The Relationship between Vitamin D Status and Rotator Cuff Muscle Strength in Professional Volleyball Athletes

This study aimed to examine the vitamin D status of professional volleyball athletes and to determine its correlation with shoulder muscle strength. We included 52 healthy male professional volleyball players (23.2 ± 4.5 years), who were categorized by vitamin D status (<20 ng/mL: deficiency, 20-30 ng/mL: insufficiency, and >30 ng/mL: sufficiency). We examined the strength of the internal rotator (IR) and external rotator (ER) muscles of the shoulder by using an isokinetic dynamometer. Fourteen players (26.9%) had vitamin D deficiency, 24 players (46.2%) were vitamin D-insufficient, and 14 players (26.9%) were vitamin D-sufficient. There was no significant correlation between vitamin D level and shoulder muscle strength at 60°/s (IR, r = 0.159, p = 0.26; ER, r = 0.245, p = 0.08) and at 180°/s (IR, r = -0.093, p = 0.51; ER, r = -0.037, p = 0.79). Moreover, the isokinetic shoulder strengths were not significantly different across the three groups in all settings. In conclusion, vitamin D insufficiency was common in elite volleyball players. Though not being associated with isokinetic muscle weakness, vitamin D levels should be regularly monitored, and vitamin D should be supplied to young elite athletes, considering its importance for musculoskeletal health.