Validity of the Fitbit Inspire 2 heart rate monitor: impact of device placement, wrist movement, and sport activities
Authors: Vermunicht P , Verberckt C , Makayed K , Laukens K , Knaepen L , Naessens S , Hens W , Van Craenenbroeck E , Desteghe L , Heidbuchel H 
Published: June 2024

Abstract

Background/introduction
Previous studies on wrist-worn photoplethysmography (PPG) heart rate monitors (HRM) have generally reported acceptable accuracy. However, accuracy can vary due to factors like sensor placement and motion artefacts. Guidelines for correct wearing of wrist HRMs are lacking.

Purpose
This study assessed the impact of device placement, wrist movement, and sport activities on Fitbit Inspire 2 validity, while also exploring potential adaptations for enhanced accuracy.

Methods
Eleven healthy volunteers participated in either a protocol involving wrist movements (n=5) or sport activities (n=6) while wearing the Fitbit Inspire 2 (Figure 1). The first protocol was performed with the Fitbit at two different distances from the wrist joint: one or three fingers away. Based on these findings, the sport activities protocol was performed with the optimal wrist joint distance. Additionally, heart rate (HR) was monitored during two training sessions of cardiac rehabilitation (CR) in five patients. The Polar H10 chest-strapped HRM served as gold-standard HR reference in all situations. Accuracy of Fitbit was assessed using root-mean-square-error (RMSE) and the percentage over/undershooting versus Polar.

Results
When the Fitbit was worn three fingers from the wrist, a lower RMSE (13.03) and percentage over/undershooting (32.88%) were observed compared to one finger (18.39 and 37.88% respectively). Considering all sport activities, the Fitbit was adequate in three out of six participants (i.e. RMSE&lt;10.00 and over/undershooting&lt;25.00%). There was a high percentage of undershooting, particularly during running, cycling, and rowing (Figure 2).
The five CR patients had similar skin types (all light skin), and no patterns were observed regarding age, BMI, and average training HRs. During the monitored training sessions, two patients had adequate Fitbit readings, while the third patient had underestimated HR during one session but adequate results during the second session. Two other patients showed inadequate agreement between Fitbit and Polar and were advised to either wear the Fitbit more tightly or further from the wrist, without any significant improvement (RMSE decreased from 30.11 to 25.79, and from 31.75 to 20.12; over/undershooting decreased from 82.46% to 74.73%, and from 36.18% to 26.02%).

Conclusions
The Fitbit Inspire 2 demonstrates greater agreement with gold-standard Polar chest-strap HR measurements when worn three fingers from the wrist, but it still underestimates HR in half of the participants during sports activities. The reasons for success or failure of HRMs remain elusive. No technical solutions could be identified in this study or existing literature to enhance Fitbit's validity, highlighting the need for further research. Fitbit measurements may not be a fit for all patients, emphasizing the importance of a run-in phase before relying on its measurements.