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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 …