Lots of threads on this topic, but for vo2max to be updated regularly/accurately it need a pretty steady run (not huge hills or wind or trail that will effect the correlation of pace vs heart rate). You need a weight entered and a maximum HR that is accurate for you. You need to get your HR above a certain point before it will give you a score, maybe it is 70 or 75%% I don't recall. Needs to be at least 10minutes , the manual outlines some of the details and tips also.
