| In a healthy joint, stresses in the soft tissues and articular cartilage are attenuated by coordinated voluntary and reflexive muscle activation. This muscular contribution to joint stability is especially important when external loading occurs in secondary degrees of freedom (DOF), such as the frontal plane of the knee, that are primarily constrained by passive tissues. To facilitate muscle activations that mitigate soft tissue stresses, the central nervous system utilizes afferent feedback derived from skin, muscles, tendon, and joint mechanoreceptors, which reflect the intrinsic mechanical properties of the joint. The central theme of this research was to examine the interaction between intrinsic joint mechanics and neurophysiological manifestations of afferent feedback in a constrained DOF. Specifically, controlled frontal plane joint rotations were applied at the human knee, while assessing both motor and perceptual responses. In the first set of experiments, gender differences in passive joint stiffness and muscle activations in response to frontal plane loading were examined. Compared to male participants, females demonstrated significantly decreased passive joint stiffness and reduced reflexive muscle activation, both in terms of the frequency and intensity of muscle contractions, in response to rapid valgus loading. Diminished reflex responses in females may indicate that the position based valgus loading produced an insufficient mechanical stimulus to elicit reflexes, due to reduced passive joint stiffness. In the second set of experiments, the conscious perception of joint movement and position (proprioception) was explored across the frontal and sagittal planes of movement in healthy and osteoarthritic (OA) knees. A differential proprioceptive sensitivity, favoring the frontal plane, was noted in healthy participants, perhaps indicating the need for more precise neuromuscular control in constrained DOFs. However, this relationship deteriorated in knee OA participants, who demonstrated impaired proprioception compared to healthy subjects in all directions. Proprioceptive deficits in knee OA were only weakly associated with reduced passive joint stiffness, suggesting that neural, rather than mechanical, primarily factors affect proprioception in knee OA. Overall, these studies elucidate the complex interactions between intrinsic joint mechanics and motor and perceptual responses to imposed joint loading and provide insights into the neuromechanical contributions to frontal plane knee stability. |
