[scott_s]

My point is that I am training both, even though my anaerobic capacity is the real determinant of the outcome. But inbetween bursts, I am still active, just at low to moderate levels, and this occurs over a long period of time.

If your sprint workouts involve sprints with very light jogging inbetween then you will work both.

[dworrad]

Many people will tell you that when you go in to "sugar burning" (approx 140bpm+) mode then you are doing very little to improve your aerobic capacity... Regardless of your heart rate being lower at certain points. Read about Mark Allen's (Ironman champion) experiences with this. To train your aerobic system you need to start in the aerobic zone and stay there religiously. IMHO