Knee muscle contributions to joint rotational stiffness.

The purpose of this paper was to investigate total joint rotational stiffness (JRS) and relative muscle contributions to JRS with varying extensor moment demands. It was hypothesized that greater co-activation of the flexors at lower levels of moment would result in greater JRS, relative to moment demands. It was also hypothesized that the flexors would have greater relative JRS contributions at lower moment levels. Twelve male participants generated isometric extensor moments about the knee at varying intensities, during plateau and ramping (up and down) conditions. Electromyography was used to estimate individual muscle forces, which were used to calculate JRS about three orthogonal axes. Orthogonal trend analysis revealed a linear relationship (p<.001) between moment and JRS, about all axes. The vastus lateralis provided the greatest JRS about all axes. Of the flexors, the semimembranosis provided the most JRS about the flexion/extension (FE) and axial (AX) axes. However, while most muscle activity increased linearly, the gastrocnemious lateral (GL) had an interaction between condition and moment. Despite an extensor moment, the GL had minor contributions about the FE axis (1.2%), and it was postulated that this increase in activation was to stabilize about the VV axis, where its contribution was as high as 18.9%.