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. 2021 Nov 10;12(12):7445-7457.
doi: 10.1364/BOE.439893. eCollection 2021 Dec 1.

Monte Carlo analysis of optical heart rate sensors in commercial wearables: the effect of skin tone and obesity on the photoplethysmography (PPG) signal

Ajmal  1 , Tananant Boonya-Ananta  1 , Andres J Rodriguez  1 , V N Du Le  1 , Jessica C Ramella-Roman  1   2
Affiliations

Monte Carlo analysis of optical heart rate sensors in commercial wearables: the effect of skin tone and obesity on the photoplethysmography (PPG) signal

Ajmal et al. Biomed Opt Express. .

Abstract

Commercially available wearable devices have been used for fitness and health management and their demand has increased over the last ten years. These "general wellness" and heart-rate monitoring devices have been cleared by the Food and Drug Administration for over-the-counter use, yet anecdotal and more systematic reports seem to indicate that their error is higher when used by individuals with elevated skin tone and high body mass index (BMI). In this work, we used Monte Carlo modeling of a photoplethysmography (PPG) signal to study the theoretical limits of three different wearable devices (Apple Watch series 5, Fitbit Versa 2 and Polar M600) when used by individuals with a BMI range of 20 to 45 and a Fitzpatrick skin scale 1 to 6. Our work shows that increased BMI and skin tone can induce a relative loss of signal of up to 61.2% in Fitbit versa 2, 32% in Apple S5 and 32.9% in Polar M600 when considering the closest source-detector pair configuration in these devices.

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Conflict of interest statement

The authors declare no conflicts of interest.

Figures

Fig. 1.
Fig. 1.
Schematic of the heart rate sensor specifications in Apple Watch S5.
Fig. 2.
Fig. 2.
Schematic of the heart rate sensor in Fitbit Versa 2.
Fig. 3.
Fig. 3.
Schematic of the heart rate sensor in Polar M600.
Fig. 4.
Fig. 4.
Sample geometric illustration of Monte Carlo input configuration.
Fig. 5.
Fig. 5.
PPG signal simulated from non-obese (NOB) skin optical properties for Fitzpatrick skin type 1 and skin type 6 at detector 1 (NOB D1S1 and NOB D1S6) in (a) Apple S5, Fitbit Versa 2 and Polar M600, (b) Normalized waveform for the signal from skin type 1 and skin 6 recorded at closest S-D pair in Apple S5, (c) Normalized PPG waveform showing the AC and DC components of the signal.
Fig. 6.
Fig. 6.
PPG signal simulated from obese skin optical properties for Fitzpatrick skin type 1 and skin type 6. Waveforms extracted from skin type 1 and skin type 6 for Apple S5 detector 1, Fitbit Versa 2 detector 1 and Polar M600.
Fig. 7.
Fig. 7.
AC/DC percentage ratio of PPG signal from Fitzpatrick skin type 1 and skin type 6 for non-obese and morbidly obese cases in Apple S5, Fitbit V2 and Polar M600. (a) Closest S-D pair, (b) Farthest S-D pair.
Fig. 8.
Fig. 8.
Comparison of PPG waveform between skin type 1 in non-obese case (NOB-S1) and skin type 6 in morbidly obese case (MOB-S6) for the closest S-D pair in Fitbit Versa 2.
Fig. 9.
Fig. 9.
AC/DC percentage ratio of PPG signal from Fitzpatrick skin type 1 and skin type 6 in non-obese and morbidly obese cases simulated for IR sources in (a) Apple S5 and (b) Fitbit Versa 2
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