EMG-Driven Phase-Locked Spinal Cord Stimulation Physiologically Modulates Essential Tremor

Essential tremor (ET) is the most prevalent movement disorder, yet existing pharmacological and surgical treatments often lack efficacy or involve significant risks. Phase-locked neuromodulation offers a targeted approach to disrupt pathological oscillations, but its implementation through non-invasive spinal cord stimulation (tSCS) has remained largely unexplored. In this study, we present a novel closed-loop tSCS framework for ET based on real-time tremor phase estimation from surface electromyography (EMG). The system implements a Fourier Linear Combiner (FLC) algorithm to track the demodulated EMG signal sample-by-sample, enabling short-latency stimulation delivery while mitigating the phase lag inherent in traditional filtering or decomposition methods. The framework was validated in ET patients performing an isometric precision pinching task, with stimulation delivered across eight equidistant phases of the tremor cycle. The results demonstrate the technical feasibility of the system, which achieved a peak-to-trough tremor amplitude modulation of approximately 20% relative to the trial mean. Furthermore, the observation of phase-dependent frequency modulation in a small subset of participants suggested that tSCS may interact with tremorgenic timing mechanisms via spinal afferent pathways without compromising voluntary motor control. These findings provide an initial demonstration that closed-loop tSCS can produce phase-dependent modulation of tremor in individual patients, where physiological modulation is driven by the precise temporal synchronization of stimuli with the patient's underlying neurophysiology.