Pain effect on monosynaptic and polysynaptic reflex inhibition

Abstract

Objective: Determine the electrophysiological effects of anterior knee pain using monosynaptic and polysynaptic reflexes. Design: A pretest/posttest control group experimental design with repeated measures. Setting: All tests were performed in a university laboratory. Participants: Six subjects suffering from patello-femoral dysfunction (PFD) composed the experimental group. These patients were diagnosed and referred by medical and chiropractic doctors. Six healthy subjects were recruted for the control group. Intervention: Pain relief by cold application (10 minutes) on the knee. Main Outcome Measures: Monosynaptic reflex: H reflex of the superficial portions of the quadriceps femoris. Polysynaptic reflexes: nociceptive flexion response (NFR) of the long head of the biceps femoris. Results: Cold application, producing significant pain relief ( p < 0.05), did not change H-reflex amplitude ( p > 0.05) for both groups and did not show modulation of the NFR. However, the PFD group showed significantly lower pain ( p < 0.05) and NFR thresholds ( p < 0.05) than the control group. Finally, nociceptive stimulation was applied at the sural nerve during an isometric extension of the knee (20% of maximum electromyographic contraction). After this stimulation, uni-articular extensors, especially the distal fibers of the vastus medialis, showed a longer and stronger inhibition than the bi-articular rectus femoris. Conclusions: Pain episodes do not appear to affect monosynaptic responses. NFR threshold differences found between PFD and control group could be explained by the increased excitability of the nervous system in subjects experimenting painful bouts. Functional implications of these findings are discussed.