A study was conducted to investigate changes in the excitability of the ankle extensor and flexor motoneurons during voluntary contraction of the quadriceps femoris muscle (Quad) . For this purpose, we used a reaction time task and the H-reflex technique. Subjects lay in the prone position on a bed and performed isometric contraction of the Quad. The strength of the contraction force was about 30% of maximum.
In all subjects we observed strong facilitation of the soleus (Sol) H-reflex, which occurred from the EMG onset of vastus lateralis muscle (VL) . The pretibial muscle (TA) H-reflex was also facilitated at almost the same time as that seen in the Sol H-reflex in nine out of ten subjects. The peak of Sol and TA H-reflex facilitation appeared between 50 and 100 ms after the EMG onset of the VL, and then these facilitations gradually decreased. Weak but constant activities of the Sol and medial gastrocnemius muscle (MG) were observed on the full-wave rectified and averaged EMG record after 50 to 80 ms from the EMG onset of VL. No such EMG activity was observed in TA.
These results suggest that excitatory inputs including those of both descending and peripheral origin induced by voluntary contraction of Quad are responsible for facilitation of the ankle extensor and flexor motoneurons. In addition, removal of presynaptic inhibition of the Ia terminal of the motoneurons by descending motor command might explain the present results.

Premovement facilitation of spinal monosynaptic reflex was investigated under different movement modalities using both visually guided tracking movement and the H-reflex technique. Subjects performed fast step and slow ramp movements under reaction time (RT) conditions and self time determining (Self) conditions. For each of the conditions, the motor task was bilateral simultaneous dorsiflexion.
Under both RT and Self conditions, the onset of premovement H-reflex facilitation (OPHF) was significantly prolonged in the slow ramp movement (RT, 37, 3±11.8 ms ; Self, 79.4±21.8 ms) compared with the fast step movement (RT, 24.4+6.2 ms ; Self, 43.4±14.2 ms) . In addition, OPHF was significantly prolonged under Self conditions in both the step and ramp movements. Movement time (MT) did not differ significantly under RT and Self conditions except in three subjects. The peak value of torque change (PV) was larger under Self conditions than under RT conditions in four out of ten subjects.
From these results, we conclude that OPHF is modulated not only by changes in movement speed but also by changes in movement modality. It is suggested that this contextual dependency of OPHF might be controlled by the supraspinal motor center.