Zone 2 Session Mechanics (needing to reduce load during sessions)

Hi there,

I‘m new to the zone 2 training world but wondered about a the following dynamic.

In Peter Attias podcast with Dr Iñigo San Millan, Peter mentions that can essentially set the watts on his bike (something approx 200-250 can’t remember the exact number) and that with the fixed wattage he essentially stays in zone 2 for an hour or two.

Now I‘m new to the topic and certainly not as well measured as him (I use a garmin fenix with an garmin HRM PRO chest band). I use the Karvonen formula with max heart rate 183 and resting heart rate 43 and get 128 to 141 bpm as my zone. Now when I start a normal run or cycle exercise, I can easily stay in that window say at 5 min per kilometre pace or at 215 watts on the bicycle however after 45 minutes I need to significantly reduce pace say to 6.5 min per kilometre or 160 watts on the cycle (this of course happens continuously).

Now my question is:

Is it normal that I need to continuously reduce load to stay in my calculated zone 2 window during a single session? Or does it seem more that maybe the formula doesn’t give a good indication of zone 2 to me (e.g., my range should have a far lower upper limited)?

To me it seems as if the effort I‘m exerting might be too high and thus it could be more anaerobic than my calculated zone 2 implies, meaning my body is burning more carbohydrates and not fat and lactate is rising in my muscles which means the body needs to remain working as hard but can’t deliver the same power output.

Any thoughts on this would be super interesting? Of course if this is covered somewhere else please redirect me?

Thanks a lot for the help!
Best regards,

How hard do these sessions feel? Both from a description and on a 1-10 scale? Do you think you could continue with that 215W pace for several hours (if you ignore HR)? I would probably suspect not. Try starting much easier. Like start out with whatever power you end up having to drop to and then build from there.

For me, based on the 7 zone power model, the top of my Z2 would be something like 235-240W. But if I tried to do my Z2 work at that power I would be cooked. So I usually end up doing it somewhere around 190-215W depending on duration, fatigue, next session, heat, motivation, etc.

Endurance riding is best done too easy rather than too hard so I would start toward that lower end, then ride by feel, keeping it easy and chill and see how you end up.


Super helpful, so I understand for you at that lower wattage you can easily retain the effort without having to increase your heart rate over time.

In terms of effort at the level o train currently based on heart rate I could easily hold a conversation but the other person would recognise that I‘m doing a workout. I would maybe give it 6.5 / 10.

Appreciate the response! I will update my window probably 10bpm down and see how it works and will report back!

I don’t necessarily subscribe to the ISM Z2 idea, but have gone through the process of looking for LT1 and my “Z2”.

I have similar HR numbers (177MHR, 40RHR) and my “Z2” number is 126.

I think you’re working way above LT1, especially if you can only sustain it for 45min.

I suspect you’re in high tempo, low Sweet Spot


Tried it today, tested with max bpm of 132 and was easily able to handle more than 1.5 hours at same load. Seems the zone 2 calculation was quite off.


It looks like that Karvonen Formula works a little different, being based on percentages of your heart rate range. I ran into a similar issue because I used Friel’s HR zones, which puts zone 2 above LT1, which was fine (I understood to work below z2), until I started using Coggan “zones” (levels) for power and needed to switch my HR zones to be compatible.

The first thing that popped to my mind is heartrate decoupling. To a certain degree we expect the heartrate to drift up even though the power we’re putting out is staying the same. Some of this is fatigue creeping in, some of this can be the effect of dehydration or other factors (for instance I think fueling can affect it as well, but I’m not an expert). So you might need to drink more water. Or maybe you need more base before going for a 2 hour ISM Z2 ride (because ISM Z2 should be hard). To some degree, though, decoupling will just happen.

You mentioned that you could pass the talk to test so maybe you were starting at Z2 and needed to stay there. Or maybe your were hitting your ISM Z2 limit.

I am personally wary of putting too much importance on heartrate. I will certainly say that heart rate zone calculations can be way off. Rather than go for a method based on my maximum heartrate, I’ve used a calculation based on the last 20 minutes of a 30 minute effort which should give you your threshold heart rate and you can calculate from there. That seems to be better aligned than any calculation based of my max hr (or max hr and resting heart rate). Though I did not do the test perfectly, it seems to more or less match what I see when I’m training at a specific zone–but only when I’m fresh and only indoors. Outdoors it does go higher. If I’m not fresh, it will be lower. And it’s also always important to remember that it takes some time to get up to heartrate (though I’d expect that it should be up to speed well before 45 minutes have passed).

When I’m outside and fresh and trying to do an ISM Z2 ride I go by power and the talk test–and honestly by RPE, but only after I got used to setting that by power and the talk test. My heartrate will typically go over my calculated Z2 heartrate, at least in the second half of the ride (though not by very much).

None of that is to say that you’ve got the right ISM Z2 power, but I would suggest seeing what happens if you sustain your power/pace for longer. If I read you right, you were lowering your power to stay within your heartrate zone, not because you couldn’t keep going at that power/pace for longer. It might be worth just seeing how long you could maintain that power. I’d be more worried if you stop passing the talk test, or if you just can’t keep putting out the power (though time at ISM Z2 is not forever–at some point you will just desperately crave an end to it)

That said, it probably doesn’t hurt to do endurance efforts below ISM Z2. I think ISM has said that people shouldn’t be too religious about that exact level of effort. I’d just stop worrying about heartrate for a while. It has it’s place, but it’s easy to overemphasize.

1 Like