The present study was undertaken to examine the effects of acoustic stimulation on human spinal motoneuron excitability. For this purpose, we used the soleus (Sol) H-reflex as a test reflex, and three different types of acoustic stimuli as conditioning stimuli. The features of the acoustic stimuli were as follows, 1) click sound (CS), 2) tone burst composed of 11 click sounds (TBL, interstimulus interval 10 ms), 3) tone burst composed of 21 click sounds (TBH, interstimulus interval 5 ms) . The intensity and frequency of each sound was 110 dB and 0.5 kHz, respectively.
Significant facilitation of the Sol H-reflex occurred at conditioning-testing (C-T) intervals of 50 ms in all subjects when the TBL stimulus was used (mean±S.D.; 50.3±8.2 ms) . This facilitatory effect appeared early and later when TBH (43.7±3.7 ms) and CS (59.2±4.5 ms) stimuli were used, respectively. The maximum facilitatory effect appeared at a C-T interval of 100 ms (mean and S. E.; 98.0±0.8 ms) and the amount of peak facilitation at that time was 156.1±1.4% (relative to the control value) . Thereafter, the amount of facilitation decreased sharply up to a C-T interval of 200 ms. However, slight but significant facilitation was observed continuously up to a C-T interval of 500 ms. No significant inhibition of the Sol H-reflex was observed between a C-T interval of 0 and 500 ms in all subjects.
Irrespective of whether the magnetic cortical stimulation was given before or after the Sol H-reflex at a short interval (-2 to 2 ms), the acoustic facilitation of the Sol H-reflex was not changed with a C-T interval of 50 ms. Additional facilitation due to magnetic cortical stimulation was, however, obtained in one of the four subjetcts when a C-T interval of 100 ms was used.
These results suggest that the acoustic facilitation of the Sol H-reflex is composed of three different facilitatory mechanisms; 1) a pathway with a high threshold and shorter latency (about 40 ms), 2) a pathway with a lower threshold and medium latency (about 60 ms), and 3) a pathway with a relativelyhigh threshold and longer latency (longer than 200 ms) . In addition, we discussed the mechanisms that underlie the facilitatory effects of the Sol H-reflex.

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.

The purpose of the present study was to determine electrophysiological differences in muscular fatigue between the soleus (Sol) and tibialis anterior (TA) muscles in normal human subjects (n=5) . The subjects were asked to make four 20-sec maximum voluntary contractions (MVCs), each separated by 3-min intervals of rest. A 3-sec MVC (V_task) or a 3-sec supramaximum electrical stimulation (E_task) was imposed at 1-min intervals during the resting period. The plantar flexion and dorsiflexion forces were significantly decreased throughout both the V_ and E_tasks. In particular, the decrease in the dorsiflexion force during the V_task was found to be very steep. However, wide inter-subject variations were found in the time course of the decrease in the MVC force in all tasks. Changes in mean power frequency (MPF) of the electromyographic (EMG) recordings in the Sol were found to be small in both tasks. In contrast, the MPF was significantly decreased during the 20-sec MVC in the TA. The root mean square (RMS) of the EMG gradually declined during both the V_ and E_tasks. The ratio of the root mean square of EMG (RMS) and the exerted force (RMS/F) was also determined during both tasks and for both muscles. The RMS/F was markedly increased during the V_task in the TA. An increase was also found in the Sol, but the magnitude of the increase was small. A small but consistent decrease in the M-wave was found in the V_task. The Sol H-reflex was decreased until the second 20 s MVC, then reached a plateau, and further decreased at the end of the fourth 20s MVC. It was suggested that the electrophysiological differences in the Sol and TA during muscular fatigue induced by the repetitive 20-sec MVC reflected differences in the physiological properties of these muscles. The RMS/F was suggested to be a useful parameter for determining the local muscular fatigue in intact human lower leg muscles.