Optimization of Interphase Intervals to Enhance the Evoked Muscular Responses of Transcutaneous Neuromuscular Electrical Stimulation

Neuromuscular electrical stimulation (NMES) is a widely used technique for clinical diagnostic, treatment, and research. Normally, it applies charge-balanced biphasic pulses, which several publications have reported to be less efficient than monophasic pulses. A good alternative is the use of interphase intervals (IPI) on biphasic pulses that allows to achieve similar responses than those evoked by monophasic stimulation. This study analyzes the enhancing mechanism of the IPI and provides guidelines on how to optimize the IPI in order to reduce secondary effects such as the electrode corrosion. The tibial nerve was excited by NMES biphasic pulses with different IPI durations and polarities. Then, the elicited responses were recorded on the soleus muscle via electromyography. When cathodic-first pulses were applied, the responses increased proportionally to the IPI until the duration of 250 mu s, where the increase saturated at 30% of the original amplitude. The responses evoked during anodic-first were 6% to 30% smaller than those evoked during cathodic-first pulses and continuously increased until the IPI duration of 2500 mu s, where the responses reached an increase of around 30%. The results suggest that when a cathodic-first pulse is used, the IPI could be optimized (based on the setup geometry) to allow the action potentials to travel out of the hyperpolarization zone induced by the anodic phase. When anodic-first stimuli are applied, the IPI duration allows the fiber to recover from an apparent insensitive state induced by the anodic phase. The use of IPI is a viable option to improve the efficiency of actual stimulation systems, since only small modifications are required to significantly reduce the electrical charge required and boost the stimulation efficiency.