title basically sums it up: what are the benefits of training at 90% ftp for something like 2x30 vs 100% ftp? I have heard people say that it is primarily a mental thing (i.e. 90% is much easier to handle mentally) but assuming that one could tolerate the discomfort of 100% ftp repeats, will repeated interval sessions AT ftp lead to burnout?
Yes.
And 90% FTP workouts (aka Sweetspot) is a lot easier to accumulate lots of time there then done at 100% (or more) - at least for me.
Of course, the longer the intervals (i.e. 2 or 3 x 20’s versus 5 x 5’s) the “easier” they should be with the “gold standard” being 2 x 20’s right at FTP (ouch of me at least on a trainer)
Read training and racing with power by coggan
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Try not to look an workouts on their own but rather as a block of workouts that progress week after week.
How much work can you do consistently day after day before your body needs to back off. This is where the sweet spot stuff shows its value. It is hard enough to be beneficial yet it can be done often enough to benefit your fitness.
Also what that other guy said about Coggan.
At threshold, Lactate clearance has already broken down.
This makes it difficult for those tissues, processes, etc to adapt and improve. Given the choice, 90% threshold is a good target. Sometimes you should go harder.
http://jap.physiology.org/content/114/11/1593
To understand the meaning of the lactate threshold (LT) and to test the hypothesis that endurance training augments lactate kinetics , we studied six untrained (UT) and six trained (T) subjects during 60-min exercise bouts at power outputs (PO) eliciting the LT. Trained subjects performed two additional exercise bouts at a PO 10% lower (LT-10%), one of which involved a lactate clamp (LC) to match blood lactate concentration (b) to that achieved during the LT trial. At LT, lactate Ra was higher in T (24.1 ± 2.7) than in UT (14.6 ± 2.4; P < 0.05) subjects, but Ra was not different between UT and T when relative exercise intensities were matched (UT-LT vs. T-LT-10%, 67% V̇O2max). At LT, MCR in T (62.5 ± 5.0) subjects was 34% higher than in UT (46.5 ± 7.0; P < 0.05), and a reduction in PO resulted in a significant increase in MCR by 46% (LT-10%, 91.5 ± 14.9, P < 0.05). At matched relative exercise intensities (67% V̇O2max), MCR in T subjects was 97% higher than in UT (P < 0.05). During the LC trial, MCR in T subjects was 64% higher than in UT (P < 0.05), in whom %V̇O2max and b were similar. We conclude that *1)*lactate MCR reaches an apex below the LT, 2) LT corresponds to a limitation in MCR, and *3)*endurance training augments capacities for lactate production, disposal and clearance.
Sweet spot is more repeatable and almost the same benefit.
Sweet spot won’t give you the really high end (mentally and physically) you get from L4/L5 efforts.
FTP and above
Is not as repeatable.
May receive marginal gains compared to SST.
Can’t do high volume of it over an extended period without needing a break.
Personally I found it harder to motivate for on a regular basis.
Many people report illness more often when doing too much work above FTP for long training blocks.
Based on experience I would recommend 8-10 week blocks (depending on the athlete) before programming a lower intensity period of 2 weeks if doing regular FTP work. Burnout is something more than tiredness. It occurs when you feel flat and tired and go out and keep doing it for an extended period. Most people can fix their issues in a couple of weeks. It can take a whole season to get over. I know more than a few elite athletes this has hapenned too.
At threshold, Lactate clearance has already broken down.
This makes it difficult for those tissues, processes, etc to adapt and improve. Given the choice, 90% threshold is a good target. Sometimes you should go harder.
http://jap.physiology.org/content/114/11/1593
To understand the meaning of the lactate threshold (LT) and to test the hypothesis that endurance training augments lactate kinetics , we studied six untrained (UT) and six trained (T) subjects during 60-min exercise bouts at power outputs (PO) eliciting the LT. Trained subjects performed two additional exercise bouts at a PO 10% lower (LT-10%), one of which involved a lactate clamp (LC) to match blood lactate concentration (b) to that achieved during the LT trial. At LT, lactate Ra was higher in T (24.1 ± 2.7) than in UT (14.6 ± 2.4; P < 0.05) subjects, but Ra was not different between UT and T when relative exercise intensities were matched (UT-LT vs. T-LT-10%, 67% V̇O2max). At LT, MCR in T (62.5 ± 5.0) subjects was 34% higher than in UT (46.5 ± 7.0; P < 0.05), and a reduction in PO resulted in a significant increase in MCR by 46% (LT-10%, 91.5 ± 14.9, P < 0.05). At matched relative exercise intensities (67% V̇O2max), MCR in T subjects was 97% higher than in UT (P < 0.05). During the LC trial, MCR in T subjects was 64% higher than in UT (P < 0.05), in whom %V̇O2max and b were similar. We conclude that *1)*lactate MCR reaches an apex below the LT, 2) LT corresponds to a limitation in MCR, and *3)*endurance training augments capacities for lactate production, disposal and clearance.
You are aware of the issues with using labeled lactate as a tracer, aren’t you?
If not, you might want to catch up on the literature in this area, starting with Katz’s study of rats back in the 1980s. That will help you understand why nobody but Brooks tends to conduct such experiments.
My personal experience is that training at SST makes me better at riding at SST. However, if my FTP is already at a certain level, it does not appear to help push my FTP higher. For that I have to train at FTP or more. Like another poster, I also speculate that bi- or tri-weekly training sessions at FTP makes me more prone to catching viruses (VO2max intervals perhaps even more so). OTOH no problems with “burn-out” for extended blocks of such training.
That’s a very well put response and I cannot agree more. Well done. There is also something about mental fatigue from suffering too often. It should not be neglected at all, besides the physical side of a prolonged block of 100% work. EN was a great example where this was tested and in my case totally confirmed your finding. SST is much more repeatable in my case. Though I am 45 and not a young stud. 
For younger riders, cyclist only, 100% should be manageable. What we tend to forget here is how does 100% work tie into run and swim frequency/volume/intensity for triathletes. So, in our case, I found SST allowing for progression in the overall plan. This is not to say that I don’t do 100% FTP work. I do, but in much smaller amounts and usually shorter intervals…
My personal experience is that training at SST makes me better at riding at SST. However, if my FTP is already at a certain level, it does not appear to help push my FTP higher. For that I have to train at FTP or more.
PPP: Sooner or later, you have to increase the power.
Yes is the answer to the question in your title 
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title basically sums it up: what are the benefits of training at 90% ftp for something like 2x30 vs 100% ftp? I have heard people say that it is primarily a mental thing (i.e. 90% is much easier to handle mentally) but assuming that one could tolerate the discomfort of 100% ftp repeats, will repeated interval sessions AT ftp lead to burnout?
You need to ride above FTP to increase FTP, at least efficiently. Doing intervals below threshold is nice for endurance, but you’re leaving gains on the table.
Some of both. Depends on how you feel that day. Dogma is the enemy of improvement.
At threshold, Lactate clearance has already broken down.
This makes it difficult for those tissues, processes, etc to adapt and improve. Given the choice, 90% threshold is a good target. Sometimes you should go harder.
http://jap.physiology.org/content/114/11/1593
To understand the meaning of the lactate threshold (LT) and to test the hypothesis that endurance training augments lactate kinetics , we studied six untrained (UT) and six trained (T) subjects during 60-min exercise bouts at power outputs (PO) eliciting the LT. Trained subjects performed two additional exercise bouts at a PO 10% lower (LT-10%), one of which involved a lactate clamp (LC) to match blood lactate concentration (b) to that achieved during the LT trial. At LT, lactate Ra was higher in T (24.1 ± 2.7) than in UT (14.6 ± 2.4; P < 0.05) subjects, but Ra was not different between UT and T when relative exercise intensities were matched (UT-LT vs. T-LT-10%, 67% V̇O2max). At LT, MCR in T (62.5 ± 5.0) subjects was 34% higher than in UT (46.5 ± 7.0; P < 0.05), and a reduction in PO resulted in a significant increase in MCR by 46% (LT-10%, 91.5 ± 14.9, P < 0.05). At matched relative exercise intensities (67% V̇O2max), MCR in T subjects was 97% higher than in UT (P < 0.05). During the LC trial, MCR in T subjects was 64% higher than in UT (P < 0.05), in whom %V̇O2max and b were similar. We conclude that *1)*lactate MCR reaches an apex below the LT, 2) LT corresponds to a limitation in MCR, and *3)*endurance training augments capacities for lactate production, disposal and clearance.
You are aware of the issues with using labeled lactate as a tracer, aren’t you?
If not, you might want to catch up on the literature in this area, starting with Katz’s study of rats back in the 1980s. That will help you understand why nobody but Brooks tends to conduct such experiments.
Im not, will look into it when I get to the office.
I find there is a pretty big difference between riding at 90% and 100% FTP. I’ve never really bought into the whole SST, I’ve only ever experienced mediocre results. Where I’ve had consistent gains is when I go back to the tried and tested 5x5 min >115% FTP 2-3 x per week, and the rest of my riding all in Z2. For TT’s I’ve found nothing better.
I find there is a pretty big difference between riding at 90% and 100% FTP. I’ve never really bought into the whole SST, I’ve only ever experienced mediocre results. Where I’ve had consistent gains is when I go back to the tried and tested 5x5 min >115% FTP 2-3 x per week, and the rest of my riding all in Z2. For TT’s I’ve found nothing better.
I’m the opposite. I’m a volume responder for TT. Doing a lot of 4-8x10 minute @ 90-95% seems to do the trick. And, of course, 10-15 hours/week of “Z2”
SST has its place in my opinion. I just finished a 12 week block of SST training and have seen substantial gains. It’s been 10 hours a week with about 5 hours a week sweet spot, dialing back to 7 hour and 4 hours every 3rd week and doing a 20 min test. Power is about 20w higher now than at last seasons peak, and never felt better on the bike. Using training peaks to monitor progress and fatigue with a focus on taking rest when needed.
All this said, I think it is time to step it up to sets of shorter intervals above cp60. This week starts 7-12 min intervals at higher power to see if further progress can be made.