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Training Basics: Training Stress Score, Fitness, Fatigue, and Form

Updated: Aug 28, 2019

The training basics series will be a series of posts that explain some of the basic metrics and concepts JM Coaching uses in planning and understanding training. This post goes through one of the most foundation and important concepts, the Fitness, Fatigue, and Form models used by many coaches to track training. I’ll be going through its benefits, it's a limitation, and the bottom line, what is it’s value to you as an athlete.

Underlying the entire concept is the idea that when you train you gain fitness, and that when you train it makes you tired. These are concepts that are extremely familiar to athletes. If you’ve ever exerted yourself, you know this first hand from experience. We know that when you train, you have to recover from your efforts before you can see the benefits of that training. What the Fitness, Fatigue, and Form models try to do is place objective quantities on these affects. The model attempts to take some of the guessing out of calculations a coach or athlete would make when evaluating their current abilities and position in the season. The model uses 4 primary numbers to track this; Training Stress Score (TSS), Fitness or Chronic Training Load (CTL), Fatigue or Acute Training Load (ATL), and Form or Training Stress Balance (TSB).

First let's look at Training Stress Score. TSS is an attempt to normalize and quantify a training session into a single number. This score takes into account duration and intensity. Ignoring the calculation for a moment, TSS says that if you go for longer, you increase the stress score, and if you go hard, you increase the score. These two factors to varying degrees influence the overall score for a particular session. The concept that it tries to isolate is the idea of stress. The driving force behind the Fitness/Fatigue/Form model described in the paragraph above is stress. Human adaptation is driven by stress. Stress wears us down but the body responds to that wear and tear by building back, reconstructing in a way that makes itself more suited to handle that stress. So if we can understand the stress being applied or the initial cause of the wear and tear on the body, we can begin to understand the bodies response.

Next let's take a look at Fitness and Fatigue or CTL and ATL. These two numbers are good to look at in tandem because they function in much of the same way. Think back to the basis of the model. We go out for a training ride which applies stress, and we know that when we train we gain fitness and we get tired. The gain in fitness and getting tired are both accounted for in CTL and ATL respectively. What separates the two is the amount and effect they have over time. Fitness tends to be slower but more sustained effect and fatigue is more sudden, but fades over time. So we need some way to effect an athletes fitness and fatigue in that way. We also need to account for the persistence of fitness and fatigue. In the same way that fitness and fatigue increase with training, but they also don’t just suddenly drop, so we need to take into account the previous fitness and fatigue. Lucky there is a well established model that suits this, exponential growth and decay. Without getting into deep math, exponential growth and decay functions allow the rate of change to depend on the current value as well as a time or interval constant. They closely follow the experience that athletes and coaches find in training. CTL works a time basis of 42 days, a long term measure, while ATL works on a time basis of 7 days. These simulate the duration of effect that fitness and fatigue have.

Finally let's look at form. Form is what we all chase. We want to have lots of fitness, but we want to have the rest we need to be fast. Using the metrics we already have, we can describe this exact situation. We can create an equation by adding up our fitness and fatigue. We want lots of fitness so we’ll make that positive, and we don’t want fatigue, so we’ll make that negative. The result of fitness minus form is training stress balance (TSB) or what many programs call form. What form of TSB are we looking for when training and what do they mean? TSB is a comparison between fitness and fatigue so when fitness and fatigue are equal you would have TSB of 0, more fitness than fatigue and your TSB would be positive, likewise more fatigue than fitness and your TSB is negative. Everyone's a little bit different, but generally speaking during training you shoot to have a TSB that stays negative and goes no lower than -20. When looking to peak for a race, or increase your chances of having a good day, then a positive TSB is ideal, usually between 0 and +10.

As you can see, the ability to predict your form and performance is extremely powerful. But there are some major limitation and things we need to keep in mind when using this model. The first and most important to remember when planning is that these numbers represent a probability more than a certainty. When the conditions are correct, the athlete would be more likely to perform well, but that performance is not guaranteed. And in the same way, if the conditions are such, an athlete is likely to perform poorly, but not always. Another big limitation is the underlying stress model. Training Stress Score (TSS) is more or less valid depending on how accurate your threshold value is set. Even with a perfectly set threshold value, there are accuracy issues with the TSS model itself because it’s normalized to a group of people based on lab testing. The takeaway message is that the model needs some calibration and experience. It once was held up as a revolution in the way we look at training, but in reality it’s just a more accurate way to understand the training but it still needs to be understood at a personal level. Each ride needs to be understood using more than just a fitness and freshness model. With a better understanding and some experience, the model can be tweaked or the numbers can be put into more context to help you get more from the information. But even when that's done, we still need to remember that it’s only a likelihood of performance, not an assurance. The Fitness and Freshness model do a better job of describing your average potential performance more than anything else. And when using it to guide and understand your training, keep that in mind.

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