I’ve gotten three questions like the following in the last week so some of you must just be getting power meters and wondering how to use them.
Question: I heard in a discussion about the concept of using power levels for pacing in the bike portion of a half or full Ironman. I can’t remember all of the details and was wondering if you had any insight into this idea. For an IM race it was something like not exceeding 60% of your CP 60 power levels during the bike ride so that you wouldn’t burn out your legs prior to the run. I am not sure on the numbers but was wondering if you had any thoughts on this. I was also wondering if there were similar numbers that you could use for a half IM race.
Answer: I’ve had the Ironman athletes I coach using power meters to regulate intensity in their races for the last five years. It works well. A heart rate monitor may also be used but the excitement of the day usually creates inflated numbers the first half hour or so of the bike. This just happens to be the most critical part of the entire race, I believe. Go out too fast now and you run a very high risk of having a poor performance or not finishing. I see this happen at races all the time. It contributes to the stomach problems that so many IM athletes experience.
Most can’t understand why their nutrition plan worked on all of the long rides but not in the race. One reason is they simply started much, much faster in the race. The faster you go, the more limited your nutritional choices become. In a one-mile running race it is difficult to get water down. If you are doing RAAM (Race Across America) you can eat a hamburger and French fries on the bike (not that I’d recommend it).
So the bottom line is that a power meter will help you keep the pacing under control so that it matches up nicely with the foods you used in training. Of course, there’s more to the IM nutrition issue than just pacing. But that’s a whole other post for a later date.
So the question then is, what power should you ride at? Rule of thumb: The slower you go the lower your power must be. Not Earth-shaking, huh? What I’m really saying is that an eight-hour IM biker and a four-and-a-half-hour rider are not doing the same race. They shouldn’t train the same way or use the same percentage of Functional Threshold Power (FTP) which is the same thing as CP60 (mean maximal power for 60 minutes).
The sub-five-hour racer will usually ride at 70 to 80 percent of FTP. The eight-hour rider will do something like 50 to 60 percent of FTP. In between finish times will probably be 60 to 70 percent of FTP. Why the difference even though the slower rider has a lower FTP? Simply because you can’t hold a high percentage of a high output for a long time. If the sub-five athlete was going eight hours in a longer distance event he too would have to ride at a lower power output in the neighborhood of 50 to 60 percent of FTP. That’s what I mean when I say that the fast rider and the slow rider are not doing the same race even though both are in an IM.
So what about the power for a half Ironman? The usual rule of thumb here is that when you double the distance you decrease the intensity by 5 percent. So the converse would seem to indicate that you would increase the power by 5 percent over what you would do in an IM. That works really well for short events like 40k vs. 20k time trials. In reality it might be a bit more than five percent from IM to HIM since a two-and-a-half hour, 52-mile race is probably much less challenging than a five-hour, 112-mile race. So perhaps about 8 percent.
A friend of Gordo Byrn once showed me a chart he had put together which resolved these issues of how much power as a percent of FTP based on predicted finishing time. I can’t find it, nor do I have permission to use it. I haven’t checked, but Gordo may have it on his website. I’m sure someone out there is aware of this and can fill us in.