I understand the logic behind the 3 zone model with the divisions being defined by physiologic breakpoints. But I also was wondering if grouping all middle intensity efforts into a single Z2 is oversimplifying things too much. Another way I have heard Z1 vs Z2 described is a metabolic cost to power relationship. In Z1 there is a fairly linear increase in metabolic demand to power output. Once one crosses into Z2 the marginal increase in power comes with a non-linear increase in metabolic costs as nicely represented by a classic lactate curve. This would then imply that the being at the top of Z2 vs the bottom can have very different impacts on recovery across relatively small differences in power output.
So my question is whether there is any data on the polarized vs threshold debate which looked at intensity distribution within Z2. I could imagine that outcomes might be different for athletes spending most of their time at the top of Z2 vs the bottom.
This question was really sparked by the very helpful thread on Steve Nealās adaptive tempo workouts. I find his approach to spend time at lower end of Z2 with careful attention to capping HR to be a compelling idea to build aerobic base while reducing the risk of over doing it in the more classical SS/threshold model.
Iāve always thought of @stevenealās tempo as working on the area where the curve just starts to increase - so you are essentially working to push the flat area of the curve further to the right. I donāt know if thatās right. Generally, I think your assessment is accurate. Everything exists on a continuum.
Great question, and I agree with @kjeldbontenbalās comment. In the big picture, it probably doesnāt matter too much. However, as you stated, the problems can start when you do too much in that middle zone, too close to threshold. Thatās where you start creating a lot of overload that requires more recovery than we might expect or want to give. Trevor details that nicely around minute 4 in the Intervals 101 Workshop.
I also havenāt seen (or at least canāt recall any) data comparing intensity distribution within zone 2 specifically. And yes, I agree that when we are in that middle zone we have to be aware of where we are working and why. Steveās approach to capping the HR is a great way to manage that load. Trevor and I have discussed this a bit too where I like to promote tempo even in the base phase, but there is sometimes a fine line for an athlete that is not very aerobically developed - putting them even into the low end of that range long enough will not actually allow them to maintain a steady physiological response. Hereās one example from a recent test with an athlete that has a pretty tight zone 2; not much room for error in there, so we were more conservative with his zones for base training since he has a habit over overdoing it with threshold work.
Thatās a really good question. I feel like Iāve encountered a study or two looking at metabolic responses to different workloads each within the heavy intensity domain, and comparing fatigue development. Iāll dig around for them.
There is loads of research looking at different workloads within severe intensity domain, but not as many within heavy, as far as I am aware. I think due to individual variability and methodological constraints in verifying that every subject is indeed working within and therefore eliciting a āheavy domainā metabolic response. For example, the literature on ānon-respondersā suggests that some of the non-responding subjects may be working outside of the target intensity domain based on a flat eg. 75% VO2peak or 60% Wpeak exercise prescription.
Also as @ryan is showing, there can be a relatively small range of workloads that elicit a sustained heavy-domain response. Especially considering that the domain will functionally shrink over longer durations (CP can be reduced by prolonged heavy domain exercise)
Interestingly the general fatigue response in heavy domain is kind of a combination of, or a transition between predominantly peripheral neuromuscular fatigue (ie. contractile dysfunction with metabolic disruption at the muscle) in the higher severe domain, and central fatigue (reduced neuromotor activation) + glycogen depletion that act in the lower moderate domain.
So it would make sense that there could be a sliding proportional effect of those contributions to fatigue in an intensity- and duration-dependent manner in the heavy domain, which could certainly affect fatigue on a day-to-day recovery-from-training basis.
In terms of how that would affect adaptations, I have no idea!
Thanks all for these points. I guess the challenge is that the middle zone is that the point about individual variability really comes into play as Ryan and others mention. Perhaps this would be hard to study easily since riders need to be self governing and dial back efforts not only on HR caps but also to keep the RPE within the intended range. One idea I can think of would be to look at discrepancies between those who have their % max HR matching up with the expected power output for the lower range of the Z2 (3 zone model) vs riders who are doing threshold by HR but tempo by power. My hypothesis based on the discussions on the forum/podcast would be riders falling into the later category are the ones paying the non-linear ācostsā while only getting the watts/TSS a few points higher.
