If one is a Coggan follower and subscribes to the validity of TSS as determining a training stress score and fatigue then a TSS of 100 generated in either zone 2 or zone 3 provide the same amount of fatigue. Zone 3 produces the fatigue in a shorter period of time. Coggan is a smart guy so it he would have thought that zone 3 produces more fatigue, he would have used a different formulation for calculating TSS. By formula, the only way to increase TSS faster is to have normalized power increase for greater than 30 seconds. That adds a quadratic component to the TSS equation versus having TSS solely generate by linear function - force X time.
As I remember, in the bookTraining and Racing with a Power meter, the formula for NP was about the rate of change of lactate at efforts above FTP.
And another fact that I am sure Coggan is aware of is that as power increases there is a point where more glycogen is burned for a unit of ATP produced. I assume that occurs somewhere in high zone 2 / zone 3. Does that effect generate more fatigue. If Coggan thought it did, then he could have included it in the formula.
What do I think? I just ride these things. My personal experience is that zone 2 is just as fatiguing as zone 3 with the proviso that I ride at constant power with few if any "mini rests".
When I climbed Mount Hamilton with constant force in my muscles in both zone 2 and zone 3 it kicked my ass. Riding flat to rolling terrain in silicon value with traffic lights, stop signs and etc zone 2 was easy.
IMO, I self select zone 3 when I need more / higher constant power. For me, it is the constant effort that generates more fatigue not the zone.
Time trials ridden in zone 4 which is very fatiguing but once again, may produce the same TSS as a 2 hour zone 2 ride. However, during a TT, I try to make every pedal stroke count which tends to be quite different from a ride through Silicon Valley in zone 2.
And then there is the San Millan zone 2 and the protocol he requires during the ride. For me, riding a San Millan zone 2 ride where I am producing power at a point where talking is just starting to feel labored is a tough and fatiguing ride.
Hopefully, the above provides enough right and wrong information to provide all the zone 2 v zone 3 nazis something to complain about.
