Slow-SPEED UK: A Double-Blind Randomised Feasibility Trial

Change in average daily step count from baseline (weeks 0-4) to the final four weeks of the 18-month study period (approximately weeks 75-78), measured continuously using the Fitbit Charge 6 wearable activity monitor. Average daily step count will be calculated across four-week periods and analysed descriptively as a measure of engagement and sustainability in keeping with the feasibility aims of the study.

Composite feasibility outcome assessed by: (1) proportion of eligible participants who consent to take part (recruitment rate); (2) proportion of randomised participants who complete the 18-month study period (retention); (3) adherence to the intervention, measured by app engagement metrics and Fitbit wear time; (4) completeness of remote and in-person assessment data; and (5) frequency and nature of adverse events (safety).

Change in mean daily minutes spent at or above 64% of age-predicted maximum heart rate (HRmax), measured continuously via the Fitbit Charge 6 photoplethysmography sensor. Distribution of time across intensity bands will be described and the feasibility of capturing sustained heart-rate data over 18 months will be assessed. Analyses are exploratory only.

Feasibility of obtaining a remote estimate of aerobic fitness (VO₂max) via the Fitbit algorithm throughout the study period. Analyses will focus on completion rates, participant tolerance, and signal stability of remote VO₂max estimation. Resting heart rate and heart rate variability (HRV) will also be summarised as the average daily resting values to assess feasibility and stability of remote cardiovascular monitoring.

Feasibility of repeated blood pressure measurement using standard sphygmomanometry, assessed at baseline and 18-month in-person visits. Supine and standing readings will be taken in accordance with American Autonomic Society recommendations to assess orthostatic hypotension. Collected for safety monitoring and feasibility assessment only; not used for clinical diagnosis.

Feasibility of repeated remote motor task completion using the Roche Mobile Application V2 smartphone platform. Active tasks include finger tapping, shape drawing, reaction time, information processing, executive functioning, balance, gait, and upper limb performance, administered every 6 weeks throughout the study period. Feasibility assessed by number of data points received, proportion of expected submissions completed, and drop-off patterns across predefined 6-week windows from 0 to 78 weeks.

Feasibility of administering in-person structured motor assessments including the Movement Disorder Society Unified Parkinson's Disease Rating Scale Part III (MDS-UPDRS Part III), the Functional Gait Assessment (FGA), and the Mini-Balance Evaluation Systems Test (Mini-BESTest), with and without cognitive dual-tasking. Examinations will be video recorded with participant consent to allow baseline and follow-up comparison. Videos are not used for clinical decision-making.

Feasibility of administering digital upper limb motor assessments at in-person visits, including: (1) the Bradykinesia Akinesia Incoordination (BRAIN) test, a validated computer-based keyboard tapping task; (2) the Digital Finger Tapping test; and (3) the Manus Neurodynamica digital pen, an AI-based handwriting and fine-motor assessment device recording kinematic and dynamic features of writing and drawing. These tools are included as exploratory assessments of upper limb motor function.

Feasibility of collecting continuous raw accelerometer data from a 6-axis inertial movement sensor (Axivity AX6) worn on the dominant wrist for 7-day bursts at specified time points. Data will provide wearable-derived digital mobility and gait metrics in free-living conditions. Analyses will assess completion rates, wear-time compliance, and data quality across assessment windows.

Feasibility of administering a standardised high-frequency binocular eye-tracking battery (NeuroClues) at in-person visits, including prosaccades, antisaccades, fixation, and pursuit tasks. Key endpoints include prosaccade latency (ms), antisaccade latency (ms), antisaccade error rate (%), fixation stability, and saccadic intrusions. Included as an exploratory assessment of oculomotor control only.

Feasibility of remote long-term sleep monitoring using the Fitbit Charge 6, capturing sleep onset latency (SOL), total sleep time (TST), sleep efficiency (SE), and duration of sleep stages (deep, light, REM, and awake). Complemented by the validated Pittsburgh Sleep Quality Index (PSQI) questionnaire administered at each assessment time point. Feasibility assessed by data completeness, wear-time adherence, and questionnaire completion rates.

Feasibility of repeated administration of the Hospital Anxiety and Depression Scale (HADS) and the Oxford Happiness Questionnaire (OHQ), a 29-item self-report measure of subjective wellbeing. Feasibility assessed by questionnaire completion rates and missing data patterns. Mood and autonomic symptoms are also captured via MDS-UPDRS Part I at in-person visits.

Feasibility of administering the Montreal Cognitive Assessment (MoCA) at in-person visits to ensure participants can engage appropriately with study procedures and to assess the feasibility of repeated in-person cognitive testing in this population. Not used for clinical diagnosis or prediction.

Feasibility of repeated administration of the validated Scales for Outcomes in Parkinson's Disease-Autonomic (SCOPA-AUT) questionnaire as a general autonomic function measure. Feasibility assessed by completion rates across repeated remote and in-person administrations.

Feasibility of repeated olfactory assessment using the 40-item University of Pennsylvania Smell Identification Test (UPSIT) at in-person visits. UPSIT scores will be recorded to assess the feasibility and stability of repeated olfactory testing across the study period. Results are not used for individual risk prediction or clinical diagnosis.

Feasibility of administering the World Health Organisation Quality of Life assessment (WHOQOL-BREF) and Lawton's Instrumental Activities of Daily Living (iADL) scale at annual assessment time points. Feasibility assessed by completion rates, missing data patterns, and participant burden ratings.

Feasibility and acceptability of the Slow-SPEED smartphone application assessed using the System Usability Scale (SUS). Participant ratings will be used to evaluate usability and inform improvements to the user experience for a future definitive trial.

Quantitative assessment of participant interaction with the Slow-SPEED application, measured by total number of app opens across predefined time windows: baseline period (0-1 month), and post-randomisation periods of 1-6 months, 6-12 months, and 12-18 months. Reported separately by study arm.

Compliance with the intervention assessed by: (1) proportion of participants achieving an increase in mean daily step count relative to baseline at 18 months, categorised as 0-25%, 26-50%, 51-75%, and 76-100% increase; (2) proportion achieving an increase in mean daily MVPA minutes relative to baseline, using the same categories; (3) total number of weekly step goals completed across the 18-month period; and (4) total number of weekly intensity goals completed across the 18-month period. Reported separately for both study arms.

Proportion of randomised participants remaining in the study without withdrawal, assessed at 6, 12, and 18 months following randomisation. Reported separately for the intervention and active control arms.

Feasibility of administering a self-designed questionnaire to explore participant-reported motivators and barriers to engagement with the physical activity programme. Responses will inform the design and participant support strategies for a future definitive trial.

An exploratory composite research index aggregating standardised scores across four domains already collected within the study:

Physical activity domain-average daily step count and mean daily MVPA minutes, derived continuously from the Fitbit Charge 6 Self-reported symptom domain-scores from HADS, PSQI, WHOQOL-BREF, SCOPA-AUT, and Lawton iADL scale Sensory domain-UPSIT olfactory score adjusted for age and sex Digital engagement domain-Fitbit wear time, Slow-SPEED app opens, and Roche Mobile Application V2 completion rates

Domain scores will be standardised and combined descriptively to characterise overall patterns of engagement and function across the study period. The RCS is a research-only instrument. No individual-level scores or predictions will be generated, communicated to participants, or used for clinical decision-making. Results will be used solely to inform the design of a future definitive trial.