SEGMENTAL REFLEXES AND ANKLE JOINT STIFFNESS DURING COCONTRACTION OF ANTAGONISTIC ANKLE MUSCLES IN MAN

The size of soleus H-reflexes and short-latency stretch reflexes was m easured at different levels of plantar flexion or co-contraction (simu ltaneous activation of dorsi- and plantar flexors) in seven healthy su bjects. In four of seven subjects the short-latency stretch reflex was smaller during weak co-contraction than during isolated plantar flexi on at matched background electromyogram (EMG) levels in the soleus mus cle. In three of these four subjects the stretch reflex was larger dur ing strong co-contraction than during plantar flexion, whereas it had the same size during the two tasks in the last subject. In the remaini ng subjects the stretch reflex either had the same size or was larger at all levels of co-contraction than at similar levels of plantar flex ion. In contrast, the H-reflex was found to decrease with co-contracti on at all contraction levels in all subjects. The decrease in the refl exes during weak co-contraction might be caused by presynaptic inhibit ion of Ia afferents. It is unclear why only the H-reflex decreased dur ing strong co-contraction. The stiffness of the ankle joint was measur ed from the torque increment following the stretch of the plantar flex ors divided by the stretch amplitude. In all subjects the total stiffn ess of the ankle joint was larger during strong co-contraction than du ring plantar flexion of similar strength. The stiffness was smaller du ring weak co-contraction than during weak plantar flexion in three out of seven subjects. The medial gastrocnemius muscle was more active at a given level of soleus activity during the co-contraction task than during the isolated plantar flexion task. It is suggested that the inc rease in the stiffness during co-contraction as compared to isolated p lantar flexion was mainly due to the mechanical contribution of the ac tivity in the tibialis anterior and medial gastrocnemius muscles. The decrease in stiffness during weak co-contraction was, in contrast, mos t likely mainly caused by modulation of reflex stiffness.