This article presents an overview of fifty-eight articles dedicated to the evaluation of physical activity in free-living conditions using wearable motion sensors. This review provides a comprehensive summary of the technical aspects linked to sensors (types, number, body positions, and technical characteristics) as well as a deep discussion on the protocols implemented in free-living conditions (environment, duration, instructions, activities, and annotation). Finally, it presents a description and a comparison of the main algorithms and processing tools used for assessing physical activity from raw signals.

New technologies, such as wearable sensors, allow the quantitative assessment of gait alterations due to Parkinson's disease (PD) through Digital Mobility Outcomes (DMOs). These DMOs have the potential to complement traditional clinical assessments but must be relevant, reliable, and representative of the patient's overall condition. Real-world monitoring offers a valuable approach for this type of day-to-day evaluation of patients. This systematic review has four primary aims: 1) To identify trends in protocol design for real-world gait monitoring using wearables in patients with PD. 2) To detail the analysis of inertial data and the computation of DMOs. 3) To summarize the clinical scales and symptoms studied. 4) To outline trends in the conclusions and limitations reported by authors in this field. Three databases (MEDLINE via PubMed, Cochrane, and EMBASE) were systematically searched between September 1, 2013, and September 15, 2023. Eligibility criteria included studies involving adults with a PD diagnosis, the use of a wearable device with at least one accelerometer or gyroscope, and gait analysis conducted in real-world settings. Sixty-three studies were selected. Overall, wearables successfully provide clinically meaningful information on gait impairment in patients with PD. Stride speed as a DMO is well-established and clinically meaningful, while other metrics, such as stride length, stride duration, and cadence, show great promise for routine clinical practice and research. However, the lack of consensus on the methods of investigation and the small sample sizes remain significant barriers that must be addressed to facilitate broader adoption in clinical practice and research.