Polar (heart hate monitors) did pioneering work with HRV in the late 90's and their heart rate monitors have have a mode that HRV to to determine best heart ra

Polar (heart hate monitors) did pioneering work with HRV in the late 90's and their heart rate monitors have have a mode that HRV to to determine best heart rate limits for each exercise period for almost two decades. Heart rate itself is a good indicator of stress. Aerobic training using HR limits helps to avoid overtraining (and undertraining). If you are stressed out in one day your HR is up and you exercise slower. HRV gives even more detailed information. HRV is high during a rest and starts to decrease when the heart works closer to maximum. Measuring HRV change during a rest and gradually increasing the level of exercise gives more info than HR alone. Validity of the Polar V800 heart rate monitor to measure RR intervals at rest https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4751190/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4751190/