I am curious about heart rates as a measure of fitness. Some running programs prescribe running in certain zones for particular workouts. These zones are supposed to be within a certain percentage range of your max HR. Out of curiousity I started wearing a hear rate monitor to see where I compared to the zones. What's interesting to me is there seems to be little difference in my HR between zones 2-4. For example, I am supposed to be in zone 3 with 7:41 pace, and zone 4 with a 7:11 pace. However, when I look at my stats after the run, there is only a small avg HR difference between the two - around 5 bpm difference on avg. Should I be concerned? What does this suggest about my fitness? thanks in advance for any ideas.