The goal of this clinical trial is to investigate the feasibility if a remotely administered smartphone app can increase the volume and intensity of physical activity in daily life in patients with isolated Rapid Eye Movement (REM) sleep behaviour disorder over a long period of time (24 months). Participants will be tasked to achieve an incremental increase of daily steps (volume) and amount of minutes exercised at a certain heart rate (intensity) with respect to their own baseline level. Motivation with regards to physical activity will entirely be communicated through the study specific Slow Speed smartphone app. Primary outcomes will be compliance expressed as longitudinal change in digital measures of physical activity (step count) measured using a Fitbit smartwatch. Exploratory outcomes entail retention rate, completeness of remote digital biomarker assessments, digital prodromal motor and non-motor features of PD, blood biomarkers and brain imaging markers. Using these biomarkers, we aim to develop a composite score (prodromal load score) to estimate the total prodromal load. An international exercise study with fellow researchers in the United States and United Kingdom are currently in preparation (Slow-SPEED). Our intention is to analyse overlapping outcomes combined where possible through a meta-analysis plan, to obtain insight on (determinants of) heterogeneity in compliance and possible efficacy across subgroups