SummaryThis observational study assessed vertical impacts experienced in older adults as part of their day-to-day physical activity using accelerometry and questionnaire data. Population-based older adults experienced very limited high-impact activity. The accelerometry method utilised appeared to be valid based on comparisons between different cohorts and with self-reported activity.IntroductionWe aimed to validate a novel method for evaluating day-to-day higher impact weight-bearing physical activity (PA) in older adults, thought to be important in protecting against osteoporosis, by comparing results between four cohorts varying in age and activity levels, and with self-reported PA levels.MethodsParticipants were from three population-based cohorts, MRC National Survey of Health and Development (NSHD), Hertfordshire Cohort Study (HCS) and Cohort for Skeletal Health in Bristol and Avon (COSHIBA), and the Master Athlete Cohort (MAC). Y-axis peaks (reflecting the vertical when an individual is upright) from a triaxial accelerometer (sampling frequency 50 Hz, range 0–16 g) worn at the waist for 7 days were classified as low (0.5–1.0 g), medium (1.0–1.5 g) or higher (≥1.5 g) impacts.ResultsThere were a median of 90, 41 and 39 higher impacts/week in NSHD (age 69.5), COSHIBA (age 76.8) and HCS (age 78.5) participants, respectively (total n = 1512). In contrast, MAC participants (age 68.5) had a median of 14,322 higher impacts/week. In the three population cohorts combined, based on comparison of beta coefficients, moderate-high-impact activities as assessed by PA questionnaire were suggestive of stronger association with higher impacts from accelerometers (0.25 [0.17, 0.34]), compared with medium (0.18 [0.09, 0.27]) and low impacts (0.13 [0.07,0.19]) (beta coefficient, with 95 % CI). Likewise in MAC, reported moderate-high-impact activities showed a stronger association with higher impacts (0.26 [0.14, 0.37]), compared with medium (0.14 [0.05, 0.22]) and low impacts (0.03 [−0.02, 0.08]).ConclusionsOur new accelerometer method appears to provide valid measures of higher vertical impacts in older adults. Results obtained from the three population-based cohorts indicate that older adults generally experience very limited higher impact weight-bearing PA.Electronic supplementary materialThe online version of this article (doi:10.1007/s00198-016-3810-5) contains supplementary material, which is available to authorized users.

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We read the recent paper by Hong and colleagues (1) with interest because the title of this original article suggests an inverse association between fat mass and bone mass on hip geometry in adolescents. This is in direct contradiction to three large studies we have carried out on the Avon Longitudinal Study of Parents and Children (ALSPAC) cohort using different methods to examine the association between fat and bone in adolescents. First, we used combined cross-sectional and prospective analyses of 3503 children from the United Kingdom and reported a clear and independent positive association between fat mass and bone mass or size measured by dual-energy X-ray absorptiometry (DXA). (2) Second, we used peripheral quantitative computed tomography (pQCT) to estimate cortical bone mass on the same cohort and again reported an independent positive association between fat mass and cortical bone mass (greater in girls than in boys). (3) Third, we used a Mendelian randomization approach to further categorize the association between fat and bone in childhood and again reported an independent positive association. (4) We are therefore surprised by the title of the article by Hong and colleagues. However, we believe that it is misleading and that their conclusion is erroneous. When analyzing relationships between body composition variables, one has to account for the fact that important relationships exist between fat and lean mass and that the latter has a very strong positive relationship with bone mass. Therefore, when exploring associations between fat mass and bone mass specifically, it is essential that the term for fat mass is independent of lean mass. We achieved this is our previous studies by analyzing associations between fat mass and bone mass after adjusting for lean mass. Unfortunately, Hong and colleagues analyzed associations between bone mass and percentage of fat adjusted for body weight, which does not allow the effects of fat mass to be analyzed independent of lean mass because those with a higher percentage of fat adjusted for body weight of necessity will have a lower percentage of lean mass. Therefore, their observation of an inverse association between percentage of fat adjusted for body weight and bone mass is likely to be driven by the strong positive association between lean mass and bone mass that is well described. We believe that Hong and colleagues should reanalyze their data using percentage of fat mass adjusted for lean mass or height, not body weight.

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