title: Habitual levels of higher, but not medium or low, impact physical activity are positively related to lower limb bone strength in older women: findings from a population-based study using accelerometers to classify impact magnitude.
abstract: This study assessed the effect of accelerometry-measured higher impacts resulting from habitual weight-bearing activity on lower limb bone strength in older women. Despite higher impacts being experienced rarely in this population-based cohort, positive associations were observed between higher vertical impacts and lower limb bone size and strength.
  
  
  
  We investigated whether the benefit of habitual weight-bearing physical activity (PA) for lower limb bone strength in older women is explained by exposure to higher impacts, as previously suggested by observations in younger individuals.
  
  
  
  Four hundred and eight women from the Cohort for Skeletal Health in Bristol and Avon (COSHIBA), mean 76.8 years, wore tri-axial accelerometers at the waist for a mean of 5.4 days. Y-axis peaks were categorised, using previously identified cutoffs, as low (0.5-1.0 g), medium (1.0-1.5 g), and higher (≥1.5 g) impacts. Mid and distal peripheral quantitative computed tomography scans of the tibia and radius were performed, as were hip and lumbar spine Dual X-ray Absorptiometry (DXA) scans. Regressions between (log transformed) number of low, medium and high impacts, and bone outcomes were adjusted for artefact error grade, age, height, fat and lean mass and impacts in other bands.
  
  
  
  Eight thousand eight hundred and nine (4047, 16,882) low impacts were observed during the measurement week, 345 (99, 764) medium impacts and 42 (17, 106) higher impacts (median with 25th and 75th quartiles). Higher vertical impacts were positively associated with lower limb bone strength as reflected by cross-sectional moment of inertia (CSMI) of the tibia [0.042 (0.012, 0.072) p = 0.01] and hip [0.067 (0.001, 0.133) p = 0.045] (beta coefficients show standard deviations change per doubling in impacts, with 95 % confidence interval). Higher impacts were positively associated with tibial periosteal circumference (PC) [0.015 (0.003, 0.027) p = 0.02], but unrelated to hip BMD. Equivalent positive associations were not seen for low or medium impacts.
  
  
  
  Despite their rarity, habitual levels of higher impacts were positively associated with lower limb bone size and strength, whereas equivalent relationships were not seen for low or medium impacts.
authors.affiliation: Musculoskeletal Research Unit, University of Bristol School of Clinical Sciences, Southmead Hospital, Bristol, BS10 5NB, UK.
authors.forename: K
authors.initials: K
authors.lastname: Hannam
authors.affiliation: Musculoskeletal Research Unit, University of Bristol School of Clinical Sciences, Southmead Hospital, Bristol, BS10 5NB, UK.
authors.forename: K C
authors.initials: KC
authors.lastname: Deere
authors.affiliation: Musculoskeletal Research Unit, University of Bristol School of Clinical Sciences, Southmead Hospital, Bristol, BS10 5NB, UK.
authors.forename: A
authors.initials: A
authors.lastname: Hartley
authors.affiliation: Musculoskeletal Research Unit, University of Bristol School of Clinical Sciences, Southmead Hospital, Bristol, BS10 5NB, UK.
authors.forename: U A
authors.initials: UA
authors.lastname: Al-Sari
authors.affiliation: Musculoskeletal Research Unit, University of Bristol School of Clinical Sciences, Southmead Hospital, Bristol, BS10 5NB, UK.
authors.forename: E M
authors.initials: EM
authors.lastname: Clark
authors.affiliation: University of East Anglia, Norwich, UK.
authors.forename: W D
authors.initials: WD
authors.lastname: Fraser
authors.affiliation: Musculoskeletal Research Unit, University of Bristol School of Clinical Sciences, Southmead Hospital, Bristol, BS10 5NB, UK. Jon.Tobias@bristol.ac.uk.
authors.forename: J H
authors.initials: JH
authors.lastname: Tobias
mesh_terms: D015502:Absorptiometry, Photon; D061725:Accelerometry / Q000379:methods; D000368:Aged; D000369:Aged, 80 and over; D000886:Anthropometry / Q000379:methods; D015519:Bone Density / Q000502:physiology*; D016723:Bone Remodeling / Q000502:physiology; D015331:Cohort Studies; D015444:Exercise / Q000502:physiology*; D005260:Female; D006112:Gravitation; D006621:Hip Joint / Q000503:physiopathology; D006801:Humans; D008159:Lumbar Vertebrae / Q000503:physiopathology; D011884:Radius / Q000503:physiopathology; D013977:Tibia / Q000503:physiopathology; D014057:Tomography, X-Ray Computed; D016474:Weight-Bearing / Q000502:physiology
keywords: G-force; Hip BMD; Tibial pQCT