Effects of a complex balance task on soleus H-reflex and presynaptic inhibition in humans.

Spinal modulation of motoneuron excitability has been extensively investigated during various tasks in humans. Previous studies have revealed that balance tasks induce a decrease in Ia-motoneuron communication which has been attributed to increased levels of presynaptic inhibition (PI). Moreover, this depression in Ia-motoneuron connectivity takes place subsequent to the elevation of muscle activity. Therefore, it is hypothesized that motor learning has inhibitory effects on the spinal mechanisms in spite of the increased muscle activity during a motor task. The purpose of this study was to explore the effects of a complex balance task on the H-reflex. Soleus H-reflexes were measured from 11 healthy adult subjects both before and after 20 minutes of a complex balance task. A commonly reported H-reflex conditioning technique was applied in order to measure changes in spinal PI: an electrical volley to the heteronymous Ia (common peroneal nerve conditioning, CPN). Subjects stood on a custom designed balance board and performed a continuous series of plantar and dorsiflexion. To ensure that the task was performed similarly between subjects, auditory cues for movement were given by a metronome with a frequency of 1 Hz. The initial amplitude of the unconditioned soleus H-reflex was set at 50% of H-max, and unconditioned and conditioned (PI) reflexes were recorded before, during, and after the balance task. The unconditioned soleus H-reflex was significantly decreased 59% after the balance task and PI was increased by 50%. Further, during a period of rest following the task (20 minutes) the unconditioned H-reflex returned to near baseline levels whereas the PI conditioned H-reflex was not altered The results suggest that the initial depression in motoneuron excitability immediately after the balance task is accompanied by an increase in PI, but also that the recovery of the depressed H-reflex after the task appears to be independent of PI.

Implications for using H-max/M-max ratio in H-reflex parameters for elderly subjects compared with young subjects.

The H-max/M-max ratio has long been used to understand motoneuron excitability induced by the Ia fibers. However, we contend that when comparing young and elderly subjects, this measurement can be controversial. We compared the modulation of the soleus H-max and M-max at five different muscle lengths between elderly and young subjects. The young subjects demonstrated a significant modulation of both H-max and M-max between dorsiflexion (DF) and plantarflexion (PF) positions. In contrast, the elderly subjects demonstrated no difference in H-max and M-max between DF and PF positions. However, it was more interesting to note that those elderly subjects who had similar H-max/M-max ratios to the young subjects at 0 deg showed identical modulation of H-max and M-max to that of the young In this paper, we discuss that H-reflex comparisons between young and elderly subjects may be confounded by the initial H-max/M-max ratio. This finding has implications for both the statistical analysis of this data as well as the theoretical interpretation of H-reflex measurements.

Conditioned patellar tendon-tap reflexes in patients with ACL reconstruction.

The patellar tendon-tap stretch reflexes were examined in six neurologically healthy young subjects (mean age = 27.1 yrs) who had developed persistent quadriceps strength deficit due to ACL reconstruction. Each subject was tested on two separate days. A specially designed apparatus was used to examine the unilateral and conditioned patellar tendon-tap reflex response utilizing three different conditioning intervals: 25 ms, 75 ms, 150 ms, and a unilateral reflex (control). Peak isometric force and contraction time were measured by using a strain gauge. Also, peak-to-peak EMG was measured by using bipolar surface electrodes which were placed over the middle of the rectus femoris. All data were collected with a microcomputer (sample rate = 1 kHz). Due to the small sample, the Kruskal-Wallis nonparametric analysis of variance was performed. All subjects demonstrated quadriceps strength deficits in the ACL leg when compared with the contralateral leg. This analysis determined that for both the ACL leg and the Non ACL leg the size of stretch reflex was facilitated at the long-latency conditioning intervals (75 and 150 ms), whereas it was inhibited at the short-latency conditioning interval (25ms). However, the ANOVA model failed to reveal any differences in the conditioned stretch reflex between the ACL leg and the Non ACL leg. Also, no differences were observed at the unilateral condition. Taken together, these results indicate that ACL reconstruction results in significant strength deficits, but does not alter unilateral or conditioned reflex profiles.