Towards the Development of Wearable Tremor Suppression Systems Combining Functional Electrical Stimulation and Mechanical Actuation
Abstract
Parkinson's disease (PD), the second most prevalent neurological disorder, is characterized by motor and non-motor symptoms including tremor. Wearable tremor suppression devices (WTSDs), have shown promising results for tremor reduction, using functional electrical stimulation (FES) or active actuators. However, results and study procedures are not consistent in the literature. Although stimulations below and above motor threshold can suppress tremor, the mechanisms of the first is still not clear, and the second might lead to muscle fatigue and discomfort over time. While active actuators have shown better results for tremor suppression, they are often heavy and bulky for daily use. Lastly, while many studies have analyzed characteristics of essential tremor (ET) under different circumstances, the difference between the pathophysiology of ET and PD suggests a need for better understanding of parkinsonian tremor characteristics.
To this end, a hybrid approach using electrical stimulation and mechanical suppression has been proposed. This system reduces the required motor torque and stimulation intensity for tremor suppression by using the other mechanism simultaneously, decreasing motor size, muscle fatigue, and discomfort. The hybrid approach improved tremor suppression by 12\% and reduced voluntary motion tracking error by 57\% compared to the FES-only approach in simulation. A case study showed that this approach could reduce the weight of a device with electric motors to about one-third of its initial weight.
Secondly, a systemic approach was proposed and tested to evaluate the effectiveness of various FES settings in parkinsonian tremor. Initially, individuals' tremor, sensory, and motor thresholds were evaluated. These measurements were used to generate stimulation combinations for tremor suppression at the wrist. Results showed that tremor suppression using FES highly depends on the tremor intensity, with a tremor power suppression ratio (TPSR) of 80.1 $\pm$ 2.9\% and 59.55 $\pm$ 2.9 \% for low to medium and higher tremor power intensities, respectively. Stimulations around the motor threshold showed an overall TPSR increase of 10\% and 4\% compared to below and above motor threshold stimulations, respectively.
Finally, approximate entropy (ApEn), frequency, power spectrum density, and magnitude of parkinsonian tremor were evaluated under different circumstances. An increase in ApEn from 0.74 $\pm$ 0.13 at baseline compared to 0.81 $\pm$ 0.22 with FES suppression aligns with previous studies, using surgery or medication for tremor suppression.
Understanding the effectiveness of different FES combinations on parkinsonian tremor can be used in further development of the hybrid approach, while findings of the last study are beneficial for the design of an adaptive controller.