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Reflex responses to ligament loading: implications for knee joint stability

To assess the neuromuscular reflex responses to loading of knee ligaments, we applied an abducting positional deflection to the fully extended knee using a servomotor, and recorded EMG activity in pre-activated quadriceps and hamstrings muscles with surface electrodes. To establish that the reflex response elicited by the perturbation was not simply a form of muscle stretch response, a tendon tap was applied to the muscles at the same setting and the reflex responses recorded from the same subject. Contrary to the typical muscle stretch reflex, which is of short duration and has a short latency (28-35 msecs for knee muscles), the abduction response was characterized by long latency responses with an initial EMG peak followed by sustained muscle activity throughout the duration of the step perturbation. The latency of the abduction response was at least twice the latency of the tap reflex suggesting a different (possibly ligamentous) origin (60-85 msecs for knee muscles). This reflex was consistent throughout the set of trials performed at the same speed and amplitude of perturbation. Our investigation showed that the sustained activity of the abduction reflex is dependent on the amplitude of the perturbing stimulus. Furthermore, the knee muscle contractions elicited were sometimes selective, depending on the magnitude of the angular perturbation.