Changes in the gain of the soleus H-reflex with changes in the motor recruitment level and/or movement speed

The behaviour of the soleus H-reflex is consid- ered to be motor task-dependent. However, the speed of movements as well as the motor recruitment level is altered when motor tasks are changed. Therefore it is ambiguous to what extent the motor task-dependent changes found between walking and running, for exam- ple, are simply due to changes in these two parameters. The purpose of this study was to investigate how move- ment speed and motor recruitment level separately influence the soleus H-reflex behaviour when the motor task is unchanged. Soleus H-reflexes were elicited during pedalling at different cadences and crank loads, by which movement speed and muscle recruitment level were modified separately. The H-reflex gain was expressed as the slope of the linear relation between the reflex ampli- tudes and the background electromyelogram (EMG), and the reflex threshold was expressed by the intercept. The results showed a decrease in reflex gain by 54% (P =0.001) when the speed of movement was doubled from 40 to 80 rpm (repetitions per minute) without changes in the level