Depression and recovery of reflex amplitude during electrical stimulation after spinal cord injury.

OBJECTIVE: The aim of this study was to quantify, for the first time, H-reflexes evoked during prolonged trains of wide-pulse neuromuscular electrical stimulation (WP-NMES) in individuals with chronic spinal cord injury (SCI). We hypothesised that after the first H-reflex, reflex amplitudes would be depressed (due to post-activation depression), but would recover and this recovery would be enhanced after a "burst" of 100 Hz WP-NMES. METHODS: Soleus M-waves and H-reflexes evoked during WP-NMES (1 ms pulse width) of the tibial nerve were quantified in nine individuals with SCI. WP-NMES was delivered in two patterns: "constant-frequency" (15 or 20 Hz for 12 s) and "burst-like" (15-100-15 Hz or 20-100-20 Hz; 4 s each phase) at an intensity that evoked an M-wave between 10% and 15% of the maximal M-wave (M(max)). RESULTS: During constant frequency stimulation, after the initial depression from the first to the second H-reflex (1st: 57% M(max); 2nd: 25% M(max)), H-reflexes did not recover significantly and were 37% M(max) at the end of the stimulus train. During the burst-like pattern, after the initial depression (1st: 62% M(max); 2nd: 30%), reflexes recovered completely by the end of the stimulation (to 55% M(max)) as they were not significantly different from the first H-reflex. M-waves were initially depressed (1st: 12% M(max); 2nd: 7% M(max)) then did not change throughout the stimulation and were not significantly different between stimulation patterns. An analysis of covariance indicated that the depression in M-wave amplitude did not account for the depression in H-reflex amplitude. CONCLUSIONS: Relatively large H-reflexes were recorded during both patterns of NMES. The brief burst of 100 Hz stimulation restored H-reflexes to their initial amplitudes, effectively reversing the effects of post-activation depression. SIGNIFICANCE: For individuals with chronic SCI, generating contractions through central pathways may help reduce muscle atrophy and produce contractions that are more fatigue-resistant for rehabilitation, exercise programs, or to perform activities of daily living.

Neuromuscular electrical stimulation: implications of the electrically evoked sensory volley.

Neuromuscular electrical stimulation (NMES) generates contractions by depolarising axons beneath the stimulating electrodes. The depolarisation of motor axons produces contractions by signals travelling from the stimulation location to the muscle (peripheral pathway), with no involvement of the central nervous system (CNS). The concomitant depolarisation of sensory axons sends a large volley into the CNS and this can contribute to contractions by signals travelling through the spinal cord (central pathway) which may have advantages when NMES is used to restore movement or reduce muscle atrophy. In addition, the electrically evoked sensory volley increases activity in CNS circuits that control movement and this can also enhance neuromuscular function after CNS damage. The first part of this review provides an overview of how peripheral and central pathways contribute to contractions evoked by NMES and describes how differences in NMES parameters affect the balance between transmission along these two pathways. The second part of this review describes how NMES location (i.e. over the nerve trunk or muscle belly) affects transmission along peripheral and central pathways and describes some implications for motor unit recruitment during NMES. The third part of this review summarises some of the effects that the electrically evoked sensory volley has on CNS circuits, and highlights the need to identify optimal stimulation parameters for eliciting plasticity in the CNS. A goal of this work is to identify the best way to utilize the electrically evoked sensory volley generated during NMES to exploit mechanisms inherent to the neuromuscular system and enhance neuromuscular function for rehabilitation.

Changes in corticospinal excitability evoked by common peroneal nerve stimulation depend on stimulation frequency.

The afferent volley generated during neuromuscular electrical stimulation (NMES) can increase the excitability of the human corticospinal (CS) pathway. This study was designed to determine the effect of different frequencies of NMES applied over the common peroneal nerve on changes in CS excitability for the tibialis anterior (TA) muscle. We hypothesized that higher frequencies of stimulation would produce larger increases in CS excitability than lower frequencies. NMES was applied at 10, 50, 100, or 200 Hz during separate sessions held at least 48 h apart. The stimulation was delivered in a 20 s on, 20 s off cycle for 40 min using a 1 ms pulse width. The intensity of stimulation was set to evoke an M-wave in response to a single pulse that was 15% of the maximal M-wave. CS excitability was evaluated by the amplitude of motor-evoked potentials (MEPs) in TA evoked by transcranial magnetic stimulation. MEPs were recorded immediately before and after the 40 min of NMES and in each 20 s "off" period. For each subject, MEPs recorded during three successive "off" periods were averaged together (n = 9 MEPs), providing a temporal resolution of 2 min for assessing changes in CS excitability. When delivering NMES at 100 Hz, MEPs became significantly elevated from those evoked before the stimulation at the 24th min, and there was a twofold increase in MEP amplitude after 40 min. NMES delivered at 10, 50, and 200 Hz did not significantly alter MEP amplitude. The amplitude of MEPs evoked in soleus and vastus medialis followed similar patterns as those evoked simultaneously in TA, but these changes were mostly not of statistical significance. There were no changes in the ratio of maximal H-reflex to maximal M-wave in TA or soleus. These experiments demonstrate a frequency-dependent effect of NMES on CS excitability for TA and show that, under the conditions of the present study, 100-Hz stimulation was more effective than 10, 50, and 200 Hz. This effect of NMES on CS excitability was strongest in the stimulated muscle and may be mediated primarily at a supraspinal level. These results contribute to a growing body of knowledge about how the afferent volley generated during NMES influences the CNS and have implications for identifying optimal NMES parameters to augment CS excitability for rehabilitation of dorsiflexion after CNS injury.

Relationship between ventilatory constraint and muscle fatigue during exercise in COPD.

Dynamic hyperinflation and leg muscle fatigue are independently associated with exercise limitation in patients with chronic obstructive pulmonary disease (COPD). The aims of the present study were to examine 1) the relationship between these limitations and 2) the effect of delaying ventilatory limitation on exercise tolerance and leg muscle fatigue. In total, 11 patients with COPD (with a forced expiratory volume in one second of 52% predicted) completed two cycling bouts breathing either room air or heliox, and one bout breathing heliox but stopping at room air isotime. End-expiratory lung volume (EELV), leg muscle fatigue and exercise time were measured. On room air, end-exercise EELV was negatively correlated with leg fatigue. Heliox increased exercise time (from 346 to 530 s) and leg fatigue (by 15%). At isotime, there was no change in leg fatigue, despite a reduction in EELV compared with end-exercise, in both room air and heliox. The change in exercise time with heliox was best correlated with room air leg fatigue and end-inspiratory lung volume. Patients with chronic obstructive pulmonary disease who had greater levels of dynamic hyperinflation on room air had less muscle fatigue. These patients were more likely to increase exercise tolerance with heliox, which resulted in greater leg muscle fatigue.