Physical activity, musculoskeletal traits and fractures in childhood and in old men

Background: Physical activity (PA) levels in children need to be increased. This may be done by interventions, but the activity-stat theory suggests that the total duration of PA in children is constant and cannot be altered. There are findings that PA interventions during childhood are associated with beneficial developments in various musculoskeletal traits, but such beneficial effects may attenuate over time. Aims: To assess whether a daily school-based PA intervention program is associated with increased duration of total PA, and when provided from before until after puberty (Tanner stage 1 to 5), with beneficial gains in musculoskeletal traits and a musculoskeletal composite score. Also, whether a composite score in old men can predict fractures, and whether the level of PA is associated with the composite score and fracture incidence. Methods: The Pediatric Osteoporosis Prevention (POP) study is a population-based prospective, controlled PA intervention study that includes children from four schools. In one of the schools (intervention school), we increased PA from 60 minutes/week to 200 minutes/week. The remaining three schools continued with 60 minutes/week of PA. We invited children with school start 1998-2000 and followed them from school start until the last year of compulsory school (grade 9). We evaluated duration of PA and sedentary screen-time by questionnaires, bone mass and lean mass by dual-energy X-ray absorptiometry (DXA), muscle strength by a digital dynamometer and bone mass/quality by quantitative ultrasound (QUS). To estimate the overall effect of PA on musculoskeletal development we calculated a composite score as the mean Z-score of lumbar spine bone mineral content (BMC) and bone area (BA), total body lean mass (TBLM), calcaneal speed of sound (SOS) and knee flexion muscle strength. MrOs Sweden is a population-based prospective observational study that includes elderly men. At baseline, we measured bone traits by DXA and QUS and muscle strength as hand grip strength by a handheld dynamometer. We then calculated a musculoskeletal composite score as the mean Zscore of femoral neck BMC and BA, TBLM, hand grip strength and SOS. We used the Physical Activity Scale for the Elderly (PASE) questionnaire to estimate the level of PA. We registered radiographically verified fractures during the follow-up period (median 9.6 years). Results: The PA intervention program was associated with higher duration of total PA in intervention compared to control children, after both three and mean seven years (both p<0.001), while we found no statistically significant group differences in sedentary screen-time. From Tanner stage 1 to 5, the intervention boys and girls both gained significantly more bone mass and better musculoskeletal composite scores than their respective controls (both p<0.05). In old men, a musculoskeletal composite score predicted incident fractures similar to, but not better than, a standard aBMD measurement. Level of PA was associated with both the musculoskeletal composite score and incident fractures. Conclusions: A school-based PA intervention program was associated with increased duration of total PA (thereby refuting the activity-stat theory), with beneficial gains in musculoskeletal traits as well as in a composite score. In old men, a musculoskeletal composite score could predict fractures and level of PA was associated with both the composite score and fracture incidence.