Does Vitamin D Affect Muscle Strength and Architecture? An Isokinetic and Ultrasonographic Study

The objective of this study was to explore the association between 25-hydroxyvitamin D (25(OH)D) and muscle strength/architecture. Thirty patients (27 F, 3 M) were allocated into Group I (N=15, mean age; 44.4±9.4 years) and Group II (N=15, mean age; 39.0±9.9 years) according to the median of 25(OH)D (＜13.7 ng/mL vs ＞13.7 ng/mL, respectively). Peak torque/body weight of the knee flexor/extensor muscles at 60°/sec and 180°/sec and those of ankle flexor/extensor muscles at 30°/sec and 90°/sec were evaluated by using a Biodex System 3 Pro Multijoint System isokinetic dynamometer. A 7-12 MHz linear array probe was used to evaluate thickness (MT), pennation angle (PA) and fascicle length (FL) of medial gastrocnemius and vastus lateralis muscles. Mean of 25(OH)D was 9.4±2.5 ng/mL and 20.7±8.3 ng/mL in Groups I and II, respectively. Although all isokinetic strength parameters were lower in Group I, significant differences were found in knee flexion at 180°/sec (p=0.007), knee extension at 30°/sec (p=0.038) and 180°/sec (p=0.001), and ankle extension at 30°/sec (p=0.002) and 90°/sec (p=0.007). On the other hand, no significant difference was found between the groups regarding MT, PA and FL values (all p＞0.05). In light of our results, we can argue that 25(OH)D is associated with muscle strength but not with muscle architecture. Further studies concerning the long-term follow-up effects of 25(OH)D treatment on muscle strength are awaited.