Training with a power meter is the most precise and detailed way to get results. But the power meter is only part of the equation, you need to know what the numbers mean and how to interpret them. There are a few certain key terms that you need to understand in order to take full advantage of this amazing tool.
1. Functional Threshold Power (FTP)
Popularized by Hunter Allen and Andrew Coggan in their book, “Training and Racing with a Power Meter, the term Functional Threshold Power, or FTP, refers to the maximum power you can maintain through an hour’s effort without fatiguing. It can be determined during a lab test, or by performing a field test.
There are many ways to determine your FTP in the field. One of the most common is a a test created by Hunter Allen and is a simple way you can test your FTP. Perform this field test on a flat course relatively free of stop signs or traffic, and if possible one that has a steady, uphill grade of less than 5 percent. The uphill grade helps you to maintain a constant output, as opposed to rolling terrain where it’s difficult to keep your power up on the downhills.
Do at least a 15 minute endurance paced warm up, then do 3 x 1 minute fast pedaling drills of at least 100 rpm with 1 minute recoveries in between each. This is to warm up the muscles without taking away energy for the 20 minute test itself. Then perform a 20 minute time trial all by yourself— no training partners, and not in a race. Your effort should be done as if it was a race for the entire 20 minutes- all out, but sustainable for the 20 minutes.
Your average power for that 20 minutes, less 5 percent, is an approximation of your FTP. For example, if your average power for the 20 minute test was 250 watts, then after subtracting 5 percent, your FTP would be 238 watts.
2. Normalized Power® (NP®)
No ride is perfectly steady. Your power output varies due to factors like terrain, wind, following attacks, surges, etc. This is where Normalized Power becomes a critical metric to follow. The algorithm for NP is somewhat complicated, but importantly it incorporates two key pieces of information. First is the fact that physiological responses to rapid changes in exercise intensity are not instantaneous. Instead, they follow a predictable time course. Second, many critical physiological responses, like glycogen utilization, lactate production, stress hormone levels, are curvilinearly, rather than linearly, related to exercise intensity.
An example is if you pedal at 200 watts for 1 hour, you have an Average Power (the numerical average power) and a Normalized Power of 200. If you pedal at 100 watts for 20’, 200 watts for 20 and 300 watts for 20 you will still have an AP of 200 watts but your NP will be approximately 239 watts.
Your NP is an estimate of the power you could’ve maintained for the same physiological “cost” in terms of glycogen utilization, lactate production, stress hormone levels, and neuromuscular fatigue would have been for a given effort or ride if your power output had your ride been perfectly constant rather than variable. As such, NP is always higher than average power.
NP is a better indicator than average power of how metabolically challenging the workout was. It emphasizes power surges which require a lot of glycogen (carbohydrates) and therefore contribute to an increase in fatigue. It is important that you view NP on your cycling computer rather than AP.
3. Variability Index (VI)
Variability Index is a way to measure how smooth or “variable” your power output was during the ride. It is calculated by dividing your Normalized Power by your Average Power. A steady and even output, like during a triathlon, should have a VI of 1.05 or less. During a criterium race your VI may be as high as 1.2 or more.
4. Intensity Factor® (IF®)
Intensity Factor is simply the ratio of the Normalized Power to your FTP. For example, if your Normalized Power for a long training ride done early in the year is 210 watts and your FTP at the time is 280 watts, then the IF for that workout would be 0.75. However, if you did that same exact ride later in the year after your threshold power had risen to 300 watts, then your IF would be 0.70. Therefore, IF provides a valid and convenient way of comparing the relative intensity of a training session or race either within or between riders, taking into account changes or differences in FTP.
5. Training Stress Score® (TSS®)
Based off of the duration and intensity of your session, Training Stress Score is a single, numerical value for the effective load of your training session. You earn 100 TSS for an all out, 100 percent, 60-minute workout. Since your FTP value is the wattage you can hold for 1 hour, holding that value for 1 hour equates to 100 TSS points.
Of course, most workouts are not completed at 100 percent, so most workouts will accumulate less than 100 TSS per hour. For example, if your FTP is 250 watts and you hold 200 watts for 1 hour you would have a TSS score of 64. You can earn more than 100 TSS within a single workout, as long as it is longer than an hour, but never more than 100 TSS per hour. If you find that you are accumulating more than 100 TSS points per hour you have either bad data readings or your FTP needs to be raised. TSS values are critical to using the Performance Management Chart to track long term data trends.
Combining detailed data with knowledge of a few key terms allows you to analyze each ride with greater precision and understanding. When you know these terms and how to apply them, you’ll be able to quantify the true load of your training- for each ride and over time. Don’t just look at your numbers and wonder, use them to become a faster rider.