Training Stress Score
It all starts with your Training Stress Score (TSS). Every workout you do is assigned a training stress score based on duration and how intense the workout was relative to your threshold. Here we see a red dot representing each day’s workouts for an entire season. The red dots at the bottom of the screen indicates that there was no workout that day.
Fatigue (Acute Training Load, or ATL)
By taking an exponentially weighted average of that stress from the past 7 days we are able to calculate your Fatigue, or an estimate of your fatigue accounting for the workouts you have done in the past two weeks. Here you can see on days with a workout with a high training stress score the Fatigue climbs sharply in response. You do a hard workout today and you will feel it in the coming days.
Fitness (Chronic Training Load, or CTL)
We also use each day’s TSS to calculate Fitness. Fitness is an exponentially weighted average of the last 42 days of training and reflects the training you have done over the last 3 months. However, the workouts you did 15 days ago will impact your fitness more than the workouts you did 6 weeks ago. You may notice that as Fitness goes up so does Fatigue, only at a greater rate. The PMC helps to tell a story, consistent training is marked by a steady rise in Fatigue and Fitness where as a sharp drop might indicate time off due to sickness or injury.
Form (Training Stress Balance, or TSB)
Finally, by subtracting yesterday’s Fatigue from yesterday’s Fitness we come up with the yellow line, or the Form. Just because you are fit does not mean that you are ready to race at your best. A negative Form would indicate that you are carrying a lot of fatigue and are not on form. However, by tapering you can shed fatigue at a greater rate than you lose fitness and come into form on the day that matters most… race day! In short, Fitness minus Fatigue equals Form.
There is no single Form that works for every athlete, but as a general rule of thumb you would want to be slightly negative up to positive 25. If your Form becomes too high it may indicate that you tapered too much and have lost fitness.
Looking at Form can also give you clues into how much stress you can handle before getting sick or injured. Going forward you would know to take some recovery days prior to reaching that negative a number.