The purpose of this study was to determine differences between three different difficulties of the same balance task, their influence on H-reflex amplitude and to find out if there is a conection in modulation of H-reflex amplitude and proficiency of balance task. The research included 10 voluntary participants. Each of them performed walking over of three different beam widths in random order. Participants walked across one beam per day. The rest period between walking over different beam width was at least two days. Participants performed six walking trials across the same width with their arms crossed over their chest. Modulation of spinal mechanisms was evaluated with soleus H-reflex. Corresponding background EMG of soleus muscle was obtained before and after (ten seconds, five minutes and ten minutes) six trials of each beam. Anything else except normal walking was considered a balance failure (stepping off, moving arms from a fixed position, etc.). Once a balance failure was observed during a trial, that trial and the collection of walking distance were stopped and the next trial started.
Walking distance was statistically different between three different beam width (p<0,001). Correlation between background EMG of soleus muscle (bEMG) and H-reflex modulation shows that individuals with bigger normalized activation when standing, modulated H-reflex less from prone to standing and vice versa (p=0,01, p=0,438, p=0,624). But there was no correlation between bEMG m. soleus and H-reflex modulation following walking across all beam widths. Although modulation of H-reflex from soleus muscle was not observed following walking ascross all beam widths, H-reflex amplitude was significantly lower following narrow beam compared to wide beam (Po; p<0,001; Po 5; p<0,01; Po 10; p<0,05). Analysis of correlation showed that participants, whose H-reflex modulation decreased from prone to closed-legged standing, walked greater distance on narrow beam (p=0,038). There is also a tendency that participants that walked a greater distance across narrow beam, showed a greater decrease in H-reflex amplitude after walking on narrow beam (p=0,080). Therefore, we suggest that regulation of H-reflex could be a reliable measure of dynamic balance abilities. Thus, the results of our study imply that the ability to down regulate the spinal excitability is one of the main mechanisms, which influence the static and dynamic balance tasks probably due to increased presynaptic inhibition of Ia afferents.