I've been re-reading Joe Friel's legendary triathlon training book The Triathlete's Training Bible in order to answer the question "What should the purpose of my workouts be?" While trying to answer that question, I happened across a section on determining race readiness. The protocol calls for doing an “aerobic threshold” workout for a given amount of time, depending on desired race distance and then looking for “decoupling”. For a half-iron race an athlete should be able to run for 90 minutes in low zone 2 without heart rate decoupling between the two halves of the run.
Decoupling refers to the phenomenon of heart rate tracking unpredictably with exertion. For example, it might seem reasonable that if you maintain your power level for a certain amount of time…your heart rate should also stay the same. In practice, as you fatigue, your heart rate will start to inch higher even if your pace remains exactly the same and the terrain is flat.
In order to test race readiness, then, you can keep your power output level and see if your heart rate stays even (See chapter four, table 4.6). I did exactly such a test this afternoon. In the interest of brevity, I only ran for 1.5 hours. First, I warmed up for a few minutes, then set my treadmill to a 10 minute pace and started my watch.
For 90 minutes, I ran the 10 minute pace. This put me squarely in zone 2 (average was 2.5). At this intensity I was burning a 50/50 blend of fat and carbohydrate (80% of HRmax roughly equals 65% of VO2max, which is about a 50/50 burn). Although my heart rate moved around a bit, the overall averages between the two halves of the workout were well within the 5% drift indicating poor event fitness. For the first half of the run, my average heart rate was 154.2 BPM. For the second half, it was 155.7 BPM, a difference of only 1.5 beats per minute.
Data: Cardiac drift test