Effects of grip contraction with different finger recruitment and different wrist positions on dynamic medial elbow joint stability.
BACKGROUND Medial ulnar collateral ligament (MUCL) injuries are increasing among baseball players. Flexor-pronator muscles, particularly the flexor digitorum superficialis (FDS), are crucial for dynamic medial elbow stability. The FDS consists of superficial (third and fourth digits) and deep (second and fifth digits) layers; however, their isolated roles in medial elbow stability remain unclear. The wrist position also differs among pitch types, such as fastballs and curveballs, potentially affecting medial elbow stability. This study aimed to examine the effect of excluding the second- or third-digits during grip contraction on medial elbow stability, and to clarify the effect of wrist position on grip-induced medial elbow stability. METHODS Twenty healthy right-handed men (mean age: 22.4 ± 1.6 years, mean height: 172.7 ± 5.5 cm, mean weight: 64.4 ± 6.8 kg) were included. The nondominant arm was fixed at 60° shoulder abduction, 90° external rotation, and 90° elbow flexion using a Telos device. Medial elbow joint space (JS) was measured using ultrasonography under three conditions: rest (0 N), valgus stress (60 N), and 60 N with 50% maximal voluntary contraction (MVC) grip. In Experiment 1, we compared three grip tasks: all-digits, no-second digit, and no-third digit. In Experiment 2, medial elbow stability was examined in three different wrist positions during gripping: neutral, dorsiflexion, and palmar flexion. Dynamic stabilization value ([JS at 60N valgus stress] - [JS at 60N valgus stress with 50%MVC]) was calculated in each condition and compared using repeated-measures ANOVA or the Friedman test (p < 0.05), with larger values indicating better dynamic stabilization function. RESULTS JS significantly increased from 0 to 60 N (ΔJS: 0.8 ± 0.2 mm, p < 0.001) and decreased at 60 N with 50% MVC under all conditions (ΔJS: -0.5 ± 0.3 mm, p = 0.003). In Experiment 1, the dynamic stabilization value was smaller in the no-second-digit condition than in the all-digit condition (p < 0.001), whereas no significant difference was observed between the all-digit and no-third-digit conditions. In Experiment 2, the dynamic stabilization value was smaller in dorsiflexion and palmar flexion than in neutral (p = 0.022 and p < 0.001, respectively) and smaller in palmar flexion than in dorsiflexion (p = 0.022). CONCLUSION Recruitment of the second-digit flexor muscle is essential for medial elbow stability, suggesting that strengthening the second-digit FDS may enhance medial stability Additionally, grip contraction with palmar flexion did not sufficiently stabilize the medial elbow joint. LEVEL OF EVIDENCE Basic Science Study; Kinesiology.