Sensor-Based Assessment of Quadriceps EMG-Amplitude-to-Torque Ratios at Different Knee Angles: An Exploratory Cross-Sectional Study.

Surface electromyography (sEMG) normalized to mechanical output can describe electrical activation amplitude relative to torque, but such ratios should not be interpreted automatically as neuromuscular efficiency. This exploratory cross-sectional study compared quadriceps activation-to-torque ratios at 45° and 75° knee flexion and examined hypothesis-generating associations with countermovement jump (CMJ) outcomes. Seventeen participants performed two brief unilateral maximal isometric knee-extension trials at each angle. Wireless sEMG from the vastus lateralis (VL), vastus medialis (VM), and rectus femoris (RF) and Biodex torque were acquired separately and manually matched by participant, knee angle, and test set. Ratios were calculated as Noraxon-exported mean EMG amplitude divided by Biodex average peak torque. Ratios were higher at 45° than at 75° for VL (mean difference = 0.74 μV/(N·m), BCa 95% CI: 0.34-1.34; Holm-adjusted p = 0.020), VM (0.52, 0.09-0.72; p = 0.009), and RF (0.69, 0.22-1.32; p = 0.028). Exploratory Spearman correlations with CMJ outcomes ranged from -0.38 to 0.32 and were nonsignificant after false-discovery-rate correction. The findings indicate protocol-specific, angle-dependent differences, whereas CMJ associations remain inconclusive and require larger, reliability-focused studies with synchronized acquisition and fully documented signal processing.