Decline of Conduction Time May Lead to Central Nervous System Adaptions That Are Important for Postural Control and Gait

Introduction and Objectives: Slowed time course of the efferent / afferent pathway and slow processing time at the central level have been considered as the main causation of decreased postural control in aging population. However, there is no report of the correlation between the time course of pathway and measures of postural control. Therefore, this study is aimed at examining the correlation between different time courses of neuro-pathway (lower extremity sensory / motor nerve conduction velocity, and conduction time of H-reflex) and signal processing (characterizes of H-reflex) as well as the measures of postural control in aging population. Methods: Six participants (one male, five female) were recruited. The proximal sural nerve conduction nerve conduction velocity (SCV) test, tibia nerve conduction velocity (MCV) test, plantar pressure sensitivity (PPS) tests, and H-reflex test at soleus muscles, were conducted. Based on the results of the H-reflex tests, we have also calculated the peak magnitude of H-wave (H), the peak magnitude of M-wave (M), H / M ration (H/M) and H-index. H-index was calculated using the equation of H-index = the square of (the height of the participants in cm / H-M latency in ms) X 2. For physical performance, the participants performed quite standing trials with their eyes open and closed, as well as a 6-minute walking (6MW) test and a timed-up and go test (TUG). Pearson correlations were used to examine the correlations among the measures of nerve function test as well as the correlations between the measures of nerve function tests and the outcomes of the physical tests. Results: Age (70.5 ± 7.4 years old), body mass (82.4 ± 25.3kg), and height (162.7 ± 7.5 cm) of participants were recorded. The nerve function tests were conducted while the skin temperature was above 31°C. PPS (4.5 ± 0.84) among the participants were close to 5 points and indicates minimal loss of foot sole sensation. Therefore the following results were not under the influence of foot sole numbness as often the case for many elderly. The observed MCV (29.3 ± 2.2, range from 26.2 to 31.5 m/s) and SCV (22.7 ± 3.5, range from 17.2 to 25.7 m/s) are lower than the 40 m/s, the recognized healthy lower threshold for healthy elderly reported in the literature. The observed average conduction velocities were lower for sensory than that of the motor nerves. The observed range for sensory (~37%) was wider than that of the motor (~18%) conduction velocities. The observed H-index (75.6 ± 16.8, ranging from 53.0 to 97.3) has the largest variation (58.6%) among the three conduction time related measures. The other H-reflex related values were: H (1.00 ± 0.74 mv), M (4.57 ± 1.99mv), and H/M (0.25 ± 0.23). For the physical performance tests, participants exhibited greater 95% sway areas (A95%) during eyes closed conditions (205 ± 246 mm*mm) comparing to the eyes opened conditions (177± 115 mm*mm). But average sway velocities (VAVG) were comparable for eyes closed (317 ± 72 mm/s) and eyes opened (O: 321 ± 76 mm/s) conditions. On average, participants walked (523 ± 116 m) in 6 minutes, and spend (7.07 ± 1.44ms) during TUG tests. Those values indicate normal locomotion capacities among participants. The correlation results are very revealing. SCV was positively correlated with H-index (r =0.811, P\u3c .05), where correlation between MVC and H-index were not significant (P\u3e .05), suggesting H-index value were mainly influence by sensory nerve among this population. Further, SCV was negatively correlated with H/M (r = -0.972, P\u3c .001) and H (r = -0.951, P\u3c .001), but positively correlated with M (r =0.805, P\u3c .05). These observations, slower sensory conduction velocity accompanied by greater H-wave, could mean that sensory nerve impairments lead to increase spinal facilitation of H-reflex, as a compensatory mechanism. Along the same line, H-index was negatively correlated with H/M (r = -0.749, P\u3c .05), but positively correlated with M (r = 0.823, P\u3c .05). Although we have failed to observe significant correlation between H-index and any physical performance, but H/M was positively correlated with VAVG (eyes open: r = 0.808, P\u3c .05; eyes closed: r = 0.786, P\u3c .05). The observations indirectly links the greater sway velocity with slower H-index since them both (VAVG and H-index) correlated with greater H/M value. Conclusion: Slowed sensory nerve conduction velocity with increased conduction time of H-reflex were accompanied by increase the magnitudes of H and H/M, may indicates central nervous system adaptation to peripheral impairments during aging. These changes in the tested nervous system were not directly related to postural control and locomotion tests without observable foot sole numbness. The lack of correlation here may suggest H-reflex related pathway is not the primary reflexive mechanism for postural control and locomotion when cutaneous sensation is intact