Task-related changes of transmission in the pathway of heteronymous spinal recurrent inhibition from soleus to quadriceps motor neurones in man

An H reflex conditioning technique was used to monitor the transmission of heteronymous recurrent inhibition from soleus to quadriceps motor neurones of the human lower limb. Inhibition declined during quadriceps muscle contraction under all conditions examined, falling to zero at around one-third of the maximum voluntary contraction. Inhibition declined during soleus muscle contraction in sitting, standing and bicycling tasks. The level of inhibition assessed at a given (weaker than 30%) level of quadriceps contraction was reduced during postural tasks involving quadriceps and soleus co-contraction (standing and late-stance phase of walking) when compared with sitting and performing matched voluntary muscle contractions. The level of inhibition during the mid-power stroke of a bicycling task, which also involved co-contraction of quadriceps and soleus, was greater than during matched voluntary muscle contractions while sitting. It is concluded that the pathway of heteronymous recurrent inhibition from soleus to quadriceps motor neurones is under at least two types of control: one related to the task, which sets the operating range, and a second which couples inhibition to the level of muscle contraction. Multiple control pathways are consistent with the diverse effects on recurrent inhibition reported in subjects with upper motor neurone lesions.