Musculotendon translational stiffness and muscle activity are modified by shear forces

BackgroundIn 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.MethodsWhile seated on a Biodex, 12 male participants performed knee extensor moments against a cuff at three different distances (265 mm, 310 mm, and 355 mm) 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.FindingsAnterior–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.InterpretationWe 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.