Exactly how difficult is a workout? Well, you may have noticed two acronyms with corresponding numbers nestled right underneath the workout graph in the SYSTM app’s video list: TSS and IF. These stand for Training Stress Score and Intensity Factor, and are meant to help you determine how demanding a given workout is. While they serve that purpose well in a FTP-driven environment, with 4DP these values lose some of their clarity. Let’s take a look at why.
TSS and IF were developed by Dr. Andy Coggan and Hunter Allen—authors of Training and Racing with a Power Meter— as a way to better understand and estimate the demands of training. Up until Coggan and Allen created this framework, estimating training load was fairly primitive, using things like overall training time, distance, and, to some extent, time spent in different heart rate zones. With the invention of the power meter, all this changed and TSS and IF were created to more accurately determine how difficult a given session was. Together, these two numbers shed much better light on the true intensity of a workout, and how sessions compare to one another, even if they are completely different.
Intensity Factor (IF)
What it is: IF allows you (or your coach) to assess how intense a workout is by comparing the demands of the session to your ability to produce steady power. In this way, we can easily see the difference between an easy recovery ride and a set of full-gas intervals.
How you calculate it: IF is calculated by dividing the Normalized Power (NP)* for a workout by your FTP**. While this value is typically calculated after a workout is completed, many apps can calculate the “planned” IF of a given workout based on the power targets. When power targets are set exclusively off of FTP, the IF for a workout will be the same for everyone, regardless of differences in FTP.
For example: Two riders doing the same workout could see NP values of 176W and 264W, but if their FTP is different—say 200W and 300W respectively—they will both see an IF of 0.88.
What it looks like in practice: Simply looking at the power graph of a workout won’t necessarily give you a good idea of how hard a session might be. However, comparing the “Planned IF” for two or more workouts will give you a better idea about which one is more difficult. You can also compare the Planned IF to the actual IF calculated at the end of your ride. This can be a quick and useful way to see if you were generally exceeding the power targets during a workout, or struggling to hit them.
Training Stress Score (TSS)
What it is: TSS allows you to more accurately compare the training load of a session by considering both the intensity and the duration.
How you calculate it: This one is a bit more complicated, so get out your calculators. To get TSS, you square the IF, multiply it by the duration of the session in hours, and multiply the result by 100 (IF² x Time of Session in Hours x 100 = TSS)
What it looks like in practice: TSS gives a more complete picture of how difficult a given ride is because it takes both duration and intensity into account. TSS will show you that the training load from a 2-hour recovery ride is lower than a 45-minute all-out interval session. You can then look at your TSS over time using a “Performance Management Chart” (another key piece of Dr. Coggans work that) to track your overall fitness levels.
The Shortcomings of IF and TSS
The common denominator of every IF and TSS calculation is an athlete’s FTP. When looking at longer sessions where you spend a majority of the time below FTP, IF and TSS are useful, rock-solid metrics to track.
Once you go over FTP for extended periods, however, these metrics become less useful as they give the time spent above FTP equal “weight” for all riders, regardless of their ability to produce power above FTP. When the main goal of a workout is high-intensity intervals well above FTP, IF and TSS lose a bit of their usefulness.
Let’s look at an example of two riders: Rider A can hold 200% of their FTP for 1 minute, and Rider B can hold 150% of their FTP for 1 minute. If those riders were to do a session involving 1-minute repeats at 140% of FTP, they would both be given the same IF and TSS values, regardless of their respective FTPs. However, Rider A, with a greater Anaerobic Capacity, would have found that session much easier than Rider B. Even though their TSS is the same, how each rider feels afterwards would be quite different.
IF and TSS In a 4DP World
SYSTM workouts aren't based on FTP alone, but on your complete 4DP™ power profile including Neuromuscular (NM), Anaerobic Capacity (AC) and Maximal Aerobic (MAP) efforts. The SYSTM app looks at your unique ability in those areas to set targets for shorter, high-intensity efforts, optimizing the intensities of each effort to ensure you get the intended training stimulus and benefit that your body needs.
4DP™ allows you to take advantage of a principle that elite coaches and sports scientists have been using for years: effective workouts must look beyond FTP to consider an athlete’s complete power profile. Since IF and TSS are dependent only on FTP, your completed IF and TSS for a given SYSTM video will likely be different than the Planned IF or TSS. This is especially true for riders who are better over short durations. You will see higher TSS and IF after completing a workout than you previously did. In fact, if your rider type is Sprinter, Attacker, or Pursuiter, you may even see IF values over 1.0 for workouts that are an hour or longer. It’s also highly likely that even though your completed IF and TSS will be different from another rider’s, the actual stress of that training session will be nearly identical for you both (something that is not the case for workouts based only on FTP).
The SYSTM app still displays TSS and IF for all workouts. Even though it's not an accurate representation of training stress, they can still give you a very rough idea at a glance of how intense a workout is. Just don’t worry if your numbers don’t match up after the session is over. Rest assured, you’ll get the workout you need.
Remember, the ultimate goal of most high-intensity, low-volume workouts is to push yourself to your limits to maximize your fitness gains. While the numbers can help guide and direct you, it’s not the numbers that pedal the bike. It’s you.
* Normalized Power (NP)
Normalized Power (NP) is essentially a different way of calculating average power that gives you “credit” for time spent at higher intensities. In simplest terms, the NP for a training session better reflects the “physiological cost” of that session. back
FTP is typically defined as the highest average power you can sustain in a semi steady state for an extended period of time (usually an hour). back