Hi John,
Great question. Both are analogous in terms of what they’re trying to accomplish - that is, create a numerical value that represents how much training you have historically done.
The basics:
CTL is 42 EWMA of TSS
TL is 60 day EWMA of Low XSS + 22 day EWMA of High & Peak XSS
TSS is calculated on time & some weighted average power during activities. It can only be accrued at 100/hr, assuming a full hour at FTP.
XSS is based on work performed, relative to MPA. In theory, it doesn’t have a limit in terms of XSS/hr. Practically, I’d say around ~200 XSS/hr is the highest achievable value.
For most sub-(F)TP efforts, there’s likely a very linear relationship between XSS & TSS, but this changes as efforts above (F)TP are added in. This is because Xert awards more XSS when work is performed under fatigue, unlike TSS. I often explain it like this… a 20 min FTP test with other systems awards the same TSS for the first minute of the 20 min test as the last. If you’ve ever done an FTP test, you know the last minute of that test is much harder than the first - and this is because you’re becoming limited by your MPA. Xert knows & understands this, so you’ll receive far more XSS in the last min of that 20 min test than you do for the first minute of the test.
TLDR: they’re both numbers that represent how much training you’re doing, but it’s sorta a little bit of apples to oranges, since they’re both calculated differently.