Assessing information of soleous and gastrocnemius motor unit H-reflex response to paired stimulation

In this study we compared soleous and gastrocnemius muscles responses to paired stimulation in order to reveal the embedded information in H-reflex of these muscles. Four men with ages between 20 and 26 were tested. Since there was no ideal point based on the scientific rules to record H-reflex from gastrocnemius muscle, we were to determine an ideal point and electrode placement from anatomical and physiological point of view. Then, soleous and gastrocnemius muscles were subjected to paired stimulus and H-reflex recovery curve of these muscles were recorded To record the recovery curve, 11 pairs of stimulus were delivered to the posterior tibial nerve. Time interval between the 1st pair of stimulus was 50 ms and increased by 20 ms steps in the next pairs up to 250 ms. The second H-reflex amplitude was divided to the first H-reflex amplitude and expressed in percent, then the recovery curve was drawn accordingly. While the time interval of the paired stimulus was less than 110 ms, the behavior of the curves was similar. In this stage we guessed that the muscles were impressed by the renshaw cells inhibition and the induced fatigue by the first stimulus. When the interval in a pair was 110 ms or more, the recovery percent of soleous was significantly higher than that of the gastrocnemius. This behavior may be due to the less fatigability of soleous motoneurons and less renshaw inhibition on slow motoneurons of soleous. When the time interval of a pair stimulus increases to more than 170 ms, both of the curves behave alike. This may be due to the diminishing of renshaw cell inhibition and the reduction of fatigue. Because of motoneurons physiological properties, when we evoke H-reflex, smaller motoneurons recruit predominantly; but the comparison of the soleous (slow) and gastrocnemius (fast) recovery curves behavior shows that the information about the fast motoneuron are included in H-reflex using this procedure. So we can indirectly assess different motoneurons and renshaw cell activities.