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.
Acute Training Load (ATL)
By taking an exponentially weighted average of that stress from the past 7 days we are able to calculate your ATL (referred to as Fatigue in TrainingPeaks), or an estimate of your fatigue accounting for the workouts you have done in the past two weeks.
Chronic Training Load (CTL)
We also use each day’s TSS to calculate Chronic Training Load (referred to as Fitness in TrainingPeaks). CTL is an exponentially-weighted average of the last 42 days of training. It reflects the training you have done over the last 3 months, but because the average is exponentially weighted, 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 CTL goes up, so does ATL, only at a greater rate. The PMC helps to tell a story; consistent training is marked by a steady rise in ATL and CTL, whereas a sharp drop might indicate time off due to sickness or injury.
Training Stress Balance (TSB)
Finally, by subtracting yesterday’s ATL from yesterday’s CTL, we come up with Training Stress Balance (referred to as Form in TrainingPeaks). Just because you're fit does not mean that you're ready to race at your best. A negative TSB indicates that you're 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 (positive TSB) on the day that matters most… race day! In short, CTL minus ATL equals TSB.
There is no single TSB that works for every athlete, but as a general rule of thumb, you want to be slightly negative up to positive 25. If your TSB becomes too high, it may indicate that you tapered too much and have lost fitness.
Looking at TSB can also give 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.
You can read more details and view a short video about the PMC chart here.