Selective activation of MG in sural nerve reflexes during hopping, running and walking

Abstract

Stimulation of the sural nerve activates MG (medial gastrocnemius) more strongly than LG (lateral gastrocnemius) in acute cat experiments. In contrast, in sitting man previous studies have failed to establish such differential effects. Since some cutaneous reflexes are suppressed under passive conditions, the issue was reexamined using electrical stimulation of the sural nerve at the ankle during various kinds of rhythmic activities such as hopping, walking and running. Stimuli at two times perception threshold were applied at each of 16 phases of the cycle in five human volunteers. For all three movements the responses in MG were mostly facilitatory on the ipsilateral side while for LG the responses were almost always suppressive. The MG facilitatory responses were more prominent during running and hopping as compared to walking. During running the MG responses were most prominent when elicited in the second half of the stance phase. During hopping the largest responses were present in the period prior to contact, when background activity was low. Presumably this latter activation could contribute to 'anticipated stiffness' prior to touchdown. In contrast, only small responses were present in the second half of the contact period. Finally, the MG responses. were small or even absent in the early contact phase. In general, contralateral to the stimulus the reflex responses of medial and lateral head of the gastrocnemius muscle were quite similar, except for running where facilitatory responses were more activated in LG than in MG. It is speculated that the difference between MG and LG in the ipsilateral leg is 'hard-wired' and expressed even in movements such as hopping when a rotation moment is not expected to have functional significance. (C) 1998 Elsevier Science B.V.