We have previously shown that abduction angular perturbations applied to the knee consistently elicit reflex responses in knee joint musculature (Dhaher et al., 2003). Although a stabilizing role for such reflexes is widely proposed, there are as of yet no studies quantifying the contribution of these reflex responses to joint stiffness. In the present study, we estimate the mechanical contributions of muscle contractions elicited by mechanical excitation of periarticular tissue receptors, to medial-lateral knee joint stiffness. We hypothesize that these reflex muscle contractions will significantly increase knee joint stiffness in the adduction/abduction direction and enhance the over-all stability of the knee. To assess medial-lateral joint 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 was also recorded in both relaxed and pre-activated quadriceps and hamstrings muscles with surface electrodes. A simple, linear,second-order, delayed model was used to describe the knee joint dynamics in the medial/lateral direction. Our data indicate that excitation of reflexes from periarticular tissue afferents results in a significant increase of the joint's adduction-abduction stiffness. Similar to muscle stretch reflex action, which is modulated with background activation, these reflexes also show dependence on muscle activation. The potential significance of this reflex stiffness during functional tasks was also discussed. We conclude that reflex activation of knee muscles is sufficient to enhance joint stabilization in the adduction/abduction direction, where knee medial-lateral loading arises frequently during many activities.