Inability of Quadriceps Muscular Stiffness to Explain Variances in Torque during Isometric Knee Extension Tests Across Different Degrees of Knee Position in Healthy Individuals

Abstract Objective This study aims to assess how internal (IR) and external (LR) knee joint rotation affect stiffness and strength in quadriceps femoris (RF, VL, and VM) during isometric knee extensor tests at various knee flexion angles (30°, 60°, 90°), and to explore their relationship. Methods A descriptive observational cohort study recruited 46 healthy young adults. Participants performed maximum voluntary quadriceps contractions at three knee flexion angles (30°, 60°, 90°), with three tibial rotation conditions (internal 10°, neutral, external 10°) set for each angle. Muscle hardness of dominant lower limb RF, VL, and VM was recorded using MyotonPro during isometric knee extensor tests. Isokinetic training instrument measured quadriceps femoris strength under neutral, 10° external, or 10° internal tibial rotation. Statistical analysis employed repeated measures general linear models. Results Knee joint rotation and flexion did not interact. Muscle hardness of RF, VL, and VM significantly increased during isometric knee extension compared to resting, but rotation angle and tibial position showed no significant differences. Quadriceps femoris strength was unaffected by 10° tibial rotation but varied with knee flexion angle, notably greater at 60° flexion (P < 0.01). Moderate correlations were observed between quadriceps muscle hardness and strength in each group. Conclusion Isometric knee extension force remains unaffected by tibial position but correlates with knee flexion angle. However, quadriceps femoris muscle hardness does not vary significantly with knee rotation or flexion. MyotonPro-measured muscle hardness under different conditions may not fully explain corresponding muscle strength variations.