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 mu s, and biphasic pulses with PDs either 200 or 500 mu s, applied with/without an IPI of 200 mu s 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.

Automated summary

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.