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How to Use a Power Meter for Optimal Ironman Pacing

Wouldn’t it be nice if your coach could ride your Ironman with you and guide your pace all day? Well, in effect, they can. Perfect Ironman pacing is available with the latest power meters and software. Take your coach with you on race day through power meter pacing!

Once you have us install a new power meter, what do you do with it?  And what do you need to know to use it to your advantage on race day? Below are some of the keys to understanding how to use your power meter and Training Stress Score to optimize your performance:

What is Training Stress Score (TSS)?

Before we start, it is important to note that“TSS”, “NP” and “IF” are trademarks of Peaksware, LLC.  However, the terms are as widely used as power meters themselves.

So, what exactly is Training Stress Score? TSS uses power data to determine how much stress is put on the body during a ride. There are no shortcuts in Ironman training, but there is a proven race day pacing metric that will lead you to your best Ironman bike and run (assuming the work is put in). That pacing metric is a target of 280 TSS for the 112-mile bike leg on race day.

Understanding how to use a 280 TSS on race day could prove as important as the hundreds of hours of training you put in. How do you use TTS to your advantage? All you need is a power meter and and an appropriate understanding of the common power metrics that go into it.

How is Training Stress Score (TSS) Calculated?

TSS is calculated using Normalized Power (NP), Intensity Factor (IF), and ride duration/time. The formula is TSS = (IF²)x(100)x(ride time). While knowing the formula isn’t required on race day, using the correct inputs is.

Now to explore the variables that make up TSS in more detail.

Intensity Factor (IF)

Divide your Normalized Power (NP) of a ride by your FTP and voila, you have your IF (Intensity Factor) for that ride. For example, a rider with an FTP of 200 watts and who rides at a normalized power of 150 watts has an IF of 0.75 for that ride. The ride was done at 75% of threshold power.

Why Use Normalized Power (NP) Instead of Average Power?

Normalized Power (NP) can be confusing and I don’t want to get off in the weeds with it.

On a basic level, you can think of NP as an Average Power number that takes the metabolic costs of changing riding conditions into account more. It is the physiological equivalent power had you ridden an entire ride at a constant power. Even though you ride at varying power levels through a ride, Normalized Power “flattens” that variability out and accounts for more factors than Average Power on its own.

We could discuss Normalized Power for an entire article (or book), but not right now. The bottom line is, “Make sure you have a Normalized Power field on your computer for both training and race day and that you’re very familiar with it.”

Functional Threshold Power (FTP)

FTP is your maximum sustainable 60-minute power. You’ll be riding your Ironman bike at just a 60-70% fraction of this power, but it’s very important to nail down what this number is for you.

Estimating Ironman Bike Time

Once you have established your TSS, the challenge for many coaches and athletes is using it as an IM pace guide. You need to approximate how long the rider will be on the bike course. Approximating this within 30 minutes will do fine for starters, but the tighter the estimate the better. Course modeling software like BestBikeSplit.com can nail this down very closely as long as the inputs are correct. Experienced coaches and triathletes who know the course can help too.

The beauty of a TSS target is that it can allow for pace to be adjusted on the fly come race day.  This keeps unknowns, like weather, from throwing you off your plan and capabilities.

On race day, target finishing the bike at 280 TSS. Make sure you have an accurate FTP value stored in your bike computer and, in addition to your preferred fields, I recommend having fields for power, NP, and TSS on your display.

Establishing Target Power from TSS for Power Meter Pacing

We began with the end in mind with 280 TSS.  We still need a race day power target pace though. To establish the race day power meter pace to abide by, we use the same formula we used for TSS.

Once the ride time is estimated, we solve for “IF” (Intensity Factor) and then power. As an example, we’ll use our rider with a 200-watt FTP who projects to be on the bike course for six hours.

280 = (IF²) * 100 * (6 hours).

IF² = (280/6)/100.


IF = 0.68.

68% of a FTP of 200 watts is 136 watts.

This rider will target 136 watts for a flat IM bike ride. This is the number if you’re racing IM Maryland.  However, for many, a variable power plan for hilly courses is required.

Establishing a Power Meter Pace Target for Hilly Ironman Courses

BestBikeSplit.com, or other modeling software, can give you exact values on every segment of the ride.  However, for the “cheatsheet” version, you can cap power at 10% over target on hills and 10% under on descents then coast when you spin out. Most find power naturally drifts up on climbs and down on descents so just some simple monitoring outside of your power target is all that’s needed.

Using our rider above as example, they would target 136 watts on flat sections of the ride. For climbs this rider will allow power to raise up to 150 watts (~10% higher). False flats or minor uphills target 140-145 watts. On downhills this rider will allow power to drop down to 120 (~10% lower) watts and eventually coast once your cadence spins out.  A guide for this rider on a hilly course thus looks like the following:

Flats: 136 watts

Minor climbs: 140-145 watts

Major climbs: 150 watts

Descents: 120 watts

This rider will modulate efforts between these targets throughout the day while keeping cadence in range. There may be hills where it’s not possible to keep power low enough or cadence high enough, and that’s fine. This is an area where it’s very important to explore all gearing options ahead of race day and once best gearing is chosen do your best on the steep climbs to keep power under control then continue on plan.  If you would like help with gearing on your bike, contact us.

Putting Power Meter Pacing All Together

We only estimate bike time for race day because a wide range of race day conditions can change it. I recommend sticking to your plan for the first 28 miles, and check your TSS value. For most riders, if your TSS is around 70, then stick to the plan. If it’s over 70 you are going a little bit too hard and/or the bike will be take longer than projected. If it is over 70 you need to back off; target 5 watts less for the rest of the ride. If TSS is below 70, you are going faster than anticipated and you can calmly lift the target power that same percentage. At the 56-miles check again. TSS should be 140.  If not, adjust your power targets as needed.

Start ramping up training today with a power meter. Getting a power meter and understanding how to use it is your guide to optimizing your Ironman performance on both the bike and the run next season!

Where Do I Get a Power Meter?

While there are many places to buy a power meter, Fit Werx has been selling power meters since 2001. We are one of the most experienced shops in the country when it comes to installing power and helping riders and triathletes understand how to start using it effectively.  Contact us to discuss how a power meter on your bike can help you reach your potential and what brand/model makes the most sense for you and your bike.

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