With all of the n00b intervals.icu training talk, it's probably worthwhile to have a little primer on Load, which is the building block of much of what we're talking about.
Load for a given workout is
intensity (how hard you worked)
x time (how long you worked out, in hours)
x 100 (because decimal points suck and bigger numbers are betterer numbers)
Intensity is how hard you workout as a percentage of how hard you
can work out. So if my FTP is 250w and I have an average of 200w for a workout, my intensity is 80% or 0.80. If my workout was one hour long, the load would be 80 (.8 x 1 hour x 100). If my workout was 90 minutes, the load would be 120 (.8 x 1.5 hours x 100).
All of the Fitness, Fatigue, Form, etc is based off of Load.
Fitness is based off of a weighted average daily load over ~6 weeks. If your current Fitness is 50 and you want it to go up, you need to up your average daily load, so doing 45 mins at 85% four days a week isn't going to do it (that would roughly be an average daily load of 36)
Fatigue is similar, but it's a weighted 7 day average (basically, fitness builds/decays more slowly, whereas fatigue builds/recovers more quickly).
Form is a comparison of your Fitness and Fatigue. It's a good shorthand way of telling if you're overtraining, undertraining, etc. You're undertraining if your Fatigue is a lot lower than your Fitness. You might be in danger of overtraining if your Fatigue is too much higher than your Fitness. It might be a good idea to taper to someplace in between if you have an event coming up, etc.
Hope this helps.
