Increase in Ib inhibition by antagonistic voluntary contraction in man.

1. Ib inhibition from gastrocnemius medialis (GM) muscle to soleus (Sol) muscle was studied at rest and at the onset of phasic voluntary contraction of antagonistic pretibial muscles in seventeen normal subjects. 2. In twelve out of seventeen subjects there was inhibition of Sol H reflex by GM conditioning stimulation at rest with a latency of 1.5-3.0 ms and a threshold of 0.85-1.00 times the motor threshold (MT). The amount of inhibition at 0.95-1.05 x MT, which was calculated by subtracting the size of the conditioned reflex from that of the unconditioned one, ranged from 0.8 to 5.6% of the maximal M-response or 2.9-18.3% of control H reflex. This inhibition was ascribed to Ib inhibition, taking into account its latency and threshold. 3. On weak pretibial contraction the inhibition either increased in amount or newly appeared in all the subjects. When the strength of voluntary contraction was graded from 1 to 20% of the maximum, the increment in the amount of inhibition decreased or almost disappeared at strengths of more than several per cent. These facts imply that at least some of the Ib interneurones are facilitated to fire by descending commands alone without peripheral Ib impulses. Contralateral ankle dorsiflexion did not modify the inhibition. 4. Soleus muscle H reflex was not modulated at all by cutaneous stimulation instead of GM stimulation at rest, nor was it affected by cutaneous stimulation on ipsilateral antagonistic contraction. 5. It is concluded that activity in the Ib inhibitory pathway is facilitated at the onset of antagonistic voluntary contraction. This suggests that control of the Ib inhibitory pathway may be utilized in ordinary voluntary movement, and is presumably beneficial for smooth execution of movement.