# Medical Devices for Tremor Suppression: Current Status and Future Directions
## 3 Electrical Stimulation Systems
### Antagonistic Muscles Activation  
In contrast to TENS, which stimulates sensory nerves, functional electrical stimulation (FES) provides stimulation to motor nerves to trigger muscle contraction. FES for tremor suppression was pioneered by [PERSON] and colleagues in 1989 [72; 73] and clinically  
Figure 1: Cala Trio transcutaneous electrical nerve stimulation. The median and radial nerves, which project to the ventral intermediate nucleus of the thalamus, are stimulated by Cala Trio (Cala Health, Burlingame, CA, USA) through two working electrodes placed on the anterior surface of the wrist. assessed in 1992 [74; 75]. Briefly, FES was associated with a tremor suppression of 73% in ET, 62% in PD tremors, and 38% in cerebellar tremors [75]. The recognized limitations of these early works included the potentially unreliable placement of surface electrodes, which could lead to insufficient tremor suppression [74; 75]. Although the implantation of percutaneous intramuscular electrodes could solve this problem, this approach is invasive and reserved for patients with severe tremor [64]. Nonetheless, the results from these pilot works led to the first functional electrical stimulator developed specifically for tremor suppression [76; 77]. Comparing the previous approach using an analog filter [72; 73; 74; 75], the use of an optimized digital filter [76; 77] in a portable functional electrical stimulator, with the enabled self-tuning and adaptation of more complex algorithms, showed improvements in suppressing tremors. In six participants who were healthy or with PD tremor, the functional electrical stimulator based on a digital filter showed an 84% tremor suppression, compared to a 65% when an analog filter was used [77]. The current approaches in utilizing FES to suppress tremors echo these early works, involving primarily two strategies: out-of-phase and co-contraction stimulations [78]. The MOTIMOVE system (3F-Fit Fabricando Faber, Belgrade, Serbia), based on an out-of-phase stimulation, obtained a CE marking for use in the European Union in 2019 but has not been approved by the FDA (Figure 2). The use of the MOTIMOVE system has been studied in patients with ET, PD tremors [79], and hemiplegia [80]. Two prototypes, the TREMOR neurorobot and the Tremor's glove, have adapted the co-contraction stimulation. Both devices have been assessed in patients with ET or PD but are currently not approved for clinical use. The MOTIMOVE system consists of a multichannel stimulator that provides support to activate several electrodes, placed on the forearm and upper arm above the flexor and extensor muscle points, that enable the selective muscle activation via distributed, asynchronous electrical stimulation. The inertial sensors within MOTIMOVE deliver real  
Figure 2: MOTIMOVE functional electrical stimulation system. This device (3F-Fit Fabricando Faber, Serbia) comprises a multichannel stimulator that attaches to several electrodes placed on the flexor and extensor muscles of the forearm, enabling muscle activation. time estimation of tremulous movements to a host computer, which provides control over the stimulation of muscles. This system delivers out-of-phase stimulation by sending electrical current pulses to the flexor and extensor muscles, triggering the depolarization of motor neurons that counteracts the tremorigenic activity. A pilot study of MOTIMOVE revealed a 67% tremor suppression in six of seven patients with ET or PD [79]. One patient, however, did not respond, suggesting that out-of-phase stimulation may not work for all patients with tremor [79]. Additional clinical studies evaluating the MOTIMOVE system are claimed to be currently in progress in Serbia, France, and Hungary, which will hopefully demonstrate its efficacy in tremor suppression and feasibility. The TREMOR neurorobot and the Tremor's glove adopt a similar design as the MOTIMOVE, consisting of electrodes that provide muscle stimulation, inertial sensors that capture biomechanical characterization signals of tremor, and a controller.