Listener Question - performing intervals

We received a great question from Peter B. He asks:

I know you did a full episode on performing intervals – takeaway “keep them simple, execute well” – but I still don’t have a good sense of how hard a good set of intervals should be, and what it should feel like.

For example: 4 x 8 minute, riding on Zwift in erg mode so it is easy to hold a fixed power. Should I set the power for “legs on fire, barely able to complete the last one”? Or a more conservative “solid workout, let’s go get bagels”? :wink: Should heart rate be relatively constant interval to interval or is it normal for heart rate to climb as you progress through the set?

57-year-old male, recreational, experimenting with using these for my low volume of high intensity (no more than once/week).

Last session: Felt like I could go a bit harder:



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Hi Peter,

Thanks for the email and really good question! You’re asking the right question because just working with an interval like 4x8s, there’s a lot to figure out in order to do them right. It’s not a matter of going as hard as you can go. And unfortunately the right answer to your question is different for each type of interval. For example, at this time of year with 4x8s you don’t want to go all out and kill yourself. However, if you were doing something like 5x5s close to the season, yes, you should be ready to throw up by the end of each one. So you have to re-ask those questions for every type of interval work. That’s why I personally only have about eight interval workouts I use. They are the one’s I’ve learned how to execute well.

Addressing specifically your 4x8s, it’s always hard to see much in the Strava graphs – too short to see true heart rate fluctuations. But the way I tell my athletes to do them is they should not be a legs on fire scenario. Done right, all eight should be the same power. The first one should feel hard but should also feel like you could have gone 10-20 watts bigger. The last one should be a bit of struggle to finish.

The heart rate line should level out with each interval. If you don’t see that, you’re going too hard. Here’s a set of 4x8s I did last year where I was particularly happy with my execution:

The red line is heart rate. Notice that it’s pretty level and while I touched my threshold heart rate, I didn’t really go over it. Also notice that I bumped my power up slightly on the third and fourth because my heart rate and RPE were a little too low on the first two (with RPE actually being more important in that decision.)

See my next reply for the subsequent graph of going out too hard.

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By contrast, here’s a set I was not proud of. I was going too hard, my heart rate did not level off, and I ultimately pulled the plug on the fourth because of my bad execution:

I hope you see that it’s not dramatic differences. Clearly the first graph is more controlled, consistent, and both power and heart rate are in better places. But it’s not dramatic. That’s the difference between just “pounding out” intervals and executing well. It’s subtle and takes time to learn. But when you get it, you’ll know.

All that being said, based on the not very detailed graph you sent, yours look pretty good.

Hope that helps!



Coach @trevor, From listening the Podcasts with Dr. Seiler, I thought 4x8 were supposed to be executed as hard as you could. Looks like I misunderstood that part and probably that’s why it is important to have an experienced coach to help you understand these nuances.

Looking at the graph 1, the well performed 4x8, looks like it was executed right around your FTP, hence why your HR is near or close to Threshold. Are these better done in the base season and harder ones over-Threshold better done close to race season? Should one still do these 2x week during base season or one a week is enough?

Thanks in advance,

Alex, great points. I see you tagged Trevor, but I’ll give my two cents as well. When I think of “as hard as you could” I think of it in terms of “maximal sustainable intensity” and when we’re working around threshold, that’s a pretty fine line. We can bump 10 watts higher and see a dramatic change in our HR response, ultimately making the intervals look more like the example of the poorly executed interval session.

Considering the added recovery we might need following a poorly executed session, I find when working around threshold I try to think of “maximal sustainable” for “the total accumulated time” goal of the session. So whether that’s 4x8 minutes (32 min total) or 3x20 min (60 min total), I’m usually shooting for the same approach. In certain times I might work on a “hard start” approach to really tap into that anaerobic contribution and burn it up a bit before settling in to make it more specific to events I’m preparing for, but also realize that I may need more recovery focus after a session like that.

So to your question, I do think it’s smart to do the 4x8 controlled efforts this time of year. But you can also have some fun with your sessions and do some mid-interval surges, hard start approach, or batman style intervals to increase the engagement a bit along the way.

Coach Ryan

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Hi Alex,

Great 2 cents Ryan! Appreciate all the great advice you’ve been giving!

Alex, you’re right, there are a lot of nuances and the particular “style” of the coach also plays in. If you ask 10 coaches this question, you’ll likely get 4-6 different answers. Fortunatley, in this case, Ryan and I are in agreement, so you’re mostly just getting one.

When Dr Seiler conducted those studies, he wanted to keep the prescription simple and he wanted to see what power the athletes naturally gravitated to. So he just told them to go as hard as they could. But I’ve discussed this with him and he agrees that when prescribing them for training, they do need some control.

I agree fully with Ryan that the most important thing is having them be consistent. When I do them or give them to an athlete, I want the average power for all four intervals to be within about 5 watts of one another. When you only have 2 minutes to recover between intervals, you’re really not getting that much rest. So, it’s a little easier than a 32 minute time trial, but not much. If you did a 32 minute time trial, you’d be pretty close to threshold power. So, these intervals, in order to keep the consistency, will be close to threshold power or just a little over.

And that brings up the other important facotr in this prescription - the recovery length. Doing 5x5 minutes with 1 minute recovery is a very different workout than 5x5s with a 5 minute recovery. So, during the base season, I like to give intervals like 5x5s and 4x8s with very short recoveries to really focus on threshold power. And that’s also why at that time of year I like to restrict my athletes to their threshold heart rate. As my athlete is approaching the season, I’d do what Ryan says and add some 1 minute surges into the 8 minute intervals.

In season, if I give intervals of this length to an athlete, I’d have longer recoveries and have them push wattage that’s over threshold. But again, I’d insistent on sustainability. They’d still all have to be the same power.


Hi Alex,

Jumping along with Ryan and Trevor’s comments, I fully support what they’re saying in terms of the aim is NOT to max out the first one or two intervals and then flame out on the 3rd/4th or even quit them. The goal is the max average wattage for all 4 (or whatever you had planned) intervals. This definitely take some experience and discipline on the first couple.

The other important thing that Ryan/Trevor mentioned is the recovery period intensity and duration. There’s a big difference between a 1 min and a 5 min recovery at 100W, or recovering at 180 W instead. That’s what goes into the maxim that interval training can be both extremely simple and yet difficult to execute.

But I think Dr. Seiler will agree that, in his polarized model, these 4x8 are meant to be HARD and not ridden as sub-threshold efforts. Such workouts are saved for the hard days.

Personally, that’s why I love the workouts on Xert (I’m its Chief Sport Scientist, so I’m biased and obviously have a conflict of interest). The workout is customized to your individual fitness signature (not just FTP), and designed so that they are both hard BUT doable for all the planned intervals. They also allow you to vary the recovery duration and intensity and will adapt in real time (i.e. if you pedal at higher than target wattage, the recovery duration will be shorter).

For example, here’s an example of a 3x8 min interval workout called “Gasoline” on the system. Each 8 min effort is above my threshold power throughout and has 4x2 min segments, which starts with a higher intensity and then ramps down. I really like it for what Ryan mentions, front-loading the effort so that you ride the rest of it in a state of fatigue. I also really like having a spike in effort every 2 min to replicate what happens in real life (e.g. rolling roads). That plus I HATE steady state efforts.

NB. The purple line is Maximal Power Available (MPA), which is an index of your fatigue in real time.


My question for these 4X8 min threshold intervals (with 2 min rest) is how low of intensity/HR is too low for ideal adaptation? If my LT2 HR is 168 I’m thinking I need to be 162-168 for threshold intervals. Where is the cross over from a good “sub-threshold” interval, to a “sweet spot” interval?

Hi @robertehall1, welcome to the forums! So you may get some slightly different perspectives on this, but personally when doing/prescribing sub-threshold work, I aim for about 93-98% of threshold HR or around 85-89% of max HR. That allows you to work in a range of intensities and focus on getting a nice HR response that makes finishing the workout achievable. Sweet spot would be near and below the lower end of that, dipping into more of a tempo effort at the lower end (85-92% threshold HR). There is no light switch that will immediately go from tempo to sweet spot or sweet spot to sub-threshold, so allowing yourself some flexibility to take into consideration the sensations in your legs and overall perception of the effort will make for quality workouts. I consider these middle zones more of a dimmer switch, and for that reason never give a well-defined crossover point based on HR alone.

As @ThermalDoc said, the 4x8 that are also utilized with Dr. Seiler’s polarized approach are meant to be hard, and these would likely send you above threshold. Completely achievable, yet much different level of effort, so what I think is important, to echo what Dr. Cheung also suggested, is that this takes experience and discipline. So learning how to do these at a variety of intensities (sweet spot, sub-LT, maximal sustainable) will give you great insight into how your perception of the workload impacts your ability to finish, recover, etc.

Coach Ryan

Great advice! I like that variation on the 4x8s - I’ve given those jumps to my athletes a few times (close to the season.) They are HARD but also great for turning around race form.

I’ve been doing structured training for some time but not polarized. I’m creating some 4 x 8 intervals in Training Peaks and would appreciate any feedback you may have on the power targets I’m setting. I have WKO with more than six months of ride data starting with a set of base tests. I thought for an 8 minute interval my target power should be in the range of 90% of my Mean Max Power at 8 minutes.
Does that sound about right?

Hi @CCR_TX and welcome to the forum! Thanks for your question.

So what is the goal of utilizing specifically the 4x8 intervals? Which area of your performance are you working on improving?

I’m going to tag Coach @trevor on this one as well to get his thoughts.

Coach Ryan

I’ve been doing all types of over threshold HIIT, e.g. FRC, VO2, and max effort 30 second over/under intervals for months. I was coached by a coach with Peaks Coaching Group from June through October. I stopped because I think I’ve learned enough to self coach and I need to pay off a new bike and smart trainer.
I’ve been doing sweet spot and endurance for sub threshold work. I also do one or two 3 hour group rides/week and routinely try to improve my best peak power at different times, e.g. 5 minutes. CTL has been in the 80’s
I’m retired so time isn’t an issue.
I’ve come to the conclusion that I’m spending too much time at too high an intensity. I want to try polarized training as an alternative and Seiler is well known for his 4X8 intervals so I thought I would do one of those/week, 1 group ride/week, and 3 days of LSD. I’m also doing strength training twice/week. I’m trying to take it easier for the rest of this month and next but keep my CTL > 65.

My goals are to improve my fitness and strength on the bike so group rides are more fun and to age as gracefully as possible. I’m 69 and have no interest in racing. On group rides I like to ride at a pace that allows easy socializing but I will also go off the front from time to time to attack a climb and then sit up and let the group catch up.

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I always thought that the Seiler 4 (3-5) x 8 minute intervals were done above threshold, typically in the range 103-110% FTP. These are hard intervals. I usually start with 3 intervals and then move to 4 and then 5 before upping the power. As Seiler says extend first then intensify

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Ride with slower people :wink: ! J/K

I like 4x8s and doing them different ways. The 8 minutes are great for my strengths and my focus threshold for workouts over a longer period. I do need to work myself to go longer. But I would first think about where you need to improve and focus the 8 minute efforts there - some constant effort related to threshold with the appropriate rest or “micro intervals” within the 8 minute blocks if you need to work on shorter higher intensity efforts like getting back onto a group, etc.

Thanks. It’s more fun when I’m stronger than my friends on our group rides. The problem is that they keep getting stronger, too!
I’ve been doing intervals at all different levels; sweet spot, threshold, VO2, FRC, and neuromuscular. I identify areas that need improvement by my Power Duration Curve. I find achieving new peak power bests for a specific time or time range very satisfying.
I want to take it easier for the next five weeks. I think I’ve been applying more than the minimum effective dose of stimulus and think the 80/20 approach might be a better approach for me as I have the luxury of time.
Come February I’ll start varying the types of HIIT I do and continue to put in a high volume of LSD.
I’m interested in learning how to use the data from my PDC to choose appropriate levels of intensity for my HIIT.

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Mean max power at 8 minutes in anyone’s PDC is always going to be above threshold. I’m just curious to learn if others think that setting the interval target power at 90# of your MMP for the length of the interval is a good practice.

I guess if you’re saying 90% of your maximal 8 minute power, then if you want to accumulate minutes then go for it. The more minutes you want to accumulate then I guess you would need to reduce the percentage even furthet

With me I look at the 20 of the 80/20 is my focus for a particular block. The 8 minute intervals (or any intervals) is considered the 20%. If I working the 8 minute at “threshold” +/ 10% all the time - and I have fallen into the that trap - I turn out have some great rides where I put out great power, but I have no variation in power. Which is great when I solo but has turned out bad on group rides, especially when my fitness in the 1-3 minute range is not trained - when I would make that big push to go up a hill or get on the back, it takes me too much time to recover and can get popped the next surge.

So, in my “base” period, I tend to work those 8x4s with about 2 minute rest around 90% of my “threshold”. But I do calculate threshold oddly since I never do a full hour of power to determine my “real FTP” and if I use the 95% of my 20 minute power I over estimate since I know I cannot maintain that for 30-40 minutes on most days. I tend to look at a combo of my 3 month best 1 hour and try to find a mid-point and work there when I am doing my “hard” 4x8s with a focus of working “threshold”. If I feel that I am working hard, keeping power and cadence fairly consistent, keeping good form (if I am not keeping good form, then I am probably more in the Vo2Max efforts) , and really wanting to quit (but able to hang on) the last 4 minutes of set 4, then I know I am at the right power. If things seem too easy or hard, I know I need to adjust the next time (or since I have done it enough, I can adjust within the workout).