While trying to understand the 5 HR zones commonly described in heart rate training articles, I found this simple explanation.
Zone 1 - recovery --- you can sing ( walking and extremely slow jogging)
Zone 2 - aerobic - you can carry out a complete conversation (slow easy running)
Zone 3 - high aerobic - you can say a few single sentences ( steady pace , long runs)
Zone 4 - aerobic threshold - you can only use single words ( tempo paced runs tempo paced intervals)
Zone 5 - anaerobic -- you can say nothing! ( sprinting, fast paced race finishes. intervals)