Smartwatches are only of limited use for scientific studies as we have demonstrated in your review about papers about smartwatch accuracy:

090. Review of Accuracy Studies of Consumer Grade Heart Rate Monitors: Methods, Testing Protocols, Reference Devices

So let us build your own medical and research grade ECG device

Use a device which is based on the AD8232 ECG chip, the ADS1115 16 bit A/D converter & an ESP32 instead – the building blocks for a multi-channel ECG device for medical & research applications:

The AD8232 ECG chip, the ADS1115 16 bit A/D converter & an Arduino microcontroller – the building blocks for a multi-channel ECG device for medical & research grade applications

And let us build our own ECG Stress treadmill

Buy a treadmiss such as the ADVG Curved Treadmill is available from U.K for 3 123 USD:

Add an electric motor and some speed sensors to it as we have discussed it previously:

How to build a “Schiller CS-200 Excellence Stress System with Treadmill” which costs $26 000 for a view thousand dollars by ourselves

And do only scientific studies to about 15 km/h, and only on a treadmill. Since which really serious scientist does heart rate studies at speeds above 15 km/h outdoors? As the g.tec medical support told us: “We do not know of a manufacturer who has solved the problem of ECG motion artifacts. We can only minimize those”.