A fitness tracker can be used interchangeably with a reference method for underwater single-lead electrocardiography but not heart rate variability analysis in swimming horses.

To validate a single-lead ECG fitness tracker device (FT) against the reference multilead ECG recording device (RM) for heart rate (HR) and HR variability (HRV) monitoring of horses during routine swimming. 40 race-fit Thoroughbred racehorses were used for 5 days in May 2024. Surface ECG recordings were obtained simultaneously from the FT and RM devices in horses swimming 63.65 m across a pool. Electrocardiograms were reviewed and artifacts corrected when necessary, and correlations were analyzed between the 2 devices for HR and HRV parameters. Descriptive statistics and Bland-Altman tests were used to determine the agreement between data generated from the 2 devices. A negligible bias was observed for HR (r = 0.99) between the 2 devices. The root mean squared error calculated for HR between them was 0.28 beats/min. The time domain and nonlinear HRV parameters, except triangular index, had additional variability and bias with the FT compared to the RM. This suggests that additional testing is needed to validate the FT for assessment of HRV parameters. The FT and the RM can be used interchangeably for HR monitoring in swimming horses. The FT is not recommended for comprehensive HRV analysis during swimming before bias and variability are corrected. Our findings confirm that the FT is a practical tool to reliably monitor HR in swimming horses, which has applications for fitness and workload monitoring during training.