Monitoring heart rate response during exercise can indicate glycogen status and readiness for training.

“so this is why using that my version of heart rate variability it's quite useful I've been using it incredibly successfully for 25 years with my athletes uh where I see that hey your heart R is not going up today you know usually it's 185 190 for example when you do a active threshold for example and today it was 170 so tomorrow take it easy or pile up on glycogen I mean on carbohydrates or take an easy day and you'll see how you're going to be very responsive the next day the following day and in fact that's what happens I would say 10 out of 10 times but let's say nine out of 10 times right but I do that with myself as well”