As a track coach I can confirm Therm's assessment of how they keep their HR in that target zone.
Everyone of my athletes knows how to keep track of their HR and pulse and each day a new person is in charge of keeping the group in this target zone by monitoring their HR and that of their teammates. As soon as their HR begins to dip it's time for the next interval or rep on the track.
I do this myself with several 'sprint' workouts, some of them look like this:
1~100m repeats, 10 pushups in between each repeat, this is your 'rest'
start with just ten 100s (believe me when you get to 5 10 will suddenly feel like a lot)
2~400m sprint, jog 100m; 300m sprint, jog 100m; 200m sprint, jog 100m; 100m sprint, jog 100m

etc...............