Posture-related changes in heteronymous recurrent inhibition from quadriceps to ankle muscles in humans

The possibility was investigated that changes in heteronymous recurrent inh ibition (RI) from quadriceps (Q) to soleus (Sol) and tibialis anterior (TA) motoneurons (MNs) occur during postural tasks requiring cocontraction of Q with one of these muscles. Stimulation of the femoral nerve (FN), which el icited a Q H-reflex discharge, was used to activate Renshaw cells. The resu lting inhibition of TA and Sol MNs was assessed using three test responses: (1) the rectified and averaged ongoing electromyogram (EMG) activity in TA or Sol; (2) the motor-evoked potential (MEP) elicited by cortical stimulat ion in these muscles; and (3) the Sol H reflex. The characteristics of the depression (appearance and increase with the conditioning reflex discharge, short central delay and long duration) are consistent with a Renshaw origi n. In addition, results obtained in control experiments (no change in the E MG suppression after an ischaemic blockade of group-I afferents from the le g, time course of the FN-induced depression of the MEP similar to that of t he ongoing EMG) made a significant contribution from other pathways activat ed by FN stimulation unlikely. Posture-related heteronymous RI from Q was c ompared in different postural tasks at matched levels of background EMC act ivity: voluntary co-contraction of Q and of the relevant ankle muscle while sitting (control situation), postural co-contraction of Q and TA (while le aning backwards during stance), or contraction of Sol with (preparation for hopping) and without (standing on tip of toes and leaning forwards during stance) associated contraction of the Q. During stance, heteronymous RI fro m Q was reduced to TA (but not to Sol) while leaning backwards and to Sol i n preparation for hopping, but not in the other situations. Thus, RI from Q to TA or Sol was specifically decreased when a cocontraction of the Q and of the relevant muscle operating at the ankle was required to maintain bipe dal stance. It is argued that this control of Renshaw cells is descending i n origin and contributes to selection of the appropriate synergism in vario us postural tasks.