## 2 Materials and Methods
### Data Collection  
Before entering the pool, the subjects were equipped with a chest strap HR monitor (H10, Polar, Kempele, Finland), a wristwatch (Pacer Pro, Polar, Kempele, Finland), and a free optical sensor (OH1+, Polar, Kempele, Finland). For the present study, both the wristwatch and the free optical sensor were tested against the chest strap. It was assumed that the chest strap measured real HR values. Chest straps manufactured by this brand have been used for decades as a valid and reliable method for assessing HR [22]. Given their superior performance in detecting the QRS complex [9; 10], these chest straps are often used as the \"gold standard\" to test the accuracy of other sensors [9; 10]. Moreover, the model used in the present study has specifically been developed and validated for assessing R-R intervals [10], and are thus more accurate than the typical QRS complex detection provided by previous devices. The chest strap was placed around the subjects' chest, at the xiphoid process, with a close but comfortable fit without moving, as described and shown by the manufacturer [23]. The close fit of the strap alongside the decision to use tethered swimming aimed to ensure the quality of the HR readings. The wristwatch was placed on the top of the wrist, approximately 2 cm (a finger's width) up from the pisiform bone (commonly known as the \"wrist bone\") and a little bit in the direction of the elbow, as recommended by the manufacturer [24]. Different strap sizes were used for the male and female swimmers as the male-sized straps tend to be too large for female wrists. This ensured compliance with another manufacturer guideline that states that the device should be tightened firmly against the skin [24].  
The free PPG sensor can be worn in several places, the temple being one of them. It was placed against the temple of the preferred breathing side and attached to the goggle's straps as described in the owner's manual [25], with a small add-on: the swim cap was worn outside of the goggles strap and, therefore, the sensor stayed underneath the swim cap. In theory, this tweak could help reduce the probability of the formation of a water film between the skin and the sensor that could affect the readings. The same swimming cap and goggles were used for all participants, ensuring a tight fit of the sensor against the temple and complete coverup throughout the protocol. All three devices recorded HR in real time to their own internal storage; i.e., although available, no real-time transmission was used with either device. This has to do with the inability of Bluetooth and ANT+ technologies to properly transmit data through water. After the test, all of the data were transferred to the Polar Flow website and then transferred to a laptop in CSV format for further analyses. All devices were previously synchronized with the brand's computer software to ensure they all had the exact same time stamp. Despite the attempt to start all devices at the same time, it is virtually impossible to achieve per-second synchronization. Thus, corrections were made afterwards based on the devices' time stamp to ensure that all data analyses started and ended according to the test timings. The HR from the chest strap, temple sensor, and wristwatch were treated as \(\mathrm{HR_{chest}}\), \(\mathrm{HR_{temple}}\), and \(\mathrm{HR_{watch}}\), respectively. All recordings, by any device, that presented reading failures were discarded.