Variable Amount of Reaction Time and H-Reflex on Position Changes of the Hip Joint

Abstract

Postural changes and the difference of attitudes influence the reaction time of a limb muscle. The present study is a modification and extension of previous studies on influense of the position changes in the hip joint on the electromyographic reaction time (EMG-RT) and on the H-reflex of the soleus muscle in normal human subjects. Primarily, the aim of this study was to explore the relevant neurophysiological mechanism which control reaction time. In experiment I (Exp. I), the EMG-RT from the 20 subjects (16 males and 4 females) who reacted the plantar flexion at peep sound (1000 Hz, 100 db, 100 msec duration) were recorded from the soleus muscle at three different positions of the hip joint (neutral, externally and internally rotated positions). In experiment II (Exp. II), H-reflexes were elicited in the soleus muscle at the static three hip joint positions. In this case, H-reflexes were recorded under two conditions; one where the subjects' hip joint position was changed passively by the experimentor (20 subjects) and the other where the subjects voluntarily changed their hip joint position (10 subjects). In this way, obtained H-reflexes were measured from peak to peak. Those amplitudes of H-reflex were examined as an indicator of activation of alpha-motoneurons pool. Major results may be summarized as following: 1. The EMG-RT of internally rotated position was longer than that of neutral position, and the EMG-RT of externally rotated position was shorter than that of neutral position (Exp. I). 2. In both conditions, the H-reflex amplitudes of internally rotated position were smaller than those of neutral position, and the H-reflex amplitudes of externally rotated position were larger than those of neutral position (Exp. II). 3. Especially, this tendency was remarkable when the subjects voluntarily changed their hip joint position (Exp. II). From these results, the relevant neurophysiological mechanisms which control reaction time were discussed, and the scheme for an explanation of the relationship between the changes of reaction time and the excitabilities of alpha-motoneurons pool was suggested.