group, whose only performed 60 minute/week of PE lessons over the same period of time. In addition, they also had higher tibial cortical thickness postintervention. The authors suggested that the gain observed in lumbar spine aBMD after the seven-year period was equivalent to a 0.4 increase in standard deviation, which according to previous studies would lower the fracture risk around 25%. Gains in muscle strength were observed in both sexes in spite of lean mass did not vary significantly. The authors suggested that girls benefited more from the intervention because it is known that they are overall less active compared to boys. In addition, and based on previous findings, an intervention focused on more intense activities, rather than just moderate PA, would have probably had more impact in both sexes to the extent that lean mass could have significantly changed and had an impact on bone-related outcomes. It is important to mention that objective measurements of extra-curricular PA would have been desirable in Fritz's study as control measures. In this line and partially backing up Fritz's study, high-impact, weight-bearing PA occurring above a certain intensity and duration has been advocated as a strategy to develop healthy bones during childhood and adolescence. Research has shown that moderate intensity and readily accessible weight-bearing exercise before puberty may increase femoral bone strength 8. Bone development is dependent on the mechanical loading and processes related to bone modelling and remodelling, such as structural adaptations to the trabecular microarchitecture. According to Wolf's law, when the loading on a particular bone increases, the bone remodels itself over time to become stronger to resist that loading. Based on the previous theory, studies in animals and humans have shown that the growing skeleton adapts to the mechanical stimuli to a greater extent than mature bone due a higher osteoblast activity and bone formation, hence the importance of promoting PA from early life stages. Although it is well established that exercise can augment bone in children and adolescents, the mechanistic basis for this effect is unclear. Exercise-induced changes in bone have been attributed to increases in lean body mass. However, while altered endocrine status and markers of bone metabolism are known to be important during growth and development, their influence on bone in response to exercise in children and adolescents is poorly understood. A review of 19 cohort studies from childhood to adulthood highlighted the importance of PA during growth and how it links with bone mass later in life. The positive association was higher in males than in females at the femoral neck and lumbar spine sites 4. The results from the six-year Saskatchewan Pediatric Bone Mineral Accrual study in adolescent boys and girls showed that the most active ones acquired 17% and 9% higher total body bone mineral content (BMC) compared to their inactive peers. At the lumbar spine, the active ones displayed 18% higher BMC 9. In a seven-year longitudinal study, the authors examined whether the positive effects of PA on BMC acquired in adolescence preserved into the second and third decade of life. It was reported that both active males and females during adolescence significantly remained active during adulthood. Furthermore, active boys showed 8–10% greater adjusted BMC at total body, femoral neck and total hip in adulthood. In addition, active girls had 9–10% higher adjusted BMC at total hip and femoral neck in young adulthood 10. These results highlight the importance of PA for bone accrual during childhood and adolescence and its relation with bone status in adulthood. The findings of this study are interesting and important and support the idea of increasing PE time at schools. Further research with additional control variables and periodic measurements to assess bone changes over shorter periods of time are desirable. The author has no conflict of interest to disclose.

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