Kilohertz Frequency Alternating Current Induces Less Evoked Torque and Less Neuromuscular Efficiency Than Pulsed Current in Healthy People: A Randomized Crossover Trial.

CONTEXT: Pulsed current and kilohertz frequency alternating current are 2 types of neuromuscular electrical stimulation (NMES) currents often used by clinicians during rehabilitation. However, the low methodological quality and the different NMES parameters and protocols used in several studies might explain their inconclusive results in terms of their effects in the evoked torque and the discomfort level. In addition, the neuromuscular efficiency (ie, the NMES current type that evokes the highest torque with the lowest current intensity) has not been established yet. Therefore, our objective was to compare the evoked torque, current intensity, neuromuscular efficiency (evoked torque/current intensity ratio), and discomfort between pulsed current and kilohertz frequency alternating current in healthy people. DESIGN: A double-blind, randomized crossover trial. METHODS: Thirty healthy men (23.2 [4.5] y) participated in the study. Each participant was randomized to 4 current settings: 2 kilohertz frequency alternating currents with 2.5 kHz of carrier frequency and similar pulse duration (0.4 ms) and burst frequency (100 Hz) but with different burst duty cycles (20% and 50%) and burst durations (2 and 5 ms); and 2 pulsed currents with similar pulse frequency (100 Hz) and different pulse duration (2 and 0.4 ms). The evoked torque, current intensity at the maximal tolerated intensity, neuromuscular efficiency, and discomfort level were evaluated. RESULTS: Both pulsed currents generated higher evoked torque than the kilohertz frequency alternating currents, despite the similar between-currents discomfort levels. The 2 ms pulsed current showed lower current intensity and higher neuromuscular efficiency compared with both alternated currents and with the 0.4 ms pulsed current. CONCLUSIONS: The higher evoked torque, higher neuromuscular efficiency, and similar discomfort of the 2 ms pulsed current compared with 2.5-kHz frequency alternating current suggests this current as the best choice for clinicians to use in NMES-based protocols.

Influence of Pulse Waveform and Frequency on Evoked Torque, Stimulation Efficiency, and Discomfort in Healthy Subjects.

OBJECTIVE: The aim of the study was to determine the influence of neuromuscular electrical stimulation pulse waveform and frequency on evoked torque, stimulation efficiency, and discomfort at two neuromuscular electrical stimulation levels. DESIGN: This is a repeated measures study. The quadriceps muscle of 24 healthy men was stimulated at submaximal (neuromuscular electrical stimulationsub) and maximal (neuromuscular electrical stimulationmax) levels using two pulse waveforms (symmetrical, asymmetrical) and three pulse frequencies (60, 80, 100 Hz). Repeated measures analysis of variance and effect sizes were used to verify the effect of pulse waveform and pulse frequency on stimulation efficiency (evoked torque/current intensity) and discomfort and to assess the magnitude of the differences, respectively. RESULTS: Stimulation efficiency was higher for symmetrical (neuromuscular electrical stimulationsub = 0.88 ± 0.21 Nm/mA; neuromuscular electrical stimulationmax = 1.27 ± 0.46 Nm/mA) compared with asymmetrical (neuromuscular electrical stimulationsub = 0.77 ± 0.21 Nm/mA; neuromuscular electrical stimulationmax = 1.02 ± 0.34 Nm/mA; P ≤ 0.001; effect size = 0.56-0.66) but did not significantly differ between frequencies (P = 0.17). At both neuromuscular electrical stimulation levels, there were no statistically significant differences in discomfort between pulse waveforms or frequencies. CONCLUSIONS: The higher stimulation efficiency of symmetrical pulses suggests that this waveform would be preferred to asymmetrical pulses in clinical practice. Stimulation frequencies between 60 and 100 Hz can be used interchangeably because of similar efficiency and discomfort.

Development and reliability of a new system for bedside evaluation of non-volitional knee extension force.

PURPOSE: Neuromuscular electrical stimulation (NMES) is a widely-used technique for diagnostic and therapeutic purposes. Here we developed and tested the reliability of a new NMES-dynamometer system for bedside evaluation of knee extensor muscle function. MATERIALS AND METHODS: Thirty-two healthy participants (16 men, 16 women; 27±5 years) completed two testing sessions, 7 days apart. On day 1, a single experienced rater, who repeated the evaluation on day 2 with two other raters, completed a standardized testing procedure. Participants were placed supine, with knees flexed and legs connected to the dynamometer. Maximal voluntary knee extensor isometric force (MVF) and supramaximal twitch force (TwF) were obtained. RESULTS: High intra-rater intraclass correlation coefficients were observed for both MVF (0.91) and TwF (0.94). MVF and TwF standard error of measurements (8.2%, 5.9%) and minimal detectable changes (16%, 11.6%) were low compared to mean values. High intraclass correlation coefficients were also observed for inter-rater comparisons of MVF (0.89) and TwF (0.86). Standard errors of measurements (MVF: 8.7%, TwF: 5.5%) and minimal detectable changes (MVF: 17.2%, TwF: 10.8%) were similar to intra-rater comparisons. CONCLUSION: The good reliability of the novel NMES-dynamometer system suggests it as an appropriate tool for the bedside evaluation of knee extensor muscle function.

Echo intensity reliability between two rectus femoris probe sites.

INTRODUCTION: The ultrasound technique has been extensively used to measure echo intensity, with the goal of measuring muscle quality, muscle damage, or to detect neuromuscular disorders. However, it is not clear how reliable the technique is when comparing different days, raters, and analysts, or if the reliability is affected by the muscle site where the image is obtained from. The goal of this study was to compare the intra-rater, inter-rater, and inter-analyst reliability of ultrasound measurements obtained from two different sites at the rectus femoris muscle. METHODS: Muscle echo intensity was quantified from ultrasound images acquired at 50% [RF50] and at 70% [RF70] of the thigh length in 32 healthy subjects. RESULTS: Echo intensity values were higher (p = 0.0001) at RF50 (61.08 ± 12.04) compared to RF70 (57.32 ± 12.58). Reliability was high in both RF50 and RF70 for all comparisons: intra-rater (ICC = 0.89 and 0.94), inter-rater (ICC = 0.89 and 0.89), and inter-analyst (ICC = 0.98 and 0.99), respectively. However, there were differences (p < 0.05) between raters and analysts when obtaining/analyzing echo intensity values in both rectus femoris sites. CONCLUSIONS: The differences in echo intensity values between positions suggest that rectus femoris's structure is not homogeneous, and therefore measurements from different muscle regions should not be used interchangeably. Both sites showed a high reliability, meaning that the measure is accurate if performed by the same experienced rater in different days, if performed by different experienced raters in the same day, and if analyzed by different well-trained analysts, regardless of the evaluated muscle site.