Characterization and Evaluation of Interferential Current Stimulation for Functional Electrical Stimulation

Interferential current stimulation (ICS) has long been employed in neuromodulation and physical therapy, yet its mechanisms and potential applications in functional electrical stimulation (FES) remain under-explored. We present new data, including neural activation reported by muscle force measurement, to evaluate the potential advantages of ICS, such as selective targeting and reduced discomfort.

Experiments were conducted on human participants, focusing on ulnar and median nerve activation with stainless steel and commercial conductive hydrogel electrodes. Key parameters, including electrode configuration, frequency, current amplitude, and skin preparation, were investigated to test their effects on activation thresholds, force generation, and user comfort.

Our results revealed that ICS can achieve proportional muscle force control, although its efficiency was lower than rectangular biphasic pulse stimulation. The application of moisturizing cream and gel significantly improved comfort and reduced activation thresholds, underscoring their importance in optimizing ICS protocols. However, ICS required higher electrical power and caused discomfort during burst initiation with all electrodes, presenting challenges for its practical use in FES. Furthermore, our findings indicated that ICS did not exclusively activate neural regions at the difference or "beat" frequency but also by the carrier frequency, challenging some prior assumptions in the literature.

These results highlight the need for further research and practical measurements of neural recruitment and muscle fatigue and into the mechanisms of nerve activation and neuromuscular junction transmission with stimulation via the skin surface. Innovations in electrode design, stimulation waveforms, and protocols are also needed to enhance the efficacy and comfort of ICS.