In order to utilize the most effective methods of training and racing with power it is important to understand the critical principles of Ironman racing. Extensive endurance and muscular endurance (for more advanced athletes) are the most important components when preparing for the bike leg of an Ironman triathlon.  The key to success is specificity. Becoming extremely efficient at the intensity you will race at, roughly 60%-80% of your Functional Threshold Power (FTP – The average power an athlete can sustain for one hour), must be the focus of your training.


Nearly all record breaking times are set with negative splits. This means that the second half of the race is faster than the first, which obviously has to do with pacing. A power meter is by far the most accurate way to gauge your intensity (output) on the bike.  Your speed on the bike is highly affected by wind, heat, hills etc. but when you have a power meter, you are able to roll along at the prescribed intensity and no longer need to guess at how hard you should be going.  An Ironman athlete completing the bike leg in less than 5 hours can usually sustain a power output of 70%-80% of their Functional Threshold Power (FTP). Athletes finishing in the 5 to 8 hour range should stay between 60%-70% of their FTP and the 8+ hour rider will be somewhere between 50%-60% of their FTP. Being able to accurately ride at the correct intensity enables your training to become much more efficient and allows you to race much more effectively. The first half of the bike in an Ironman race may be the most critical part of the entire race and a power meter will help you keep your pacing under control.


The most critical component of endurance racing (specifically Ironman racing) is aerobic endurance.  An athlete’s aerobic system composed of the heart, lungs, blood and the muscles’ aerobic enzymes must be fully developed. But how do you know if your aerobic endurance is progressing and how do you know when you’ve reached an optimal aerobic endurance fitness level? The parallel relationship between input (heart rate) and output (power) is referred to as coupling.  When they are no longer parallel in a workout (e.g. one variable remains steady while the other drifts) the relationship is said to have decoupled. As aerobic endurance improves, there is a reduced rate of heart rate drift relative to constant power output.  In determining your decoupling you can use software such as Training Peaks, WKO+ or perform the following calculation:

Divide the workout into even halves. For each half the normalized power is divided by the average  heart rate to establish two ratios. The ratios are then compared by subtracting the first half ratio  from the second half ratio and dividing the remainder by the first half ratio. This produces a power- to- heart rate-ratio percentage of change from the first half to the second half of the aerobic  threshold ride.

For Ironman racing the goal should be too keep decoupling on the bike to less than 5% for a minimum of 4 hours. Once an athlete’s aerobic system is fully developed it will make training at higher intensities more effective.

Variability Index (VI)
The variability Index is a measure of how smooth or consistent an athlete’s power output was during a workout or race. The VI is found by simply taking the normalized power and dividing it by the average power. The normalized power (easily calculated by your cycling computer or software) can be thought of as your average power without the “zeros” or “spikes”.  A high VI is an indication that there was a lot of surging taking place. This can be instances such as heading into a head wind, going up and down small hills, being passed or passing someone else etc. Surges can zap your energy very quickly and wreak havoc on your body as it is trying to process fluid and fuel. Steady state pacing, especially in Ironman racing, is a far more efficient use of energy. A pacing strategy of a smooth and consistent power delivery will reduce fatigue and pay dividends on the run.

Written by Kyle Visin
Santa Barbara Triathlon Club Co-President
USAT Level 1 Coach
Certified CycleOps Power Coach
Co-Founder of KillerTri 

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