Hi, everyone!
When doing long sweetspot intervals (15/20mins) i like to do at least one of them with a lower cadence (75/80rpms), just for better climbing adaptations.
When i do that, the effort seems way easier and i increase the workout’s intensity a 5/10 percent.
Should i do that or am i training different zones?
Depends on whether the baseline workout is low sweetspot or high sweetspot. If it was high sweetspot then you’ll be pushing into threshold. Just divide the wattage you ended up riding at by your FTP and multiply by 100 and that’ll tell you where you were.
The good news is that as long as you’re recovering well (and that’s quite the caveat) you’re getting more bang for your buck. Just keep an eye on your fatigue. Some people find riding at 98% much harder to recover from than 92%. Others, not so much.
With the exception of sprints, I think you should definitely at least try different power levels at different cadences. An easy one is endurance: on a 90-minute endurance workout I spend at least 30 minutes at cadences lower than 80 rpm and at least 30 minutes at 95–100 rpm.
I think it is a great idea to extend the range of cadences you feel comfortable at. Because sometimes you don’t have the choice. And other times having more choices is better.
No, you are not training different zones. But you are shifting the burden from your muscles (at low cadence) to your cardiovascular system (high cadence).
They very possibly are, if they’re increasing the workout intensity by 5% or 10%, as per the original post. 
They’re not just talking about dropping cadence. The question is about dropping cadence and increasing power output.
I wonder whether varying the burden within a workout means that sometimes you would be not providing sufficient stimulus for adaptation to either, so that if lowering cadence at a given power reduces PE, then you should endevour to raise power in such instances to maintain PE.
Good question.
I would also think that the FTP is different from high to low cadence, as the burden is on cardiovascular vs muscles. Anybody with more insights on that ?
Anyone up for doing two FTP tests 48 hours apart? One at 60rpm, one at 90 rpm?
[Takes one large step backward]
I wouldn’t change the intensity. What you are doing is shifting to a greater metabolic cost at a lower cadences. At a higher cadences you are shifting the load more onto the cardiovascular system. They’re kind of giving you a different stimulus. I’m assuming you are doing a number of intervals so I recommend doing the first one at a lower cadence (say 75) and increase the cadence as you progress through the intervals to the last (say 90). This teaches you to not bog down and mash away when the fatigue sets in.
At lower cadences it probably feels easier because you have more metabolic fitness (i.e. dealing with lactate) than cardiovascular fitness (VO2max).
Ok, I re-read the post and you are right that it might. I misunderstood the OP here. Intensity should not be touched when you change cadence.
You should not alter the intensity based on your cadence, keep your training power-based. If a workout gets too hard, then work your way towards spending more time at a certain cadence.
The stimulus at different cadences is different, but that’s exactly the point: in practice, you will need to work at different cadences. Indeed, your self-selected cadence will not depend on power, but wheel inertia (= wheel speed = speed) and whether you are climbing or not. Getting used to working hard at different cadences and being able to put the power down is a good adaptation.
Overall, it isn’t cheating or anything. The point of workouts is that you get through. And varying the stimulus is important for when we take our bikes on/off the road.
FTP has nothing to do with cadence. FTP is a field test for your power at lactate threshold (2). You do have a self-selected cadence range, though, so given specific circumstances (speed, whether you are at an incline, etc.) will determine your self-selected cadence. At your self-selected cadence your body is at its most efficient. If you are far away from that optimum, you will significantly decrease the time you can spend at a given power level.
In my experience, you can tell more easily when you do VO2max efforts: when you are close to your limit, shift up/down a gear, and see which one feels easier. If your muscles are the limiter, shifting down a gear (= increasing cadence) might help you finish the last intervals. If your cardiovascular system is maxed out and your muscles feel alright, shift up a gear (= decrease your cadence).
Once you get to the extreme ends, that doesn’t work anymore. E. g. you can deplete your fast twitch fibers relatively quickly even at moderate power output (try to hold 150 rpm at 70 % FTP and see how long you will last).