The purpose of this study was to investigate how reciprocal Ia inhibition is changed during muscle fatigue of lower limb muscle, induced with a voluntary contraction or height frequency electrical stimulation. Reciprocal Ia inhibition from ankle flexors to extensors has been investigated in 12 healthy subjects. Hoffmann reflex (H-reflex) in the soleus muscle was used to monitor changes in the amount of reciprocal Ia inhibition from common peroneal nerve as demonstrated during voluntary dorsi or planterflexion and 50Hz electrical stimulation induced dorsi or planterflexion. The test soleus H-reflex was kept at 20-25% of maximum directly evoked motor response (M response) and the strength of the conditioning common peroneal nerve stimulation was kept at 1.0xmotor threshold. At rest, weak Ia inhibition was demonstrated in 12 subjects, maximal inhibition from the common peroneal nerve was 28.8%. During voluntary dorsiflexion and 50Hz electrical stimulation induced dorsiflexion, there absolute amounts of inhibition increased as compared to at rest, and decreased or disappeared during voluntary planterflexion and 50Hz electrical stimulation induced planterflexion as compared to at rest. During voluntary or electrical stimulation induced agonist muscle fatigue, the inhibition of the soleus H-reflex from the common peroneal nerve was greater during voluntary dorsiflexion (maximal, 11.1%) and 50Hz (maximal, 6.7%) electrical stimulation induced dorsiflexion than at rest. The inhibition was decreased or disappeared during voluntary planterflexion 50Hz electrical stimulation induced planterflexion. It was concluded that the results were considered to support the hypothesis that α-motoneurones and Ia inhibitory interneurones link to antagonist motoneurones in reciprocal inhibition. The diminished reciprocal Ia inhibition of voluntary contraction during muscle fatigue as compared to electrical stimulation, is discussed in relation to its possible contribution to ankle stability. Copyright (C) 1998 Elsevier Science Ltd.
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