# Medical Devices for Tremor Suppression: Current Status and Future Directions
## 3 Electrical Stimulation Systems
### Antagonistic Muscles Activation
Both devices adapt the co-contraction stimulation strategy, which applies mechanical loading via continuous transcutaneous stimulation to a pair of antagonistic muscles, increasing the stiffness of the limb. In turn, this filters out the mechanical manifestation of tremorigenic activity, which are oscillations in the muscle tissue. Like MOTIMOVE, the TREMOR neurorobot stimulates the flexor and extensor muscles of the forearm. This device was found to have a 52% tremor suppression in six patients with ET or PD tremors (\(p<0.001\)) [81]. Conversely, the Tremor's glove stimulates the abductor pollicis brevis and the first and second dorsal interossei muscles of the hand. In a sham-controlled randomized trial of 30 patients with medically refractory tremor in PD, the use of the Tremor's glove was associated with a significant reduction in the Unified Parkinson's Disease Rating Scale (UPDRS) score (\(p=0.001\)), suggesting improved experiences of daily living and motor complications [82]. The A-alpha (A\(\alpha\)) sensory fiber, a primary afferent nerve fiber that innervates antagonistic muscle pairs, appears to have a crucial role in the complex neural pathways that are involved in tremor pathophysiology. The reciprocal inhibition of A\(\alpha\) fibers seems to decrease the excitability of antagonist motor neurons and increase the excitability of agonist motor neurons [83]. While it is not entirely clear whether the reciprocal activation of A\(\alpha\) fibers results in tremor, intermittent stimulation of the A\(\alpha\) fibers innervating the flexor and extensor muscles via FES has been studied in patients with ET or PD, showing a 58% tremor suppression [78]. Another study of 14 patients with PD tremor also observed reduction in tremor amplitude and frequency [84]. This suggests that the excitability of antagonist and agonist motor neurons can be modulated, thereby supporting the mechanism by which FES attenuates tremor. Muscle fatigue is commonly seen as a nonserious adverse event in patients who are treated with FES, owing to the fact that both out-of-phase and co-contraction stimulations lead to the activation of joints and muscle contraction [85]. The use of the Tremor's glove can also result in numbness of the hand and burning sensation [82]. Contraindications for FES include a prior implanted electrical device, cancer, osteomyelitisitis, thrombosis/hemorrhage, epilepsy, or pregnancy [86]. In each case, it is incumbent on physicians to evaluate the risk and benefit of a FES treatment based on the patient's medical history. Functional electrical stimulators are minimally invasive and demonstrate sufficient efficacy in the suppression of tremor. However, muscle fatigue during repeated FES-induced contraction limits their long-term use. To address this limitation, several emerging technologies have been proposed to reduce or counter muscle fatigue during FES [85]. These functional electrical stimulators have only been studied in small cohorts of patients. Large scale, sham-controlled randomized trials are necessary to validate the efficacy and safety of these devices.