In Experiment A, the investigators will recruit healthy participants without motor disorders and medications influencing brain function (n = 25). The investigators will characterize inhibition of wrist flexors/wrist extensors and first dorsal interossei (FDI) and abductor pollicis brevis (APB) muscles with PES, and also use TMS to stimulate the corresponding areas in the brain and see if the movement can be reduced through PES. HD-EMG will be used to collect data and identify associated motor unit spike trains. Motor-evoked potentials (MEPs) will be recorded with EMG when TMS is targeted to the primary motor cortex. Resting state functional magnetic imaging resonance (rs-fMRI) and high-angular resolution diffusion imaging (HARDI) tractography of the brain will be recorded.~In Experiment B, the investigators will recruit patients with essential tremor (ET) and/or Parkinson's disease (PD) (n = 25 for each pathology). The investigators will repeat the same characterization of inhibition of wrist flexors/wrist extensors and PDI/APB with PES and also use TMS to stimulate corresponding areas in the brain. Additionally, the investigators will test PES conditions as a tremor reduction strategy at the wrist level. The investigators will also test PES conditions at the elbow and shoulder joints as a tremor reduction strategy. Finally, the investigators will observe and characterize the long-term effects (lasting 24h, 48h, and 7 days post stimulation) of PES via coherence between HD-EMG and EEG at the tremor frequency.
The purpose of this study is to understand the neurophysiological mechanisms of peripheral electrical stimulation (PES) in modulating supraspinal tremorogenic input to motoneurons. For this purpose, the investigators will use transcutaneous PES, high-density electromyography (HD-EMG), transcranial magnetic stimulation (TMS), electroencephalography (EEG), magnetic resonance imaging (MRI), and neuromusculoskeletal modelling. This study will be carried out in both healthy participants and patients with essential tremor (ET) and Parkinson's disease (PD).