Neuromuscular electrical stimulation pulse duration and maximum tolerated muscle torque

Abstract

Neuromuscular electrical stimulation (NMES) is a physical therapy intervention used to treat muscle weakness. NMES-elicited forces during therapy are correlated with strength gains. Patient discomfort limits NMES-elicited forces potentially compromising strength gains and the efficacy of this invention. The purpose of this study was to determine if NMES containing two different electrical stimulation pulse durations (200 or 500 μs) affected the knee extensor muscle torques subjects tolerated. Other NMES characteristics were identical in the two conditions: monophasic square-wave pulses; 75 pulses per second; and electrical stimulation train duration of one second. The primary dependent variable of interest was the percentage of maximum voluntary isometric contraction (MVIC) tolerated. The two pulse duration conditions were tested during a single session on the opposing lower extremities of 15 subjects. Subjects tolerated 49.3 ± 18.7% of MVIC torques in the 500-μs condition versus 44.5 ± 17.9% of MVIC torques in the 200-μs condition, which was a statistically significant difference (p = 0.02). Further research is needed to explore if the differences observed in this study would lead to clinically significant differences in strength gains and to see if the findings of this study can be generalized to other forms of NMES that contain other types of wave forms.

Comparison of the effects of kilohertz- and low-frequency electric stimulations: A systematic review with meta-analysis

“…Generally, longer phase durations generate greater muscle torque at least when monophasic square-wave pulses are used. [ 45 , 46 ] A recent study also showed that KFAC and PC, with the same phase duration, have similar efficiency for inducing %MVIC, and NMES with longer phase duration induces higher muscle-evoked torque regardless of the carrier frequency,[ 9 ] which indicates that phase duration plays an important role on muscle torque. Nevertheless, most of included studies used same phase (pulse) duration, which does not support the theory that KFAC had a short phase duration and produce less muscle torque compared to PC.…”

Electrically Elicited Muscle Torque: Comparison Between 2500-Hz Burst-Modulated Alternating Current and Monophasic Pulsed Current

“…As stated previously, longer phase durations generate greater muscle torques, at least when monophasic square-wave pulses are used. 14,15 Phase duration and current amplitude determine the phase charge, which is responsible for muscle peripheral nerve recruitment. Therefore, the shorter phase duration of the BMAC may require that amplitude be increased to achieve the same phase charge, which may lead to greater discomfort and subsequently limit torque production.…”

“…14,15 Phase charge, the product of current amplitude and phase duration, 1,7,13 is an important parameter that determines stimulus strength and therefore motor nerve recruitment and torque production. 6,7,14,15 Phase charge can be increased by either increasing amplitude or phase duration, 13 and thus either high current amplitudes or longer pulse durations may facilitate greater muscle force production. However, this has only been shown with pulse durations up to approximately 500 to 600 microseconds; longer pulse durations have little effect on muscle force production.…”

“…15 Our work and that of others has shown that NMES with pulse durations at the high end of the range over which force production varies allows participants to produce higher muscle forces than with shorter pulse durations. 6,7,14,15 The purpose of this study was to compare the maximum knee extensor muscle torque produced in response to 2 different waveforms, an MPC consisting of 500-microsecond square waves and a 2500-Hz BMAC, at the maximum tolerated stimulation current. We hypothesized that MPC would produce greater torque compared to BMAC at a similar maximum discomfort level.…”

Design considerations for the development of neuromuscular electrical stimulation (NMES) exercise in cancer rehabilitation

“…In support of this, Hultman et al [2] demonstrated that increasing phase duration from 150 to 500 ms achieved a 40% greater torque output. In addition, when comparing low (200 ms) and high (500 ms) phase durations, healthy young individuals have been shown to tolerate and generate higher force tetanic muscle contractions (45% v 49% maximum voluntary contraction (MVC)) with a higher phase duration [63]. However, many HF-NMES applications use phase durations of <300 ms [3], despite the evidence supporting the use of higher phase durations for muscle strengthening applications.…”