Changes in polysynaptic Ia excitation to quadriceps motoneurones during voluntary contraction in man.

Homonymous Ia facilitation of quadriceps (Q) motoneurones (MNs) was significantly larger during (and at the onset of) a very weak Q voluntary contraction than at rest. This increase in Ia facilitation only appeared with a conditioning-test interval within the narrow range of 5-9 ms, which fits the time course of the recently described polysynaptic Ia excitation to Q MNs. This suggests that interneurones mediating polysynaptic Ia excitatory effects to Q MNs receive a strong descending excitation during such a contraction. It is therefore argued that these interneurones might mediate part of the descending command to Q MNs during voluntary contraction.