Abstract
BACKGROUND: The flexor pronator mass (FPM) is an important dynamic stabilizer to valgus stress at the elbow and has been reported to protect against ulnar collateral ligament (UCL) injury. Active gripping and pronation have demonstrated reduced ulnohumeral joint space and change in material properties of the UCL when examined in vivo via ultrasound. No studies have utilized ultrasonography and shear wave elastography to characterize the medial elbow's response to FPM activation under valgus stress. This cross-sectional, repeated measures study aims to characterize medial elbow changes in UCL and FPM tissue stiffness and joint space width (JSW) during valgus stress with incremental FPM activation through gripping. METHODS: Thirteen participants (6 male, 7 female) aged 18-40 year with a BMI < 30 and no history of upper extremity injury were included in this study. Elbows were placed in a telos stress device at 30° of flexion and a 100N valgus stress was applied. Participants then activated the FPM by gripping a spherical dynamometer at 100, 75, 50, 25, and 0% of maximal grip strength. UCL thickness, ulnohumeral (UH) JSW, UCL stiffness, and FPM stiffness were measured for each condition and compared via a two-way repeated measures ANOVA and a post hoc Fischer's Least Significant Difference test. RESULTS: Men and women showed no baseline differences in UCL thickness or UH JSW. JSW was significantly wider under valgus load, 2.22 ± 0.42 mm vs. 2.99 ± 0.46 mm in males and 2.15 ± 0.41 mm vs. 2.99 ± 0.55 mm in females (p < 0.05). No statistically significant differences were demonstrated in UH JSW by gripping force magnitude and differences by sex were not observed. Additionally, no significant changes in tissue stiffness were observed during dynamic conditions for shear wave velocities for either the UCL or FPM. CONCLUSION: Gripping does not change UH JSW or medial elbow tissue stiffness in the joint testing configuration and external loading conditions applied in this study. This suggests that gripping may not be as protective during the high valgus stress observed in baseball pitching as believed, and that the influence of FPM activity may be joint position or load dependent.