Vitamin D deficiency is common in indoor athletes. The purpose of the present study was to investigate 25-hydroxyvitamin D (25(OH)D) levels in futsal players before and after wintertime and to analyse the relationship between lower limb skeletal muscle strength. Serum 25(OH)D levels together with other markers of bone metabolism were investigated at two time points in 19 professional futsal players (mean age 20.5 � 1.27 year; height 165.35 � 4.73 cm; weight 56.6 � 5.51 kg). Total 25(OH)D concentrations were quantified using IDS-iSYS (IDS plc, Boldon, UK). Serum levels of intact parathyroid hormone were measured by immunoassay. Serum calcium and phosphorus were evaluated by clinical biochemistry. Concentric quadriceps and hamstring strength were measured using a Biodex System 3 Dynamometer at angular velocities 60, 180 and 300 degrees xs(-1). The average vitamin D level was 20.03�5.75 ng/ml on November, 2016 and 14.19�4.48 ng/ml on April, 2017 among players. Serum 25(OH)D levels decreased (p= 0.000) in April (14.19�4.48 ng/ml, ranging from 7.0 to 24.6 ng/ml) compared to November (20.03�5.75 ng/ml, ranging from 7.14 to 29.6 ng/ml) . Isokinetic testing demonstrated significant decreases between November and April in concentric right and left hamstrings peak torque at angular velocities 180 and 300 degrees xs(-1), respectively. Concentric right hamstring total work significantly decreased both at angular velocities 180 and 300 degrees xs(-1). According to our results, vitamin D deficiency seems to have an effect upon concentric isokinetic knee flexion after winter.

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