title: Serum [25(OH)D] status, ankle strength and activity show seasonal variation in older adults: relevance for winter falls in higher latitudes.
abstract: BACKGROUND
  seasonal variation exists in serum [25(OH)D] and physical activity, especially at higher latitudes, and these factors impact lower limb strength. This study investigates seasonal variation in leg strength in a longitudinal repeated measures design concurrently with serum vitamin D and physical activity.
  
  
  METHODS
  eighty-eight community-dwelling independently mobile older adults (69.2 ± 6.5 years) were evaluated five times over a year, at the end of five consecutive seasons at latitude 41.1°S, recruited in two cohorts. Leg strength, serum [25(OH)D] and physical activity levels were measured. Time spent outside was recorded. Monthly falls diaries recorded falls. Data were analysed to determine annual means and percentage changes.
  
  
  RESULTS
  significant variation in [25(OH)D] (±15%), physical activity (±13%), ankle dorsiflexion strength (±8%) and hours spent outside (±20%) (all P < 0.001) was demonstrated over the year, with maximums in January and February (mid-summer). Low mean ankle strength was associated with increased incidence of falling (P = 0.047). Quadriceps strength did not change (±2%; P = 0.53).
  
  
  CONCLUSIONS
  ankle dorsiflexor strength varied seasonally. Increased ankle strength in summer may be influenced by increased levels of outdoors activity over the summer months. Reduced winter-time dorsiflexor strength may predispose older people to increased risk of tripping-related falls, and warrants investigation in a multi-faceted falls prevention programme.
authors.affiliation: School of Human Life Sciences, University of Tasmania, Locked Bag 1320, Launceston, Tasmania 7250, Australia. birdm@utas.edu.au
authors.forename: Marie-Louise
authors.initials: ML
authors.lastname: Bird
authors.forename: Keith D
authors.initials: KD
authors.lastname: Hill
authors.forename: Iain K
authors.initials: IK
authors.lastname: Robertson
authors.forename: Madeleine J
authors.initials: MJ
authors.lastname: Ball
authors.forename: Jane
authors.initials: J
authors.lastname: Pittaway
authors.forename: Andrew D
authors.initials: AD
authors.lastname: Williams
mesh_terms: D000058:Accidental Falls / Q000517:prevention & control / Q000706:statistics & numerical data*; D000367:Age Factors; D000368:Aged; D000369:Aged, 80 and over; D000375:Aging; D000843:Ankle Joint / Q000503:physiopathology*; D015415:Biomarkers / Q000097:blood; D001696:Biomechanical Phenomena; D015577:Geriatric Assessment; D006801:Humans; D057187:Independent Living; D007719:Knee Joint / Q000503:physiopathology; D016014:Linear Models; D008137:Longitudinal Studies; D008875:Middle Aged; D009043:Motor Activity; D053580:Muscle Strength; D018482:Muscle, Skeletal / Q000503:physiopathology*; D017711:Nonlinear Dynamics; D052097:Quadriceps Muscle / Q000503:physiopathology; D018570:Risk Assessment; D012307:Risk Factors; D012621:Seasons; D013648:Tasmania / Q000453:epidemiology; D013997:Time Factors; D014807:Vitamin D / Q000031:analogs & derivatives* / Q000097:blood; D014808:Vitamin D Deficiency / Q000097:blood / Q000175:diagnosis / Q000453:epidemiology*
chemical_list: D015415:Biomarkers; D014807:Vitamin D; C104450:25-hydroxyvitamin D