I'm trying to figure out how intensity minutes for an activity translate into heart rate zones. A short walk this morning (well the last uphill section I ran) shows intensity 49m total (19m moderate and 15m vigorous x2). 
Times in Heart Rate Zones 5 = 5.56m, 4 = 4.53m, 3 = 5.08, 4 = 4.01m.
How is intensity 19m and 15m (x2) derived from this?
At what point does aerobic exercise become vigorous?
