I’ve never used SF, nor will I start now.
Your post did encourage me to check them out… and it appears as though the way in which they establish zones is based on up-to-date science; it’s thus far better than using a bell curve to “guestimate” targets above threshold.
Hunter Allen, who (together with Andy Coggan) literally wrote the book on “training and racing with a power meter” , established four clear physiological zones according to the dominant metabolism - i.e. how energy is supplied to muscles. From most anaaerobic to most aerobic, in order:
- Phosphagen system
- Anaerobic glycolysis
- Aneronic AND aerobic glycolysis
- Oxidative system
It’s important to note that these all occur on a spectrum, and they all contribute to some degree, but the ratios change as you move through the zones and there is a relationship between time and depletion/exhaustion in each zone.
Here’s how Hunter’s zones and the SF zones line up:
Hunter: Neuromuscular Power, 5-15 seconds.
Sufferfest 4dp: Neuromuscular Power, 5 seconds
Hunter: Anaerobic Capacity, 30 seconds – 2 minutes
Sufferfest 4dp: Anaerobic Capacity, 1 minute
Hunter: vO2 Max, 3 minutes – 8 minutes
Sufferfest 4dp: Maximum Aerobic Power, 5 minutes
Hunter: Steady State, 10 minutes – several hours
Sufferfest 4dp: FTP, 20 minutes
From that perspective, the 4dp is FAR superior to a simple “FTP” based system.
EDIT: Because these energy systems to exist on a spectrum, one can still obtain some benefit from using the “guestimated” power targets above threshold… such as what TR uses… but they won’t be optimal. You may not be hitting targets that are high enough to cause the best stress-response (adaptation), and on the flip side you may be over-reaching and over-training.