Time vs. Energy: Using your Power Meter in Long-Course Triathlon

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What are you trying to optimize?

Time vs. Energy

In a half-Ironman, or an event in the 4-7 hour timeframe, between glycogen stores and exogenous carbohydrates ingested, most people can race without reaching a fuel-limiter.   Compared to a full-distance Ironman, it is a more forgiving event and you can have a good race without needing to take in a lot of calories.  In a race where you are less fuel-limited, than it pays off to optimize time over energy.

However, for events in the 7-8+ hour time frame, most people eventually run into a fuel limitation.   If you push too hard and burn through too much fuel, surpassing the amount you had stored and the amount you can take in and process, then you will be energy limited for the event.  Rather than attempting to get to the finish line quicker by pushing harder, it pays to optimize energy over time.  If you can gradually dole out your energy, while continuing to take in exogenous carbohydrate, you will get to the finish line faster, by slowing down less.

This is where the power meter is paramount.  Having a tool that gives you a real-time measurement of energy expenditure on the bike is an ideal way to approach the trade off of time versus energy.  In a half-Ironman, where you can afford to expend a little more energy to get to the finish line faster, your pacing strategy and power guidelines can be more aggressive.  But in Ironman, being more conservative will pay off as you dole out energy, little-by-little, with the goal of using up every last bit by the end and therefore getting you to the line in the least amount of time.   Your mental approach with the power meter can be aligned with this idea.  In a half-Ironman, use your power meter and keep your effort ABOVE your targets to keep you focused on the effort and your head in the game.  In a full Ironman, use your power meter to keep your effort BELOW your targets and keep the effort in check, making sure to focus on nutritional intake and sticking to your plan, not getting carried away.

With respect to half-Ironman, what is the optimal approach to minimize time?  There is an excellent article by Alan Couzens on the Endurance Corner website here: http://www.endurancecorner.com/Alan_Couzens/pacing_case_study.  The basic idea is to go your hardest when the race is slowest as you get more speed “bang” for your wattage “buck,” and he analyzes some power files from the Oceanside course demonstrating the point.  You get more return on your investment if you put out more effort on hills or in wind.  Your general target should be about 80% of your FTP (or somewhere between 75% and 85%).  Using that as a guideline, you can prescribe target power levels for your half-IM bike ride for specific sections of the course.  For example, if it is a hilly course, expending more energy on longer climbs, pushing your effort to and even slightly above your FTP will pay dividends.  The same thinking can be applied to windy sections on the course — working harder into the wind will pay off in more return on your investment, i.e. a faster bike split.  There is a cost, and you want to be aware of the cost, but in a half-Ironman, there is less risk of running into a nutritional or energy limitation.  If you are shooting for an IF (intensity factor) of 0.80 or 0.85, your targets for a half-Ironman could look like this:

  • Flat, steady parts of the course – 75-85% of FTP
  • Rolling hills / Wind – 90-100% of FTP
  • Longer hills – 100-105% of FTP

However, in Ironman, most people will be fuel-limited.  In this case, your IF should be somewhere in the 0.65 – 0.72 range.  And for most, the ideal pacing strategy is to use your power meter to keep the effort as steady as possible; find your target power, and work to keep the power at or BELOW that target, using them as caps.   In an Ironman, when you come out of the swim, you feel great – you are fit from all the training you did, fresh from the taper, and happy to be finished with the chaotic swim.  The tendency is to roll with the adrenalin and go too hard.  But patience pays off in Ironman and you can use the power meter to keep your effort in check.  Also, on hilly courses, conserving your energy means using all of your gears and spinning up them, conserving energy for when you need it on the back half of the run.  In addition to the energy cost, If you push hard up hills, you put more torque into the drive train (also measurable with the power meter), resulting in more peripheral fatigue in your legs; they will be shot when you really need them in the latter part of the run.  Notice the theme here – pacing on the bike should be geared towards a successful 2nd half of the run.  Minimizing the “spikes” in power (and heart rate) on your bike ride will help your body continue processing fuel and save energy for these later parts of the race when most other people are walking.  An example set of caps for the Ironman bike rice could be:

  • Flat, steady parts of the course / rolling hills / wind – 65-72% of FTP
  • Longer hills – < FTP (as low as you can while still making it up the hill)

Essentially, you want to keep a steady, constant effort as close to your target as you can without going over, treating them as caps more than targets.  It takes a lot of patience and discipline to hold back when many others around you are standing up, mashing gears up the hills, but it will pay off on the run.

After carefully determining your power targets based on your training, use your power meter to guide you accordingly in the race and to make the tradeoff between time and energy.  For half-Iron distance vents, you can afford to “optimize” time more at the expense of a more energy, but for Ironman-distance events, you are better off “optimizing” energy.  Doing this will result in the fastest time anyway.


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