A brief interphase interval interposed within biphasic pulses enhances the contraction force of the quadriceps femoris muscle

Effects of interphase interval during neuromuscular electrical stimulation of the wrist extensors with maximally tolerated current intensity

Short interphase intervals (IPIs) within biphasic symmetrical pulses enhance maximal electrically induced isometric contractions (EIC). However, IPI effects have not been previously studied in muscles stimulated at the highest tolerated current intensity. Our aim was to examine IPI effects on the strength of EIC, degree of discomfort, and sensory and motor thresholds on the wrist extensor muscles. Eighteen subjects (mean age 25.5, SD ± 3.97 years) participated in a repeated-measures trial. Five parameter settings were used to stimulate the wrist extensors: monophasic pulses with phase durations (PD) 200 µs, and biphasic pulses with PDs either 200 or 500 µs, applied with/without an IPI of 200 µs duration. Order of settings was randomized, and current was set at the maximum intensity tolerated by each subject. IPIs applied at the maximally tolerated current intensity did not increase the strength of the EICs and did not reduce the degree of discomfort or the sensory and motor thresholds. These findings were not dependent on the PDs. Insertion of an IPI within the biphasic current during stimulation of the wrist muscle yielded no advantage in term of contraction strength or degree of discomfort. These results contradict previous studies indicating stronger contractions when an IPI is inserted during stimulation at a predetermined submaximal current intensity. As some of the clinical applications of neuromuscular electrical stimulation utilize the highest intensity tolerated by the individual, these findings are clinically relevant. Additional studies involving other muscle groups, electrode size and locations, and phase parameters are warranted.

Changes in microvascular oxygenation and total hemoglobin concentration of the vastus lateralis during neuromuscular electrical stimulation (NMES)

Background and Introduction: Neuromuscular electrical stimulation (NMES) is predicated on eliciting muscle contractions and increasing muscle demand to promote increase in strength. Previous studies have shown differences in the magnitude of elicited force among various NMES waveforms but less is known about metabolic demand of muscle during NMES.Objective/Purpose: The purpose of this study was to compare elicited force and muscle metabolic demand during electrically elicited contractions using different NMES waveforms.Methods: A single-session repeated measures design was used. Electrically elicited force (EEF), microvascular oxygenation (SmO2), total hemoglobin concentration ([THC]) of the vastus lateralis, and subject tolerance (VAS score) were measured using three NMES waveforms; burst modulated alternating current (Russian), biphasic pulsed current (VMS®), and burst modulated biphasic pulsed current (VMS-burst®).Results: A significant main effect for waveform was noted for EEF (F = 12.693, p < .001), SmO2 (F = 8.340, p = .001), and VAS (F = 4.213, p = .025), but not [THC]. Compared to Russian current, VMS-burst and VMS resulted in significantly greater EEF (p = .001; p = .009) and local metabolic demand (i.e. decreased SmO2) (p = .005; p = .003), but not [THC]. VAS was significantly greater (p = .023) for VMS (4.2) compared to Russian (3.07) but not different between VMS-burst and Russian and VMS-burst and VMS.Conclusion: Greater muscle force and local metabolic demand were observed with VMS-burst and VMS compared to Russian current. These data provide novel evidence to guide clinical decision making when selecting an NMES waveform.

The effect of electrode placement and interphase interval on force production during stimulation of the dorsiflexor muscles

The aims of this study were to investigate whether introducing an interphase interval (IPI) to biphasic pulses during stimulation of the dorsiflexor muscles would affect force production and to determine whether the IPI effect is dependent on electrode position. Twelve healthy volunteers participated in the study. Each subject participated in one session during which electrically induced contraction (EIC) forces of the ankle dorsiflexors were measured with five different IPI settings ranging from 0 to 400 μs. Forces of EICs were assessed with the electrodes placed either with the proximal electrode positioned over the common peroneal nerve and the second electrode over the dorsiflexor muscles or with both electrodes located over the dorsiflexor muscles. The order of electrode placements and of the different IPI settings was randomized across subjects. The results indicated that the introduction of a 100-μs-long IPI may enhance force production when one electrode is located over the common peroneal nerve. However, increasing the duration of the IPI beyond 100 μs did not result in further increase in force production. In contrast, the introduction of an IPI did not increase force production when both electrodes were located over the dorsiflexor muscles. These findings may help to optimize stimulation settings during functional electrical stimulation to prevent foot-drop.