The purposes of the present study were (1) to develop a method to estimate the level of reciprocal inhibition (RI) between antagonist (soleus and anterior tibial) muscles in humans, and (2) to compare RI levels during rest, dorsiflexion (DF) and plantar flexion (PF). Nine healthy subjects (four men, five women) aged between 20 and 30 years were assessed. Each subject remained seated with his/her right foot strapped to a rigid foot plate coupled to a torquemeter; measurements were taken at rest and during isometric contraction of the ankle dorsiflexor and plantar flexor muscles. The soleus muscle H-wave was captured by surface electrodes. A "test" H- reflex was elicited by a stimulus (electrical pulse) to the popliteal fossa (tibial nerve). The "conditioned" H-reflex was obtained by paring two stimuli: the first applied to the head of the peroneal bone, and the second, following 1-to-3 ms, to the popliteal fossa. Peak-to-peak amplitudes of "test" and "conditioned" H reflexes were used to calculate RI. RI values obtained were 16.41%±8.68 at rest; 21.94%±5.39 in DF; and 3.12%±11.84 in PF. Reciprocal inhibition was significantly (p<0.05) lesser at PF than in the other conditions, but no differences were detected between DF and rest. Results suggest that reciprocal inhibition is modulated during voluntary activity.
H-Reflex; Movement; Muscle contraction; Neurophysiology
