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.

The effect of the ankle joint angle in the level of soleus Ia afferent presynaptic inhibition.

The factors that are responsible for the relationship between motoneuron excitability and muscle length may have both mechanical and/or neurophysiologic origins. The aim of the study was to investigate the changes in the level of presynaptic inhibition, as measured with a soleus H-reflex conditioning protocol, and muscle length. Ten healthy volunteers were measured at three different ankle angles: 30 degrees plantar flexion, neutral position (0 degrees) and 15 degrees dorsiflexion. At each position the soleus H-reflex and the maximum M-wave were measured while the limb was relaxed. The H-reflex was conditioned by a stimulation of the common peroneal nerve, 100 ms prior to the tibial nerve stimulation. The results revealed that the level of presynaptic inhibition was higher at the neutral position in comparison to the dorsiflexed or plantarflexed positions. Additionally, the HMAX/MMAX ratio was significantly decreased when the joint position was set at dorsiflexion. Further, there was a significant correlation, independent of ankle joint angle, between presynaptic inhibition levels and the HMAX/MMAX ratio. The above findings support the concept that peripheral feedback from passive, static modifications in the joint angle and consequently in muscle length, can modify the input/output threshold of the motoneurons on a presynaptic level.

Post-activation depression of the soleus H-reflex in the elderly.

OBJECTIVES: This study examined the effect of age on the spinal mechanism post-activation depression (PD) and its relationship with postural sway. METHODS: Two groups, young (n = 10, 25.2 +/- 5.2 yr) and elderly (n = 10, 74.6 +/- 6.3 yr) participated in this study. Soleus H-reflex amplitude (peak-to-peak EMG) was measured prior to and after a passive dorsiflexion about the ankle (from 120 degrees plantarflexion to 110 degrees plantarflexion at 15 degrees/s). All H-reflex measurements were recorded at an ankle joint angle of 110 degrees. For each group, stimulus intensity was set at 25% of maximal motor response (M-max) and PD measurement intervals were randomly assigned at 0, 1, 2, 3, 4, 5, 10, 15, and 18 seconds post-movement. To ascertain whether PD was related to postural sway, standing sway area (mm2) was also measured. Subjects stood motionless (20 second duration) on a Kistler force platform during two conditions: with and without vision. RESULTS: A split-plot ANOVA (Group x Interval) indicated significant differences (F(1.18) = 14.80, p < 0.05) in PD between young and elderly, as well as a Group x Interval interaction (F(10, 180) = 16.02, p < 0.05). Simple main effects identified significant (p < 0.05) intervals at 0, 1, 2, and 3 seconds. The elderly had on average 16.1% less H-reflex depression during the first three seconds after muscle activation. Additionally, a split-plot ANOVA (Group x Condition) indicated significant differences (F(1, 18) = 10.72, p < 0.05) in sway area between young and elderly, but regression analysis indicated post-activation depression and postural sway were not significantly correlated. CONCLUSIONS: It is concluded that the integrity of the Ia-motoneuron synapse as measured with a PD protocol, is different between young and elderly subjects, but that this difference is not related to postural sway.

Preoperative irradiation in operable cancer of the rectum: report of the Toronto trial.

The authors have studied the value of preoperative radiotherapy in 125 patients with cancer of the rectum. The patients were divided into two groups, control (no irradiation) and experimental (irradiation), and according to Dukes' pathological stage. The crude survival indicated no difference between the two groups, but analysis by stage and group demonstrated that for patients with a Dukes' stage C lesion, 500 rads of cobalt-60 gamma irradiation a few hours before excision of the lesion approximately doubles the probability of survival at 5 years. The prospect of surgical cure is three times higher in cancers arising above the peritoneal reflection than in those arising below it. If the practice of abdominoperineal resection is to continue the authors recommend that preoperative irradiation become a routine procedure.