Without this tool, its easy to let ones training sessions slide together into one level of intensity, in the "no-man's-land" of heart rate zones, which will yield a disastrous harvest when youre looking for that higher gear in the latter stages of a race.
Wearing a heart rate monitor, particularly for inexperienced athletes, adds a helpful degree of objectivity to training, so that you know you are training the way you are supposed to train.
What many athletes don't realize is that the use of heart rate monitors doesn't have to stop on the training track. With a little understanding of how your body works, you can use a heart rate monitor during competition in order to distribute your effort evenly, which is the surest means to optimal performance.
Among elite triathletes on the circuit, Finn Pauli Kiuri is probably the best known for monitoring heart rate during competition. I've actually heard legendary athletes criticize Kiuri for his reliance on his Polar Sport Tester, suggesting he should lose it during competition and rely more on feel.
The fact of the matter is that feel, or perceived exertion, can be a real enemy to the endurance athlete because it fails to provide any objective data during competition.
The appropriate use of a heart rate monitor can give you such objective data, which can tell you things that feel alone cannot. For example, the need to consume more fluids and/or carbohydrates during competition will be clearer.
A couple of years ago I competed in a half-Ironman event in Fiji. During that event I wore a Polar Sport Tester and recorded my heart rate for the duration of the event.
On somewhat limited training, I was able to get the most out of myself during the actual race by spreading the race load and avoiding the urge to go out too fast, chew through all my muscle glycogen and then leave nothing in the tank for the last bit of the race.
In preparation for this race I'd swum most days for the previous two months (mainly stroke work) and had done very little specific bike training. I'd kept my running going at around 120 to 140K per week, which for me (as a marathoner) was a relatively light schedule.
Using the heart rate monitor sensibly during the half-Ironman meant I was able to race to my full potential (on limited preparation) by simply following the following principles.
Generally, during the swim you'll need to race by feel because it is impractical to check your heart rate, and if youre training well, the swim simply isn't long enough to sap all of your glycogen stores, even over an Ironman distance. Once youre out on the bike and the run, however, the heart rate monitor becomes invaluable.
You should have established already your anaerobic threshold (AT2) in both the bike and run. Adjusting for the duration of the event, use these figures to dictate the pace at which you bike and run.
For example, if you're racing a sprint-distance event, and you're well conditioned, you should be able to red-line it all the way. By that I mean sit at two to five beats per minute above your AT2 for both events and just let it hurt. Here the heart rate monitor can be a real taskmaster, indicating when you're slacking off.
For elite athletes, a similar approach can be adopted over the Olympic distance, but if youre not elite I'd suggest you sit at your AT2, and surge above it every now and again (e.g., when climbing a hill or trying to break away from another competitor).
Each time you go beyond your threshold and into oxygen debt, you: a) produce lactate faster than your body can clear it, (b) burn muscle glycogen quickly, and c) will eventually need to slow down to repay this debt and clear the accumulating lactate. The better your aerobic base, the faster the clearing will be achieved.
Whats the best approach for most of us? Work at (or marginally below) your AT2 on both the bike and run, and build the intensity on each so that it is maximal toward the end of either segment. Then, if you fall across the line in a hollowed-out heap at the end of the run, it doesn't matter you've finished!