Post-tetanic potentiation of reciprocal Ia inhibition in human lower limb

The purpose of this study was to investigate how reciprocal Ia inhibition i s changed during muscle fatigue of lower limb muscle, induced with a volunt ary contraction or height frequency electrical stimulation Reciprocal Ia in hibition from ankle flexors to extensors has been investigated in 12 health y subjects. Hoffmann reflex (H-reflex) in the soleus muscle was used to mon itor changes in the amount of reciprocal Ia inhibition from common peroneal nerve as demonstrated during voluntary dorsi or planterflexion and 50 Hz e lectrical stimulation induced dorsi or planterflexion. The test soleus H-re flex was kept at 20-25% of maximum directly evoked motor response (M respon se) and the strength of the conditioning common peroneal nerve stimulation was kept at 1.0 x motor threshold. At rest, weak Ia inhibition was demonstr ated in IZ subjects, maximal inhibition from the common peroneal nerve was 28.8%. During voluntary dorsiflexion and 50 Hz electrical stimulation induc ed dorsiflexion, there absolute amounts of inhibition increased as compared to at rest, and decreased or disappeared during voluntary planterflexion a nd 50 Hz electrical stimulation induced planterflexion as compared to at re st. During voluntary or electrical stimulation induced agonist muscle fatig ue, the inhibition of the soleus H-reflex from the common peroneal nerve wa s greater during voluntary dorsiflexion (maximal, 11.1%) and 50 Hz (maximal , 6.7%) electrical stimulation induced dorsiflexion than at rest. The inhib ition was decreased or disappeared during voluntary planterflexion 50 Hz el ectrical stimulation induced planterflexion. it was concluded that the resu lts were considered to support the hypothesis that alpha-motoneurones and I a inhibitory interneurones link to antagonist motoneurones in reciprocal in hibition. The diminished reciprocal Ia inhibition of voluntary contraction during muscle fatigue as compared to electrical stimulation, is discussed i n relation to its possible contribution to ankle stability. (C) 1999 Elsevi er Science Ltd. All rights reserved.