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Weak and osteoporotic bones are an increasing cause of mortality and painful physical impairment among the elderly, especially in the Western world. Bone mineral density (BMD, g/cm2) accrual during childhood and adolescence is thought to influence an individual’s risk of osteoporosis and the related fractures.

A main aim of this thesis is to investigate the effects that various types of weight-bearing physical activity have on bone accretion in young males during their active sports careers and to study the effects that detraining has on BMD. The results suggest that bone is sensitive to loading after puberty in males, and important gains in BMD stemming from physical activity were observed during the 12-year follow-up period (papers I-III). These gains seem to be site-specific and related to the type and amount of physical activity in which individuals participate (papers I-III). For example, badminton, a sport that is characterized by jumps and rapid versatile moments in multiple directions was associated with greater gains in BMD than ice hockey was. In addition, our results indicate that with reduced training, exercise-induced bone benefits decline, predominantly at trabecular sites (paper II). In contrast, high bone density attained from previous physical loading was partially preserved at cortical bone sites after about eight years of reduced activity (papers I-II). In study IV, the associations between self-perceived health, BMD, and other lifestyle factors were studied in a well-defined group of women and men of varying ages. We found that self-perceived health was related to several lifestyle factors, such as physical activity, which were also related to BMD at the femoral neck.

In summary, BMD in young males seem to be especially sensitive to activities associated with supposed high strains in unusual directions at specific bone sites. A high bone density stemming from previous weight-bearing physical activity is largely lost at trabecular bone sites with reduced physical activity levels. Finally, self-perceived health seems to be associated with several lifestyle factors that are also associated with BMD at the femoral neck.