Hill Sprints are a form of strength training and are not Z1 training. They do not really fit into a zone category. Intensity zones are used to designate variations AEROBIC work load so you can control the intend training effect. Hill Sprints are too short (and by using a very long rest) have minimal aerobic energy contribution. As described they are relay heavily on stored ATP and to a lesser extent on the anaerobic energy production system.

If done maximally and in a high enough volume that will reduce muscle glycogen stores. I would not count on them to replace a fasted Z1-2 longer duration effort though.

Scott

The hill sprints will reduce glycogen stores for sure. I’m doubtful that they will drain your tank as much as having started the day fasted though unless you are very poorly fat adapted. Now way to know for sure from my end. Sorry.

Scott