Joint-afferent-mediated muscle activations yield a near-maximum torque response of the quadriceps

Previous work from our laboratory has shown that reflex activity is systematically evoked in a number of major knee muscles by large (>5 degrees ) (non-physiological) abduction angular perturbations of the human knee. This reflex action was shown to originate from periarticular tissue afferents. Furthermore, it was demonstrated that specific muscle activation patterns exist in knee muscles with preferential activation in medial muscles in response to the lateral perturbations. This study examines the hypothesis that in response to the mechanical stimulus, the sensory information mediated by these afferents results in activation patterns that provide the largest resisting moment by the knee muscles. It is further hypothesized that this near maximum resistance cannot be achieved by the selective activation of medial muscles alone. To examine this, the previously reported mechanically induced reflex EMG activation patterns, a stochastic 3D musculoskeletal patello-femoral joint model, and new data from selective electrical stimulation experiments were used. Using the model, the knee adduction-abduction moment in response to an applied abduction load at the knee joint for every possible random set of quadriceps activity was computed. These adduction moments were then compared to the adduction moment computed by the model when the mechanically induced muscle activation patterns were used. The data presented here illustrated that selective activation of a medial muscle alone would result in an abduction moment, regardless of the knee flexion angle. Furthermore, the findings of this study revealed that the recorded combinations of muscle activity provide a near maximum capability of the quadriceps muscles to resist externally applied abducting stimuli. It was concluded that stabilization in the abduction direction could only be achieved by a control strategy that involves activation of both medial and lateral muscles at the knee. It was also concluded that this control strategy was near optimal when mediated by joint afferents.