Musculotendon translational stiffness and muscle activity are modified by shear forces

BACKGROUND: In this study we investigate whether the nervous system is responsive to anterior-posterior shearing forces applied to the knee. Specifically, we examine whether the nervous system will increase musculotendon stiffness along the anterior-posterior degree-of-freedom via relative changes in muscle activation. METHODS: While seated on a Biodex, 12 male participants performed knee extensor moments against a cuff at three different distances (265mm, 310mm, and 355mm) from the knee joint. This was done for each combination of knee angle (20°, 45°, 70°), moment (25%, 75% of maximum), and angular velocity (0°/s, 10°/s). By altering cuff position, the magnitude of the shear force could be changed while holding moment constant. We then calculated the 6-degree-of-freedom, musculotendon stiffness matrix of the knee and interpreted the eigenvalues and eigenvectors of this matrix using stiffness ellipses. FINDINGS: Anterior-Posterior knee stiffness significantly increased with an increase in shear force at 20° and 45° of knee flexion. This coincided with significant increases in semitendinosus and vastus medialis activity. INTERPRETATION: We found that the nervous system can significantly increase stiffness along a translational degree-of-freedom in response to translational loading, and did so by activating muscles geometrically oriented to provide stiffness along the anterior-posterior degree-of-freedom.