Inoue A; Sidia Institute of Science and Technology, Brazil
Soares JPF; Antunes-Santos F; Ferreira A; Goncalves A; Alcantara JA; Dos Santos MR

JMIR Cardio. 10:e81917, 2026 Apr 16.

Background: Photoplethysmography-based smartwatches are increasingly used
for continuous heart rate (HR) monitoring. Their accuracy has been
demonstrated at rest or during low-intensity activity, but data are scarce
for maximal-intensity exercise, when motion artifacts and rapid
hemodynamic changes can degrade the photoplethysmography signal.
Validating these devices under such demanding conditions is essential
before they are applied to clinical exercise testing, athletic training,
or remote health monitoring.

Objective: This study aimed to evaluate the validity of the Samsung
Galaxy Watch6 (GW6) in estimating HR throughout a graded, maximal ramp
cardiopulmonary exercise test performed on a treadmill. A secondary aim
was to explore whether measurement error varies across 5 predefined
intensity zones (50%-60%, 60%-70%, 70%-80%, 80%-90%, and 90%-100% of the
maximum HR determined individually for each participant).

Methods: Overall, 55 healthy adults (30 men, 25 women; mean age 30.3, SD
8.2 years) completed a symptom-limited incremental treadmill protocol to
volitional exhaustion. Simultaneous HR recordings were obtained from the
GW6 (left arm) and a Polar H10 chest strap monitor, which served as the
reference standards. For each intensity zone, the following agreement
indices were computed: intraclass correlation coefficient (ICC), median
absolute error, median absolute percentage error, and root mean squared
error. Bland-Altman analysis was performed to quantify the mean bias and
95% limits of agreement between the GW6 and the Polar H10. Statistical
significance was set at P<.05.

Results: Agreement between the GW6 and Polar H10 varied across exercise
intensities. ICC indicated moderate to good agreement at low to moderate
intensities (ICC=0.71 at 50%-60%; ICC=0.89 at 60%-70%; ICC=0.54 at
70%-80%; and ICC=0.64 at 80%-90% HRmax), and at 90%-100% of HRmax the
agreement was good-to-excellent (ICC=0.90). Absolute error metrics showed
stable or reduced errors with increasing intensity, with median absolute
error consistently around 1-3 bpm and median absolute percentage error
declining from 2.90% at 50%-60% HRmax to 0.60%-0.75% at >=70% HRmax. Root
mean squared error ranged from 4.62 to 4.88 bpm across intensity zones.
Bland-Altman analysis showed that the GW6 consistently underestimated HR
compared with the Polar H10, with an overall mean bias of -2.67 bpm and
wide limits of agreement (-16.90 to 11.57 bpm). This negative bias was
present across all HR zones. The agreement was adequate for group-level
comparisons but displayed substantial individual variability.

Conclusions: The GW6 provides a good degree of validity for HR monitoring
during a maximal treadmill cardiopulmonary exercise test in healthy young
adults. Although measurement error increases modestly at near-maximal
workloads, absolute errors remain well within clinically acceptable
thresholds. These findings support the potential use of GW6 as a
convenient, noninvasive alternative for HR tracking in laboratory-based
exercise testing.