Last Updated on April 10, 2024 by pg@petergamma.org
As shown in this picture:
https://www.ehsm.admin.ch/en/sports-physiology-endurance
And most probably also the necessary 3 channel ECG devices which would be necessary for accuracy studies at a higher level than shown here:
«RR interval signal quality of a heart rate monitor and an ECG Holter at rest and during exercise»