NATURAL RECIPROCAL INHIBITION OF SOLEUS H-REFLEX DURING EARLY PHASE OF GAIT INITATION; DIFFERENCE BETWEEN SWING AND STANCE LIMB.

Abstract

PURPOSE/HYPOTHESIS: This experiment was done to investigate soleus (SOL) H-reflex for swing limb and stance limb during the pre-motor reaction time (RT) period, which was determined by onset of tibialis anterior (TA) and during the early phase of gait initiation (GI) relative to the kinetic and kinematic events in the process of GI compaired to standing. NUMBER OF SUBJECTS: Six male and seven female subjects without known medical, orthopedic and/or neurological condition were recruited in this study. Average age was 29.86 6.6 years. MATERIALS/METHODS: SOL and TA electromyogram (EMG) and SOL H-reflexes were recorded from the right limb of subjects during GI. Ground reaction force under the testing foot was also monitored. Subjects were tested in three different paradigms; standing, stance GI, and swing GI paradigms. To obtain H-reflexes for various time periods of GI, the stimulus was delivered at various times relative to the onset of light switch signal. The delay for administration of the stimulus following the onset of the light signal was vary. The beginning of GI was determined by the onset of TA EMG activity. RESULTS: Both stance and swing limb H-reflexes were significantly and gradually depressed during reaction time (RT) of GI. However, the H-reflexes in the initial RT periods were significantly lower and their depression started earlier on the impending stance limb (150 to 125ms prior to TA EMG onset; to 11.9% of standing value) than the swing limb (50 ms before the TA EMG onset; 37.9% of standing value). The H-reflexes for both stance and swing limbs after the TA EMG onset were closely modulated with changes in ground reaction forces rather than the amount of TA EMG activity. Although there was some inhibition of the H-reflex due to TA contraction during the initial G1, the effect was relatively weak, particularly when the leg was the loading limb which was responsible for the forward acceleration force. CONCLUSIONS: Such findings indicate that the SOL H-reflex inhibition was relatively weak when the leg was the loading limb, which was responsible for the major acceleration force and provides evidence that the H-reflex is centrally modulated and depends on the role of the muscle during a given task. The loading limb may need to maintain a higher gain for SOL activity to adjust to the changes in peripheral events. We speculate that GI is a preprogrammed task and the H-reflex modulation during RT and the early phase of GI is primarily under central control with possibly some contribution from peripheral afferents. CLINICAL RELEVANCE: The result of this work are related to the rehabilitation of those with central nervous system (CNS) disorders who have difficulty with precise and timely modulation of SOL excitability between the stance and the swing legs during GI. Future clinical research including both legs of people with CNS disorders would help rehabilitation clinicians enhance their knowledge of the nature of difficulties in GI, the development of effective training protocols or facilitating GI.