The graph below was produced by downloading heart rate data from my Polar S610 heart rate monitor via the Polar IR Interface to my PC, using the Polar Precision Performance software (included with the hear rate monitor).

You can easily see the four intervals. The first two are fairly flat in profile; the third and fourth build to a peak at the very end of the interval. The first interval run at my estimated max heart rate minus 35 beats (or about 140). The second interval was run at max minus 25 beats (about 150). The third and fourth were run at max minus 15 (around 160), and building to max during the last 30 seconds of the interval. As you can see, I was not successful in getting to max on the third interval.

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