The effect of ankle joint position and effort on quadriceps reflex sensitivity.

OBJECTIVE To investigate a possible ankle-knee synergy, experiments with normal subjects were performed to compare changes of the quadriceps motor pool excitability due to ankle position and effort. METHODS Vastus medialis H reflex amplitude was examined during ankle isometric contractions conditioned by different ankle positions (dorsiflexion, neutral, and plantarfiexion) with or without voluntary effort (either in the dorsiflexion or plantarflexion direction). Repeated measures ANOVAs were performed on the mean and standard deviation of the H peak-to-peak amplitude. RESULTS Mean vastus medialis H reflex amplitudes were significantly different among the trials of different ankle efforts (P<0.05), and significantly increased during plantarfiexion efforts. In contrast, mean vastus medialis H reflex amplitude did not vary with respect to changes of ankle position (P>0.05). CONCLUSIONS The data suggest that (1) the position of the ankle joint did not significantly modify the excitability of the neuromotor pool of the VM muscle, in either static or active cases, and (2) the effort effect from the ankle joint on the VM neuromotor pool is most significant during ankle plantarfiexion effort. Possible mechanisms are central motor irradiation and peripheral force-dependent pathways from the ankle joint that influence the VM neuromotor pool.