The purpose of this study was to examine the effect of hopping tempo on the stretch reflex of the soleus muscle at the landing phase during continuous hopping. The M 1 and M 2 components of stretch reflex of the soleus muscle and angular velocity of the ankle joint were recorded at the landing phase during hopping under four hopping tempos with frequencies of 1.0, 1.5, 2.0, and 2.5 Hz. The electromyogram (EMG) activities in the soleus muscle during the landing and the jumping phases were separated under 1.0, 1.5 and 2.0 Hz conditions, but EMG's for both phases were overlapped under 2.5 Hz condition. Furthermore, at 1.0 and 1.5 Hz, a non-active EMG phase was observed between the landing and the jumping phases ; but this non-active phase disappeared at 2.0 Hz. The M 1 and M 2 components at 2.0 Hz showed significantly (p<0.01) larger values than those at 1.0 and 1.5 Hz. However, there was no significant difference observed between the components at 1.0 and 1.5 Hz. The M 2 component showed a significantly (p <0.01) smaller value than the M 1 component at 1.0 and 1.5 Hz ; but there was no significant difference at 2.0 Hz. The angular velocity of the ankle joint showed no significant difference under any of the conditions. These findings suggest that stretch reflex at landing during hopping may be inhibited at the motion pattern of the inactive phase between the landing and the jumping phases in EMG's. In addition, the stretch reflex is not only related to mechanical muscle stretching, but is also under the influence of the superior nerve center.

The purpose of this study was to examine the effect of physical inactivity, produced by prolonged bed rest, on the excitability of the spinal motor neuron pool in humans, using the activity of soleus H-reflex.
Eight healthy male subjects underwent continuous bed rest with 6 degree head-down tilt for 20 days (BR group) . Ten different healthy male subjects maintained their usual daily life during the observation period for 20 days (control group) . Recruitment curves of H-reflex and M-wave were obtained from every subject. H slope/M slope (H slp/M sip) which shows the excitability of spinal motor neurons was calculated by using data in the recruitment curve.
The H sip/M sip in the BR group significantly increased after BR, compared to the pre-BR values (p<0.05) . In the control group, however, there were no significant differences in the H slp/M sip between the pre and the post-observation values. At the baseline, the H slp/M sip showed no significant differences between the BR group and the control group. After the BR, the H slp/M sip in the BR group significantly increased compared to the control group after the observation period (p<0.05) .
This study suggests that the excitability of the monosynaptic spinal motor neuron pool is facilitated by physical inactivity during BR.