JMIR Publications

ABSTRACT

Background:

Cardiovascular diseases remain a major global health burden, highlighting the need for accurate and scalable approaches to long-term physiological monitoring. Photoplethysmography (PPG) has become one of the most widely used sensing modalities in wearable devices because it enables noninvasive monitoring of heart rate, rhythm, and oxygen saturation in mobile health (mHealth) applications. Recent studies have demonstrated the potential of wearable PPG for large-scale arrhythmia screening and remote cardiovascular monitoring. However, the reliability of wearable reflectance PPG is highly sensitive to sensing conditions, among which contact force plays a critical yet often underappreciated role.

Objective:

This review aims to systematically synthesize the current evidence on how contact force influences signal quality, waveform morphology, feature extraction, and downstream physiological estimation in wearable reflectance PPG systems. By doing so, it seeks to clarify why contact force should be considered a clinically relevant system-level factor in wearable PPG, and to provide practical guidance for improving the reliability of PPG-based human health monitoring in real-world mHealth applications.

Methods:

A systematic literature search was conducted in PubMed, IEEE Xplore, Web of Science, and Scopus from database inception to March 2026. Studies investigating the relationship between contact force and PPG signal characteristics, feature extraction, or physiological parameter estimation for health monitoring were included. Eligible studies were screened and categorized according to methodological design, including contact-force control and sensing strategies, measurement sites and wearable-device configurations, experimental protocols, and outcomes at both the signal level and the health-monitoring task level.

Results:

A total of 21 studies were included in this review. The included evidence was grouped into 4 main themes: (1) contact-force control and sensing methods, (2) measurement sites and wearable-device configurations, (3) static and dynamic experimental protocols, and (4) the effects of contact force on signal quality, waveform morphology, and physiological parameter estimation for health monitoring in wearable reflectance PPG systems. Across studies, contact force showed a nonlinear relationship with PPG signal quality, typically with an inverted-U pattern. Moderate force levels generally improved signal stability and waveform clarity, whereas both insufficient and excessive force degraded performance through different mechanisms. In addition, contact-force-induced signal distortion could propagate from the signal level to health-monitoring task performance, affecting cardiovascularly relevant physiological estimates such as heart rate, oxygen saturation (SpO₂), pulse arrival time (PAT), blood pressure–related modeling, and other health-related physiological indicators.

Conclusions:

Contact force should be regarded as a clinically relevant system-level factor rather than a minor disturbance in wearable reflectance PPG. Current evidence suggests that its effects extend from signal acquisition to waveform interpretation and ultimately to the reliability of health-monitoring outputs in mHealth applications. Future wearable PPG systems will likely benefit from force-aware sensing, adaptive interface design, standardized evaluation, and context-aware modeling to improve robustness and reliability in long-term human health and cardiovascular monitoring.