I’m currently doing mostly indoor riding on Zwift, training for a 200k 4000hm in February.
As part of it I’m doing long rides in Z2 (Coggan, 68-75% FTP). To keep myself mentally fresh I’ve added 2m @ 110% FTP every 30min.
I’ve heard Dylan Johnson say (based on research by Stephen Seiler) that Z2 (Coggan, which would be Z1 in Seiler’s model) training is basically an on/off switch where doing intervals in between can harm the effect of the easy riding.
I can’t find any research to back this up, can anyone speak to the validity of this statement?
I’m having a hard time believing that 2m of intensity will negate the benefits of the half hour slow riding.
It’s not harmful at all. You are adding a different stimulus but once you are back in Z2 in terms of HR and power you are again working on your aerobic capacity
While I partially agree with David J, I would add a couple of things:
Essentially all zones have an aerobic influence… I don’t view it as a on/off switch and I’ve also not seen any evidence to back up the off/on thinking
I probably wouldn’t prescribe what looks like VO2 type intensity… but maybe up to SS. That’s just me, I’m not saying it’s horrible, but if just for mental refresh, there’s also no reason to pop up that much.
I don’t agree with @DavidHijon …
I do agree on the fact that it’s not a on/off switch and I wouldn’t call it as extreme as ‘detrimental’.
But, when intensity goes up and the energy demand can no longer be fulfilled by predominantly burning fat, your body strongly and quickly increments the chemical processes to burn carbs iso fat. The carb burning process can deliver more energy per time unit because it needs less oxygen and becomes dominant.
However, when the energy demand decreases, it takes way longer to switch back to the fat burning process as dominant.
Already from the start of your workout, no matter how low you start, it takes about 20 minutes for the fat burning system to really kick-in. So if you do high intensity every 30 minutes, the total time in the targeted zone/system will be highly reduced.
If you want to avoid boredom, introduce skill work without increasing intensity. Do zone 2 work at different cadence ranges. 70-80rpm followed by 80-90 then 90-100rpm, same power target. Follow up on what your HR does. Become efficient in a broader cadence range while still being in Z2.
You also want to avoid doing all the Z2 work at the top end of Z2, which is what lots of people do, thinking that more or higher is better. It’s not! It’s about becoming more efficient in a broader range, so all over your Z2, both low end and high end.
There’s plenty of footage on this on YouTube from Inigo San Milan, Stephen Seiler, etc…
I agree. Just from experience where if I were to run under Z2. Then if I were to increase the effort upwards of Z4. I find that my recovery back to z2 will be longer and not only that, I would feel that I’m actually unable to stay in the same Z2 level cos everything has elevated.
You may be substracting energy from different sources but it does not mean you are not working on your aerobic capacity once you stop doing VO2Max. There are four main areas of work: aerobic capacity, aerobic power, anaerobic power and neuromuscular. From your comment you seem to imply that when you do VO2Max (aerobic power) and you stop working on it you would be 20 more minutes working on that range but that’s not true. Your body may be using more glucogene than normal to recover from that effort but metabolically you are working on aerobic capacity.
I agree that from a Cardiovascular POV, it has very little to no impact. That’s why I called ‘detrimental’ a bit extreme. But as you acknowledge, your energy source changes, and the benefit of long Z2 rides, is to improve the fat burning system.
For shorter Z2 rides, below ~80min, I wouldn’t care to much. The longer Z2 work has been proven to cause adaptations that can’t be gained from other types of training.
From an efficiency POV, I would say that on those longer rides, you should maximize the outcome.
Bumped down the intensity of the 2minutes down to 90%. I prefer the feeling of a bit more high intensity over drills to break up monotony. I’ll see how that works, and I want to acknowledge your reasonings @MedTechCD.
Changed the %FTP for the Z2 blocks to cover a larger range (was 68-75%, is now 62-75%), and put the 62% after the SS interval to get back to Z2 more quickly.
The problem is that we can’t reduce those capacities to the level of energy consumption because then we have to consider the role of nutrition while riding and before and after that since what you eat has a determinant effect on how much fat you are using while doing Z2.
Not sure where to start. We might ultimately be in agreement, but some of your posts are confusing and could be misconstrued as the adaptation is driven by fat burning.
First off, one primary adaptation of low-intensity endurance training is to increasing the number of mitochondria. Another is for the heart muscle to develop more elastic, full heart beats. There’s more.
Why does increasing the number of mitochondria in leg muscles matter?
One reason is that increasing the number (and size) of mitochondria allows for more fatty acid oxidation to occur. And you’ll also increase the amount of glucose/carbs that are aerobically oxidize without producing an increase in blood lactate and muscle acidity. And you’ll raise your vo2max. Those are 3 pretty great results from one adaptation.
Are there other ways to increase the number of mitochondria?
High Intensity Interval / Repeat Training will also increase the number (and size) of mitochondria. And some of that increase is in type IIa fast twitch fibers (fast glycolytic). Versus low-intensity training which is primarily increasing in slow twitch type I fibers.
So both Endurance/Zone2 and HIIT / HIRT will increase mitochondria, which results in increased vo2max, increased fatty acid oxidation, and increased aerobic metabolism of glucose/carbs.
Further, consuming carbs while training doesn’t cause you to lose those adaptations - the adaptations are virtually independent of on-the-bike fueling (or not). Eat and ride, so that you can ride more.
Moving on, you are always burning carbs (glucose) and fat.
For myself, 50-65% vo2max is endurance (Coggan zone2) training. Conceptually its about 50% fat oxidation and 50% carb/glucose oxidation.
For example here is a 4 hour study with muscle biopsies to determine where the fatty acids and glucose are coming from (stored in muscle or blood stream).
30 minutes into the zone2 / endurance ride carb oxidation was roughly 66%, and because there was no fueling, muscle glycogen was decreasing and by 2 hours carb oxidation was down to 50% and down to 33% at 4 hours. And fat oxidation ramped up, to make up the difference. Summary from the same study:
And from other studies we know that fueling with carbs will result in better Performance.
FWIW, some geeky numbers on ATP production and oxygen requirements (because I was reviewing this morning):
One fatty acid - palmitic acid - one molecule of palmitic acid requires 23 molecules of oxygen to produce about 106 ATP and be reduced to water and CO2. Respiratory exchange ratio of ~0.7 (CO2 to O2).
One glucose molecule requires 6 molecules of oxygen to produce about 32 ATP and be reduced to water and CO2. Respiratory exchange ratio of ~1.0 (CO2 to O2).
Both fatty acids and glucose must be converted to Acetyl-CoA. And then enter the Krebs cycle and into the Electron Transport chain where Oxidative Phosphorylation converts the output of the Krebs cycle (input: Acetyl-CoA) to ATP, water, and CO2.
“At high-intensity exercise, the rapid glycolysis provides the mitochondria with excess acetyl-CoA, which is buffered by free carnitine to form acetylcarnitine. Accordingly, a fall in muscle concentration of free carnitine may reduce CPT-1 activity, and thus the ability to transport FAs into the mitochondria, and therefore, also the rate of FA oxidation.”
Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (Deed - Attribution 4.0 International - Creative Commons).
it would be interesting to have a metric to quantify the efficiency of work in Z2.
For example with arbitrary figures (is there a study on this?):
If 5 minutes of Z4 then the next 20 minutes in Z2 are junk miles (for the desired adaptations in Z2)
If 2’30" Z4, the next 10 minutes etc.
To be done with all intensities obviously
We would then have this metric in addition to the time spent in Z2, for example a ride with 90% Z2 could in reality only have an efficiency of 50% if, as we often see, athletes do not respect Z2
I am no expert but am currently following the advice in this article which suggests it may be better to add intensity towards the front or end of Z2 rides.
"We really like San Millán’s suggestion of including higher intensity work towards the end of an endurance ride in order to keep the first part of the ride to a ‘high quality’ (a.k.a low lactate levels). This is a strategy that could work well for time-crunched cyclists, doing rides in the region of 1.5-2 hours.
Another approach we like to take is to include intervals towards the beginning of a longer (2.5H+) ride. The benefit of front-loading the intensity is that you should be feeling more fresh for these efforts, and they will also deplete muscle glycogen levels, which is a key signalling pathway for a range of aerobic adaptations, including improved fat oxidation.
Of course, with this approach, fat oxidation might be suppressed for the 20-30 minutes after finishing the intervals. However, the adaptive stimulus from the remaining Zone 2 ride should be enhanced by the lowered muscle glycogen levels. "
You can use the PWR/HRZ2 efficiency to sus out some of the impact on this type of training if you have historic data and understand your fatigue during those times. It can be a little nuanced, especially during the early phase of a new training season as you’re battling things such like HR drift, but if you have a good corpus of metrics it’s pretty clear the impact different training has on it.
A couple of good ways you can pull some reasonable metrics would be to compare daily vs. form %/fatigue and your RHR. You can play around with the efficacy of 1hr vs, 1.5hr, vs. 2hr+ rides over time and it’s impact.
On a longer time scale you could look at something like your weekly max of PWR:HRZ2 and compare it to your HR distributions to gauge the impact.
Here’s one from my 2024 season that I have found pretty useful for me. The red in the top chart is total time in Z1+Z2 (up to ~152bpm out of an LTHR of 171, and a max of ~190), and the grey is time > Z2+. The efficiency range for the season is between ~1.3->1.8.
The second chart is just the weekly max of PWR:HRZ2 as it kind of nicely picks up when rested vs. not. You can also see that as I dropped the hours towards the end of the season, I was doing just enough to keep the aerobic system from dropping off too much (and was very well rested!).
Regarding changing cadence to prevent boredom:
For me - keeping the same Z2 power target - and increasing cadence by 15-20 rpms, makes my HR progressively climb higher, until I’m knocking on the door of Z3 HR and higher. As soon as I back down on the rpms, HR begins to back off as well.
Ymmv.
There is some great info shared here but none is really relevant as to why an intermediate high intensity interferes with low intensity.
I have it explained why in my blog: Endurance training – Designed By Nature
If you don’t want to read the whole piece then you can skip to the title " How to improve the quality?"
I don’t think it matters much if you do the ride long enough and don’t add too many of such intervals. In such case you do get to depleted energy reserves anyway. It is likely more important that you don’t take in energy during your Z2 training versus not doing intervals. Every 30 minutes like the OP may be too much if the ride is not long enough.
