Don't Chase the blue line: Understanding Fitness, Fatigue, and Form Metrics

How, what, and when with event CTL (Fitness), ATL (Fatigue), and TSB (form)

Athlete: “It's getting close to an event and my CTL is dropping. That means I am losing fitness right? Surely I should put more hours in to get my CTL as high as possible”.

One of the most challenging concepts of endurance training is understanding whether you are training hard enough or not. You can train too hard, not hard enough or just right. Trainingpeaks, the application we use at The Threshold to update programs and communicate with our athletes, has three distinct metrics displayed on their dashboard, Fitness (also known as CTL or chronic training load), Fatigue (also known as ATL or acute training load and Form (also known as TSB or training stress balance).

Figure 1: Dashboard metrics found on Trainingpeaks software

The implications of training too hard can result in performance decrements and illness, while training not hard enough will result in sub-optimal performance on race day. The goal of understanding these three numbers (see figure 1) allows the coach to program in a way that keeps the athlete in the goldilocks zone, where they are training as hard as possible to create adapation and not too much more.

Understanding Fitness, Fatigue and Form:

Lets first talk about Form/TSB. Form is the difference between your Fitness/CTL (how much training you have done over the last six weeks) and your Fatigue/ATL (how much training you have done in the last week). Basically, Form is a representation of how you will perform taking into consideration your current Fitness and Fatigue levels. Put it another way, when your recent training load is greater than your average training load (ie last six weeks), your Form/TSB will be negative. Looking at this visually, where an athlete's recent Fatigue/ATL (so last week of training) load exceeds the Fitness/CTL (so average of last six weeks) load, you will find the Form/TSB dipping below 0 and becoming negative. A negative training stress balance shows that you are not fully recovered from recent training and are fatigued. Most likely you will not have your best performance. For building fitness, a Form/TSB between -10 to -20 for two to three weeks is a good approach before you need to recover to allow the training to work its magic. Notice the diagram below (Diagram 2) where these various metrics cross. We call this chart the PMC or performance management chart. It helps to provide a quick visual of how fatigued or tired you are.

The reverse is also relevant. If your recent training load is much less than what your average training load has been (so Fatigue/ATL is less than your Fitness/CTL), the Form/TSB number will tend to be positive. It shows that your body is most likely recovered and what is called “fresh” and “on form”. When recovering from a training block, a good place to be after a week of recovery or a period of days is around 0 to +10. Once you begin going above +10, you are losing fitness and will equally not have your best performances.

Diagram 2: Trainingpeaks Performance Management Chart (PMC).

How to approach using the PMC as well as Fitness/CTL?

The question is a regular asked by our athletes. “Oh my CTL is 120 and my riding mate is only 80, so I am in better shape”. Its important to realize that CTL is only a reflection of training load or how much you have done over the last six weeks or so. It does not tell us how much intensity is in that mix, whether you did specific intervals required for your event. Training 5 hours a day at aerobic level will very quickly get you to CTL values above 120 over six weeks, but it does not mean that work is specific to the event you are competing at. The point here is that there is more to understanding your bike fitness than one number.

A good approach is to build your CTL to as high a number as you can, then before race day reduce your Fatigue/ATL until you are fresh, but not beginning to lose fitness (ie Form/TSB does not reach levels above +10). A good coach will understand this will be able to add the correct phasing in of intensity (which often means a stalling in the growth of CTL). Remember horses for courses. You can have a high CTL but if you do not have specific intensity in the mix at the right times before your event, you will be very fit but not very competitive. See below for CTL ramp rates that allow a safe increase in training load.

Diagram 3: CTL ramp rate for safe training load increases.

Lastly, always remember that CTL accuracy is necessitated by accurate regular testing of your threshold levels. Therefore, an FTP (functional threshold power) test is recommended every 6 weeks.

Train hard and proper

Darrin