###### Abstract  
Wearables with optical sensors for heart rate (HR) measurement are widely available in the market. However, their accuracy in water is still underexplored. The aim of the present study was to test the agreement of two different devices for HR monitoring with a chest strap while swimming at different intensities. Twenty male and ten female subjects (mean 19.6 \(\pm\) 0.7 years old, 173.3 \(\pm\) 5.4 cm, and 67.1 \(\pm\) 6.6 kg) performed an intermittent progressive protocol of 3 \(\times\) 30 s tethered front crawl swimming followed by a 1 min rest period. A chest strap, a wristwatch, and a multi-site optical sensor placed at the temple were used simultaneously. A strong association, an excellent intra-class correlation, and a low mean bias were denoted (R\({}^{2}\) = 0.85, ICC = 0.94, b = \(-\)1) between HR\({}_{\text{chest}}\) vs. HR\({}_{\text{temple}}\). Both indicators increased throughout the test, denoting an increase in accuracy from light to vigorous exercise intensity. HR\({}_{\text{chest}}\) and HR\({}_{\text{watch}}\) showed a moderate association for the whole test (R\({}^{2}\) = 0.23) but a weak association, a poor consistency, and a high mean bias stepwise (0.01 \(\leq\) R\({}^{2}\)\(\leq\) 0.06, 0.03 \(\leq\) ICC \(\leq\) 0.42, \(-\)48.1 \(\leq\) b\(\leq\) \(-\)16.1). During swimming, the HR values from the temple showed a better agreement with the chest strap than those from the wristwatch. The temple reading accuracy might be enhanced by using the device during the dryland warm-up routine.  
w _Article_  
Academic Editor: [PERSON]  
10 March 2025  
10 May 2025  
19 May 2025  
1
Footnote 1: Department of Sports and Expressions, Polytechnic of Guarda, 6300-559 Guarda, Portugal 1  
Received: 10 March 2025  
Received: 17 May 2025  
Accepted: 19 May 2025  
Published: 22 May 2025  
###### Abstract  
Wearables with optical sensors for heart rate (HR) measurement are widely available in the market. However, their accuracy in water is still underexplored. The aim of the present study was to test the agreement of two different devices for HR monitoring with a chest strap while swimming at different intensities. Twenty male and ten female subjects (mean 19.6 \(\pm\) 0.7 years old, 173.3 \(\pm\) 5.4 cm, and 67.1 \(\pm\) 6.6 kg) performed an intermittent progressive protocol of 3 \(\times\) 30 s tethered front crawl swimming followed by a 1 min rest period. A chest strap, a wristwatch, and a multi-site optical sensor placed at the temple were used simultaneously. A strong association, an excellent intra-class correlation, and a low mean bias were denoted (R\({}^{2}\) = 0.85, ICC = 0.94, b = \(-\)1) between HR\({}_{\text{chest}}\) vs. HR\({}_{\text{temple}}\). Both indicators increased throughout the test, denoting an increase in accuracy from light to vigorous exercise intensity. HR\({}_{\text{chest}}\) and HR\({}_{\text{watch}}\) showed a moderate association for the whole test (R\({}^{2}\) = 0.23) but a weak association, a poor consistency, and a high mean bias stepwise (0.01 \(\leq\) R\({}^{2}\)\(\leq\) 0.06, 0.03 \(\leq\) ICC \(\leq\) 0.42, \(-\)48.1 \(\leq\) b\(\leq\) \(-\)16.1). During swimming, the HR values from the temple showed a better agreement with the chest strap than those from the wristwatch. The temple reading accuracy might be enhanced by using the device during the dryland warm-up routine.  
Wearables, heart rate; swimming; intensity; accuracy