If you wish to track TSS, TSB, CTL, ATL, etc. you absolutely can within your Today's Plan dashboard, which all athletes can access. We do find they are interesting at a glance and establish trends when you take the 10,000-foot view. Also, Purple Patch's stance is that TSB can be interesting when you reflect on previous races, what went well, what was the load like leading in, etc.

However, here is why we don’t track, or place premium value, on these metrics:

1. It’s data-dependent. How many times have we been out for a ride and the power meter dies, or there are crazy readings? You have to carefully monitor data for accuracy if you want to use it to track. It is very tricky to take into consideration outliers with data. 
2. The algorithm is based on % of FTP or % of HR of Threshold pace or CSS. This assumes riders FTP is correct, and all of the other baselines are correct pace, HR, swim pace, etc. Simply put we cannot ensure all of these fields are accurate enough to properly track thresholds as we’ve seen how often they have been overestimated or underestimated. This is best done with a 1:1 coach overseeing and providing feedback. 
3. The algorithm assumes a standardized formula for a given athlete and doesn’t take into account individual variation. Strong swimmers, riders, runners respond differently to various loads and these metrics can’t account for that.
4. The big ones that these metrics don’t account for are SLEEP, LIFE STRESS, or any variables beyond training stress. We know stress is stress….whether work, home life, etc., it has the exact same effect on cortisol, recovery, and adaptations. Try doing a V02 max session on zero sleep, then get 8 hours and do the same session. This is an extreme example, but it illustrates the point. Tracking a metric that doesn’t account for two big variables for adaptation is at best severely flawed and leads to inaccurate reporting. 
5. It is power skewed, or peak pace/power skewed. We, for example, know that doing 4 hours endurance pace at lower intensity has many benefits to long-distance triathlon such as fat adaption, recovery, base-building, etc. We also know that over the course of this ride there are real physiological changes and stress that builds up. TSS assumes that the first minute of a 5-hour ride at 150 watts is equal to the last minute of a 5-hour ride at 150 watts. We and every athlete knows the stress is far different because of compounding load over time.
6. It leaves out the very real metabolic competent of triathlon which is an important metric. TSS doesn’t account for metabolic changes or how you are using stored energy.  For example, assume two riders with a 300w FTP. We know that two riders who have the same FTP can be entirely different in makeup. Which means you need to train differently if you want appropriate adaptations.

As to know when to take a day off, this should not be a hard and fast time on the calendar nor should you push through when there is not one built-in. It sounds simple, however, if you are training at an appropriate load and paying attention to not just the data metrics but subjective metrics such as sleep, stress, etc. then when you are tired, pull back on intensity or pull in a rest day. Listen to the body. Life is not a spreadsheet.

For real insight into what’s actually important when it comes to how to train, and how power is composed would be to look at INSCYD testing or doing metabolic testing in a lab.

We are not suggesting to throw out the baby with the bathwater because you can use these metrics, but it is just one part of the puzzle, and to Purple Patch, they are some of the least useful of all the tracking metrics, however attractive it might be to simply think "if I hit 300 CTL, then I am good." If only life and training were that simple. 

This might feel like it is a long answer, however, we wanted to share some perspective into why we decided to lean into other areas for tracking, especially for Squad members.