The Knowledge by Wahoo

Calculating Training Load: Why You Shouldn't Stress About TSS

Episode Summary

Determining training load—whether for a given workout or a training block— isn't as straightforward as it seems. There is a veritable alphabet soup of measurements out there, including NP, TSS, IF, CTL, ATL, and TSB (just to name a few). Coaches Neal Henderson and Mac Cassin take a look at the different ways of measuring training load to help you better navigate the sea of acronyms and get a deeper sense of just how hard (or easy) those upcoming workouts might be.

Episode Notes

How hard is hard? What's the difference between Normalized Power and Average Power? Is it more important to focus on Training Stress Score or Intensity Factor when choosing a workout? 

Determining training load—whether for a given workout or over the course of a season— isn't as straightforward as it seems. There is a veritable alphabet soup of measurements and calculations out there designed to help quantify just how much training load you've accumulated during the course of a workout or over time. These include NP, TSS, IF, CTL, ATL, and TSB, just to name a few. In this episode, Wahoo Sports Science coaches Neal Henderson and Mac Cassin untangle the often-confusing world of measuring training load to help you better navigate the sea of acronyms and get a deeper sense for just how hard (or easy) those upcoming workouts might be.

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Episode Transcription

Neal Henderson  0:00  

Hello, and welcome back to another episode of the knowledge podcast by Wahoo. I'm Neal Henderson, head of Wahoo sports science.


Mac Cassin  0:07  

And I'm Mac Cassin,  senior sports scientist here at Wahoo. Today we're talking about something we both find super exciting math.


Neal Henderson  0:13  

Yes. Specifically, math related to exercise and exercise physiology and training and performance and things that actually really get us super excited.


Mac Cassin  0:26  

It can also frustrate us quite frequently.


Neal Henderson  0:28  

Yeah, not all math is good math, I guess, in some ways.


Mac Cassin  0:33  

Now, it's almost like the human body is just a little too complicated for the math that you least you and I can do. I'm sure there are people out there who can


Neal Henderson  0:39  

Maybe do better?


Mac Cassin  0:40  



Neal Henderson  0:41  

All right. Well, not specifically, what are we going to talk about


Mac Cassin  0:44  

What we're talking about our numbers, math around exercise, specifically related to power and heart rate, and how you can use those values over a training session to monitor and estimate the total training load, the difficulty, the intensity of that session, to help you track your training, see your improvements here and changes. And there are a lot of numbers out there. Some of the more popular ones are things like normalized power, density factor, training, and stress score, and they're really valuable because they simplify things and give you a simple number that tells you how things are going. But like we said, it's not quite that simple. And there are a lot of instances where these numbers are super valuable and good to help guide your training. And there are other times when you're better off ignoring them and not living and dying by these simplified equations.


Neal Henderson  1:26  

Yep. And the reason that some of these provide a little additional context is some of the kind of simple ways of describing your training, like how many miles you went, how long you went, how many meters or what are those other ones of those American units


Mac Cassin  1:42  



Neal Henderson  1:42  

Yeah, how many feet you climbed, I don't know, three meters. That's really what matters. Again, Si, in the house here, system International. If you're not aware, all of those values really are quite relative to the situation, the conditions, the wind, the temperature, whether you're going uphill, whether you're an altitude at sea level, and especially if you're training indoors, well, you didn't travel anywhere. I mean, you might get an estimated distance, a calculated estimate, but really, you went zero meters if you're on a stationary bike,


Mac Cassin  2:14  

But there is a world our record for rollers, though.


Neal Henderson  2:18  

Yeah. Okay, we're not going to get into that that's a whole nother episode of silly things that can be done, but probably shouldn't.


Mac Cassin  2:26  

Fair enough. So these numbers we're talking about, they're representing your training, we already mentioned normalized power. Other ones are things like x power, which is by Dr. Skiba, and used in programs like a golden cheetah, and then Strava is a big one. And they used weighted average power. And so this first normalized power x power weighted average power, what are those trying to do?


Neal Henderson  2:46  

It is trying to give basically, a weighted power average of the physiologic cost of the ride. So the arithmetic mean, or average power doesn't always describe what happens if you go out, we live in Boulder, we got nice mountains around us, if I went uphill for half an hour, and I averaged 300 Watts, which probably can't do that today. And then I Coast downhill at say, even zero watts for 10 minutes, the average power wouldn't really describe anything that it did, the average might be 200, and some watts in that situation. But I spent basically 30 minutes at 300 Watts and 10 minutes at zero. And so the way of giving a normalized or weighted power is going to take into account the harder effort and take a little bit less away from the say coasting or low-intensity parts of a ride.


Mac Cassin  3:44  

And all of these models use some medium duration, medium duration, I mean 25 to 42nd rolling average of power. And that duration is selected because it represents a lot of basically your body heart rates a good example that I think most people can understand your heart rate doesn't immediately jump up, when you start a heart effort, it takes a while to get there. And that's the same with so many other physiological phenomena. There's a time delay where you can't look at instantaneous points. When you have something like power, you need to smooth that out over time. So that's where you get these 32nd Rolling averages.


Neal Henderson  4:20  

Also, it's taking into account the time spent above your sustainable power or FTP. Typically, there are a few other values sometimes people use as a critical power value, but we'll just use FTP to keep it a little bit simpler right now.


Mac Cassin  4:35  

But these values themselves, these equations are completely separate. When we get into intensity factor and training stress. That's where your absolute ability for sustained power comes into play. What's interesting about all these models is it's just because they're exponential. So again, this is a math episode. So we're gonna throw some math your way normalized power, you take a 32nd rolling average of that 32nd average you put that to the fourth power and then you take The average of that over the ride and you take the fourth root of it. And that's how you get your normalized power for the ride. So the average power might be 200, your normalized power might be 240. Now, because it's to the fourth power, so like two times two times two times two, that's a big number, but then you go 10 times 10, times 10 times 10, that's a much bigger number. So as your power increases, that weighted upper value that they're using becomes significantly higher. And that's how they're trying to model that small increases in the absolute output can start to create much larger changes in the physiological cost of doing it.


Neal Henderson  5:35  

So effectively big picture trying to represent what would be the potential power output that would have been held? Had it been a steady effort?


Mac Cassin  5:45  

Yes, what did your body go through to do that hour of riding and a crit? It's a lot of on-off


Neal Henderson  5:50  

very on. And if you're doing it right, very off, coasting is actually a good predictor of performance in a criterium, those who coast the most tend to do better, because they're recovering,


Mac Cassin  6:00  

And they're not braking as much they're going through corners. Well, yeah, when you watch someone who really knows how to race a crit, it's quite a sight to behold,


Neal Henderson  6:06  

Yep. And that normalized power can be well in excess of 2030, even higher percent greater than the average power.


Mac Cassin  6:15  

And that's something we'll get into where like, there's times to ignore it. But I know for myself, I've had crit, specifically with like, medium 30/42 hills in them that my normalized power is extra, you know, 40 Watts higher than anything I could do for an hour because it's on-off. And that's more my ability is, though shorter efforts. Again, we'll get into that a bit later. But so we have this value, this normalized power, the sex power, whatever that's looking at, okay, what's the relative physiological cost? If you kind of normalize things, again, because riding bikes a lot of time off, okay, you have this value. So then the next step, like you're saying is you take your FTP, some people use critical power, keeping it simple. FTP. So what do we do with this step, we have a normalized power, say I did normalize the power of 200. And my FTP is 300, divide, divide and conquer. I thought it stood together and conquer but


Neal Henderson  7:07  

Something like that, we're going to divide the normalized power divided by FTP to get the intensity factor. Okay, and simple as that. That's the easiest math we're giving you today, by the way.


Mac Cassin  7:17  

So we've got so yeah, so then we've got that number. And that becomes a good way to compare things. Because say you do, again, you do a ride one point of the year, and you do the same ride and your normalized power is the same. But if your FTP has gone up, then that relative that intensity factor, even though you did the exact same ride, exact same power, both times that intensity factor is now lower, because you're fitter, so it wasn't as challenging correct.


Neal Henderson  7:39  

And then another way to use that value to then look at describing the entire training session is to use the intensity factor, as well as the duration, specifically to come up with a value called training stress score, or TSS.


Mac Cassin  7:57  

Or if you're in Golden Cheetah, that'd be bike score, fear, and Strava. training load, there's


Neal Henderson  8:01  

different names for that. But effectively, it's the intensity factor squared times the duration in hours times 100, that will give your TSS value for the day, as a basis of this. If you ride at your FTP for one hour, your TSS score would be 100. Because your FTP divided by itself is one, one time one hour, still, one times 100 equals 100.


Mac Cassin  8:31  

And if it's, if it's that, if that one hour FTP is completely steady, yeah, yeah, that's another important component. But yeah, which is also why you can get a TSS of 100 Riding on off, your actual average power is below FTP, but that normalized is bumped up. And so that can be then TSS of 100.


Neal Henderson  8:50  

Exactly, and you can exceed 100 lots of different ways. If you ride longer at a lower intensity, you're also going to be able to achieve a much higher value than 100 TSS.


Mac Cassin  8:58  

And that brings up two points that kind of I know you and I, as coaches have run into time and time again, one is chasing a higher TSS value by just going volume because it is very volume dependent. Yep. And then to, there's been a long-standing notion that you can't have can't do over 100 TSS for an hour, you can't have an intensity factor of one for longer than an hour. And I think maybe early on when the few people who had power meters were probably more aerobic diesel engines than the data there probably suggested that but nowadays we know there are people with much better anaerobic capacity, maximum aerobic power that far exceeds their sustainable power. I've seen a legit and I didn't believe this at first. This is someone we've we've worked with for a long time and has been a great asset and kind of a thorn in the side from time to time. But for the longest time, I did not believe his test values and then training values because they were just way not they were extreme. They were beyond the chart of what we thought was possible. And then yeah, believable, believable, but then spend more time looking at more of his data knowing his background, his training background, that's just his physiology is not very aerobically strong, very anaerobically, neuromuscular, strongest extreme. So he's had like, a 1.15 intensity factor for an hour, for him is a hard hard work


Neal Henderson  10:14  

but not at the limit, right, which plenty of people can never achieve no matter what they did, if you don't have a lot of anaerobic capacity, then you're not going to be able to hit those extreme types of values in that. And so one of the things to keep in mind with any of these numbers is they're not always something that you know, that you can compare from one person to another. That is there is so much individual variability based on your rider type or rider strengths, and what you're doing in your training efforts that really might make these values even less useful in certain cases. I can tell you as a great example, back in 2008, I was using these kinds of values, looking at Taylor finished training, and he was actually still 17 years old, he didn't turn 18 until just a couple of weeks before the Olympics. So predominantly, all of his training was done as a 17-year-old high school senior who had a two-year he was in second year of training history effectively. And so he was not training 20 hours a week. And that was not appropriate for him. He had a very low training volume of 810 12, a few 1416 hour weeks here and there typically as stage races, he had an immense Max aerobic power and anaerobic capacity relative to his FTP because FTP typically takes more time and volume to kind of express that greatest potential. And it's something that we knew wasn't a focus for him as he was getting ready for an event called the individual pursuit, which was 4000 meters, and just over a four-minute long effort.


Mac Cassin  11:45  

And as a side note, that's not just Taylor was definitely training for the demands of the event. But all juniors should focus on the shorter efforts, you have plenty of years to do, yes, enough volume to


Neal Henderson  11:57  

go longer. And I'd say both of them actually parlayed their capacity in these short events like actually at junior worlds, the three kg individual pursuit in 2008, just prior to the Olympics where Taylor did take first place, he won his junior second world, second Junior World Championship title there, he took the win from another guy ended up working with later, Rowan Dennis, who was the silver medal there who ultimately went on to win two elite Time Trial World Championships so far and have some other pretty good success at longer duration, single-day events, time trials, as well as stage races, as did Taylor. So those shorter high intensity sessions didn't yield massive TSS scores on a lot of his training. So day in and day out, he might have only been doing, I'll say, quote, unquote, only been doing some heavy training sessions that were 60 to 80, maybe occasionally 100 tss, but they were very short, they were maybe an hour-long, they were really specific track type intervals for an individual pursuit or kilo. And when you looked at his training that he did in those, you know, 810 12 hour weeks with that kind of intensity was not very impressive at all, there's a value that some people look at in some of the charts called CTL, or chronic training load. And that's basically a six-week average of your daily TSS, again, rolling weighted, there's some math in there. Let's see in Strava, what do they call that they call it to train, just training, load, training load. And yeah, and so, you know, there are different ways of describing this. But comparatively, Taylor had a pretty low value, literally in the 60s, maybe got into the low 70s. And I had masters athletes who were, you know, in their 50s and 60s, who had much higher training loads, though Taylor's performance was extremely at a different level in terms of his absolute power output, the wattage he was able to hold for that. And actually, if we would have looked, I didn't have access to say somebody like Bradley Wiggins's CTL values, but I would guess he would have been close to double the training load that Taylor had, and their performance was different. Bradley Wiggins did win the Olympic gold medal there, Taylor was seventh place, and he was, you know, a little bit behind. But for double the training load, it wasn't double the performance, it was a small margin. And so something like CTL is not necessarily again, the right thing to compare one person's training to another or to say you have to hit some certain level of CTL to be ready to do the Leadville 100, or to do an Ironman or to do anything really.


Mac Cassin  14:34  

Yeah, I think that stress is a good point that we're hoping we're making throughout this whole podcast. But when you're looking at the accumulation of all these values over time, it's not a surefire predictor of how you're going to perform and that's part of that's down to the fact that you can get like we've already said you can get 100 TSS and a couple of ways you can absolutely smash yourself for an hour. Or you can do a recovery ride for like three and a half hours that gets you the same TSS for that day. which means as far as your CTL is concerned, that's the same thing, it decays, at the same rate takes the same amount out of you. And mathematically, in that model, it takes the same out of you. And that's one area where I've had 30-minute sessions that absolutely destroy me. And it will be Yeah, it'll be 6065 tss, and then, you know, I take three days to recover from that four days, right. And that whole time my CTL is dropping in on that. So it's important to yes, there's, an emphasis on using that CTL to track how your training is progressing. But understanding that just because two sessions have the same TSS does not mean you're getting the same training stimulus. And it doesn't mean they're equally appropriate for your training.


Neal Henderson  15:40  

Exactly. And so just like that, CTL, the composition of the workouts that have built up whatever that value is, for you matter a lot. And so do be careful with these numbers, look at them, for reference have an understanding of what they mean, but do not pay a requirement of doing some value in training before you think you're ready to compete or to do certain other tasks.


Mac Cassin  16:09  

There are two other components in that when you're looking at CTL. There's a tail, which is acute training load, which is just a one week,


Neal Henderson  16:14  

the seven-day rolling weighted average, kind of like that CTL.


Mac Cassin  16:18  

And then the difference between those is called your training, stress balance. And that's something where there's a lot of, I'd say, literature, there's a lot of blog posts out there stating that you need to have, you know, positive 10 TSB for these for to perform Oh, and all that stuff. And that's another area where you need to separate that value from what you're doing on the day, you can't let that value influence you. And what I mean by that is, every time I work with a new athlete, I asked them to give me a list of three to five days where they felt the best they did on the bike, I don't give any context, I say give me five days that you felt great. One of the things I do is I obviously look back at the training they did leading up to that to get an idea of what they responded to. But the second thing is, I always point out and record and show them what their tsp was. And I've yet to have worked with an athlete where every time it was between positive 10 and positive 20. TSB that's every best day they had was that some it's negative 40, some it's positive 30, some it's zero, it's so varied just for one individual, their best five days have completely different tsp values. So it's a good tool to help you in the long-term tracking of stuff. But when it comes to a single day, doesn't matter


Neal Henderson  17:24  

exactly I've seen world-class performances occur from minus 40 plus TSS levels into the plus 10 plus 20 plus 30 range. And everything in between there is no absolute rhyme or reason if somebody tells you that you have to have some certain value to be able to again, accomplish something, they are telling you a story.


Mac Cassin  17:46  

And that's where our frustration comes in. Because we love numbers so much. And we want so badly for there to just be an equation you can use that tells you everything and it's the Golden Compass. But there


Neal Henderson  17:57  

is no such thing right now. So, have references have an understanding of these values, what they mean, see how you are performing with respect to those over time, see what potential trends you might have with your training. But don't limit yourself based on any number that you do see.


Mac Cassin  18:15  

Yeah, and don't and certainly don't feel the need to compare those numbers to your friends, your training partners, your spouse, whatever it may be


Neal Henderson  18:23  

or to when you see some pro riders, you know, weekly training load or volume or CTL that kind of thing it'll blow your mind and you shouldn't be compared to that. So there's also an article that Mac wrote a while ago that you can search on the internet if you search understanding the limitations of TSS and if during high-intensity training by Mac Cassin  you'll find that you can read you know a little bit more about this and some of the background otherwise, we do hope that this little discussion has helped you understand a little bit more about some of those acronyms of normalized power and P intensity factor and TSS and how understanding and using them to help you be a better endurance athlete and how you should also not use them. Thank you for listening to the knowledge podcast by walk


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