Periarticular reflexes contribute to joint stiffness during valgus loading at the knee

To assess the contribution of periarticular tissue afferents to knee joint varus/valgus stiffness we applied an abducting positional deflection to the fully extended knee using a servomotor and recorded the torque response using a six degree-of-freedom load-cell. EMG activity in pre-activated quadriceps and hamstrings muscles were also recorded with surface electrodes. A simple linear second order delayed model was used to describe the knee joint dynamics in the medial/lateral direction. Our results revealed that periarticular tissue reflexes resulted in a significant increase of the joint´s varus-valgus stiffness. In contrast to stretch reflex action, which is modulated with background force, the periarticular tissue reflexes showed modest dependence on muscle force.