The role of pulse duration and stimulation duration in maximizing the normalized torque during neuromuscular electrical stimulation

Abstract

Study design: Controlled laboratory study

Objectives: To determine the effects of pulse duration and stimulation duration on the evoked torque after controlling for the activated area by using magnetic resonance imaging (MRI).

Background: Neuromuscular electrical stimulation (NMES) is commonly used in the clinic without considering the physiological implications of its parameters.

Methods and measures: Seven able-bodied, college students (mean +/- SD age, 28 +/- 4 years) participated in this study. Two NMES protocols were applied to the knee extensor muscle group in a random order. Protocol A applied 100-Hz, 450-microsecond pulses for 5 minutes in a 3-seconds-on 3-seconds-off duty cycle. Protocol B applied 60-Hz, 250-microsecond pulses for 5 minutes in a 10-seconds-on 20-seconds-off duty cycle. The amplitude of the current was similar in both protocols. Torque, torque time integral, and normalized torque for the knee extensors were measured for both protocols. MRI scans were taken prior to, and immediately after, each protocol to measure the cross-sectional area of the stimulated muscle.

Results: The skeletal muscle cross-sectional areas activated after both protocols were similar. The longer pulse duration in protocol A elicited 22% greater torque output than that of protocol B (P<.05). After considering the activated area in both protocols, the normalized torque with protocol A was 38% greater than that with protocol B (P<.05). Torque time integral was 21% greater with protocol A (P = .029). Protocol B failed to maintain torque at the start and the end of the 10-second activation.

Conclusions: Longer pulse duration, but not stimulation duration, resulted in a greater evoked and normalized torque compared to the shorter pulse duration, even after controlling for the activated muscular cross-sectional areas with both protocols.

Level of evidence: Therapy, level 5.

FIGURE 1

Illustration of the durations (450 and 250 microseconds) of the symmetrical biphasic pulse used in protocols A and B. Both pulses have interphase period of 50 microseconds.

FIGURE 2

A schematic diagram of the 3- and 10-second stimulation durations used in protocols A and B, respectively. Letter R denotes rise time with interval of 250 milliseconds to standardize the starting point of measuring the evoked torque in both protocols. The evoked torque was the torque maintained over a 500-millisecond window started directly after the rise time. For protocol B, the evoked torque was measured for a 500-millisecond window after the initial 250-millisecond rise time and for a 500-millisecond window ending 250 milliseconds prior to the end of 10-second stimulus.

FIGURE 3

Representative binary T2 maps of quadriceps femoris muscle from 1 participant before and after the neuromuscular electrical stimulation (NMES) protocols. (A) Pre-NMES T2 map for a single slice of left thigh. (B) T2 map for the same slice after stimulation with protocol A. (C) Pre-NMES T2 map for a single slice of right thigh. (D) T2 map for the same slice after stimulation with protocol B. (E) The activated measured cross-sectional area in each slice was multiplied by slice thickness (1 cm) and interslice space (1 cm) to calculate muscle volume that was activated (volume 1 + volume 2 +....+ volume 14).

FIGURE 4

Torque time integral (mean ± SD) of the first contraction in both protocols (A versus B). Significant differences between both protocols (P = .029).

FIGURE 5

Peak torque (Nm) versus activated skeletal muscle volume (cm³). The peak torque values of both protocols (A, blue circles; B, orange circles) were plotted against the corresponding activated skeletal muscle volume. The peak torque produced is equal to 0.08 times the activated volume plus 45 [Torque = (0.08 × activated volume) + 45] (r² = 0.51, P = .004).