Serum [25(OH)D] status, ankle strength and activity show seasonal variation in older adults: relevance for winter falls in higher latitudes

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). Conclusion: ankle dorsillexor 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.