Polar heart rate monitors have measured heart-rate variability for decades and used it to set the HR limits. Polar uses R-R intervals to adjust heart rate high

Polar heart rate monitors have measured heart-rate variability for decades and used it to set the HR limits. Polar uses R-R intervals to adjust heart rate high and low limits for for exercise period (unless you set them manually). I think they are called ownindex and ownzone. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4751190/ https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4751190/ http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100-879X2008001000004 http://www.scielo.br/scielo.php?script=sci_arttext&pid=S0100...