OpenCap vs. LESS: Sport-Specific Profiling of Dynamic Knee Valgus

Background/Objectives: This study investigated the relationships among the Landing Error Scoring System (LESS), markerless 3D (OpenCap)-derived knee valgus, and surface electromyography (EMG) of quadriceps and hamstrings. Methods: Thirty-two healthy male university students (26 athletes, 6 non-athletes) completed a drop-landing task. LESS was video-scored; knee valgus at peak knee flexion was computed with OpenCap; and bilateral rectus femoris (RF) and biceps femoris (BF) EMG was recorded. Phase-specific EMG was normalized to peak dynamic activity. Results: LESS showed an independent negative association with left knee valgus (p = 0.001). In the regression model, bilateral BF acceleration-phase activity was a significant predictor of knee valgus (p < 0.05). Exploratory comparisons indicated that athletes exhibited lower RF deceleration activity and smaller left-side valgus than non-athletes. Conclusions: These findings suggest that hamstring activity during the transition phase is linked to knee alignment. This study demonstrates the complementary value of integrating LESS with markerless 3D motion capture, although caution is warranted when generalizing group differences due to the unequal sample size.