Coach Trevor Connor takes a deep dive into an interval workout that you’ve heard us discuss many times on the Fast Talk podcast—4×8-minute intervals—and one that has been popularized by Dr. Stephen Seiler.
Coach Connor explains the physiological benefits of this workout, how to properly execute them, when to do them, how often to do them, and much more.
The workout file downloads include .ZWO for Zwift, .ERG for TrainerRoad and others, and .MRC for TrainerRoad, Computrainer, and any others.
Video Transcript
Dr. Stephen Seiler’s 4×8 Intervals
Trevor Connor 00:00
Today we’re going to take a deep dive into an interval workout that Dr. Stephen Seiler has become famous for. This is his 4×8 minute intervals, he’s done research on them showing there’s a lot of gains. So, today, we’re going to talk about how to do them, why you should do them, when you should do them, and we will show you good execution, we’re also going to show you bad execution. So, let’s dive into it.
Intro 00:34
Welcome to Fast Talk Laboratories, your source for the science of endurance performance.
Intervals for Raising That Aerobic Engine and Threshold Power
Trevor Connor 00:45
So, as I just mentioned, one of Dr. Seiler’s favorite intervals, he has done a fair amount of research on these, and you can see the results of one of his studies where he compared 4×4, by 4×8, by 4×16, and I’m not going to go too deep into this, you can actually look at the study if you want, but you can see here, these 4×8 look pretty darn good. They improved a lot of the markers that you’re looking for, if you’re trying to build that aerobic engine, so there’s your VO2 max, here’s your power at four millimoles, you can see a lot of improvements with these intervals. In his study, this was about seven weeks, so great intervals for raising that aerobic engine, raising that threshold power. One of the other nice benefits of these intervals is because it’s just a two-minute recovery between each eight-minute, you don’t really have a lot of time for that aerobic system to come back down, so in many ways, you’re getting the benefits of going out and doing a 32-minute time trial, but with those two-minute recoveries, it allows you to do each eight-minute segment a little higher quality, a little bit harder, so you’re going to get greater gains out of it.
How and Why You Should Do 4×8 Intervals
Trevor Connor 01:55
Here’s what the execution looks like, this is straight out of the fit file, which we’ll post on the website if you want to download this, but you can see we have four of these eight-minute intervals, it’s a tight range in terms of wattage, then two-minute recovery where you really want to go easy. I want to give these to my athletes, I give them both power targets, but also give them a heart rate target. So, say your threshold heart rate is 172, I tell all my athletes, you can’t break 173, I want these intervals to be controlled, I want them to be a true aerobic effort. That’s slightly different for Dr. Seiler, he prescribes them basically just saying, do them as hard as you can, but sustain, because it’s very important that whatever your wattage is on the first interval, that’s your wattage is for all four including that last one, you don’t want to see a drop. Now, if you can do them all the same wattage, I find the athletes will do them right about where I prescribe them as well because if you’re bringing a lot of anaerobic energy in those first two, you’re going to tank later on. In terms of cadence, I like to have my athletes do these around 100 RPM, especially if they’re doing them on the trainer, but as I get closer to the season, that’s where I’ll say no, do these at a race cadence, say 90 RPM. Finally, it’s important to know these intervals are not a quick turnaround, you can’t do two weeks of them and see some major gains. So, we’re looking at 8 to 14 weeks to see the benefits, I think you’re going to see most of the benefits after eight weeks, but if you want to see the full gains, you can go up to 12-14 weeks, so great bases and interval, and you should be doing them two times most weeks.
Example of Good Execution of 4×8 Intervals
Trevor Connor 03:48
So, here’s an example of good execution of these intervals. So, notice wattage is pretty consistent across all four. I think this was one of my workouts, and I’m pretty, pretty careful about that, you’ll see all four of my intervals be within two, three watts of one another. Now, look at the heart rate response, nice flatline, you’ll notice the first interval, I don’t even touch that threshold heart rate, by the second interval, I’m just starting to touch it, and then these last two, maybe just go a little bit over but not a lot. So, again, I had my threshold heart rate of 171, I don’t think any of these went over 173, so that’s important. If you’re hitting that threshold on the first one, or you’re going way over, you’re going too hard, you need to bring that power down and remember that for next time remember that feeling. Up here you’ll see a graph of watt prime, that’s the purple line, that’s basically you have that tank of anaerobic energy, and the idea is once you start going over threshold, you start depleting that tank. So, these intervals because there are aerobic, we really don’t want to see much of a movement in that line. So, you see little points where it drops down just slightly, but that’s good execution, we’re not really tapping into that anaerobic system.
Example of Bad Execution of 4×8 Intervals
Trevor Connor 05:14
So, I’m going to show you a couple of examples of bad execution. So, here you can see this athlete going way over their threshold, so that’s colored in purple, this yellow line is their threshold power. You can see already that first interval, heart rate is going over threshold, notice how it never really plateaus, it keeps rising, that’s a real good indicator that you aren’t relying mostly on aerobic systems, you’re bringing in a lot of anaerobic energy, when that happens, you see that slow creep in the heart rate throughout the effort, you really see it here, this athlete is well above threshold, really tapping into that anaerobic system, which also shows up here in that watt prime, they’re going too hard, this is not sustainable. So, notice by the third interval, that power is coming down, by the fourth interval, they got about a minute or two in and just went, I’m done. That’s not the right execution. Remember, it’s got to be a power that you can sustain across all four, this athlete didn’t do that. Here is an even worse example of that. This athlete absolutely killed themselves in that first interval, trying to get that heart rate up, you could see they completely tanked their watt prime, by the second interval it’s coming down closer to threshold, but by that third and fourth, they’re not even getting up to threshold, they are completely done, they’re basically just trying to struggle through. Now, notice, therefore I saying you have to use RPE, power, and heart rate to execute these correctly, because in this case, if you just look at heart rate, they’re hitting threshold every time they would look at this and say, “Well, that was pretty good execution,” but you can see with the power is not the case at all. In that previous example, I showed you, first two the power was pretty steady, but the heart rate was telling a different story with that slow rise throughout each interval. So, you have to look at all these things to be able to execute these intervals correctly, otherwise, this is what you’re going to end up with.
Trevor Connor 07:23
So, also really important to know that these intervals can give you a bit of an indicator of your fitness, what energy systems you’re relying on. So, believe it or not, both of these are actually pretty good execution. This one wattage isn’t quite as steady as I would like to see at certain points, but overall, a decent execution. This is a good execution up here but notice a difference in the heart rate profile. So, here again, is that nice flatline, notice after each interval, it drops down pretty far, as soon as the athletes started up again, that heart rate just comes right back up. This is an athlete with a very strong aerobic system, and they are really relying mostly on aerobic energy to do these intervals. Down here, you see something very different, I’ve already talked about this notice, slow rise throughout the interval, it doesn’t come straight up like you saw up there and never really levels out. That’s an indicator this athlete is tapping into a lot of their anaerobic energy stores to execute these intervals. Now again, I’m not going to say that’s poor execution, I’m going to say they have an underdeveloped aerobic system, and this is a really good interval workout for this athlete to do to improve that aerobic system so that when they’re doing efforts like this, they’re not going to be fatiguing themselves as quickly, they’re not going to be depleting that watt prime in a race. So, if these two athletes were racing one another, even if this power was the same, what you would see is this athlete here is never really fatiguing, they’re relying on their aerobic system. This athlete has to keep tapping into their anaerobic system, and pretty quickly this athlete is going to drop that athlete.
ERG Mode vs. Self-Pacing
Trevor Connor 09:06
So, I get asked this a lot, should you do these in ERG mode, or should you control the wattage yourself? My general recommendation is early in the base season, I always give these workouts to my athletes in the bass season, early in the bass season user ERG mode, get the job done, go home, that’s fine. As you get closer to the season, I do want them to turn ERG mode off, I really like them to get outside and execute again, but self-paced, control the pace, and again, try to get that nice, controlled wattage. So, the important thing is whether you’re doing ERG mode or whether you are self-controlled, that’s no excuse for execution, you still have to execute correctly both times. So, notice down here, this athlete was controlling their wattage, but boy did they do a good job, that almost looks like ERG mode, that’s what you’re going for. So, you can get the feel for it in ERG mode, but later on, see if you can repeat that yourself controlling your own wattage, we still want to see that good heart rate line if you’re not doing that work on your control. The same thing in ERG mode, so this again goes back to you have to use heart rate, you have to use power, you have to use RPE, you can’t just say, “Oh, well, I’m just going to lock it in at 290 watts, and it’s going to be whatever it’s going to be,” because sometimes you’re going to feel really good and 300 watts might be right other times, you’re going to feel pretty bad and 275 might be right, sometimes that 285-290 might be right, and you have to gauge that. So, here’s an example of an athlete who did this started at the wattage he had done the previous week but notice his heart rate was not even getting close to threshold in those first two intervals. So, he said, got to bring it up, so notice you see the change in the color here, that’s because he increased his wattage. So, most trainers even if they have that ERG mode, you have the plus-minus to bring the power-up, he brought it up a little bit, and notice by those last two intervals, now he’s getting up close to that threshold heart rate that we want to see. So, he was using all the indicators to say today, the wattage I use last week is not correct, I need to go higher.
Sustaining Power Throughout the Intervals
Trevor Connor 11:22
So, final notes about these intervals, they are great for raising your sustainable power, but then I think you need to do some work to learn how to actually sustain it. So, I did have an athlete who was a time trialist, he was focusing on nationals, we gave him some of these intervals, we actually gave him 5×5 minute intervals, but 4×8 could do the same thing. He got his threshold power way up a good 50 watts higher, but then when he tried to do his first 40k, he wasn’t used to sustaining that power for that length of time, and he had a pretty bad race. So, do these intervals for a while, but if you’re a time trialist is looking to do a 40k or 20k TT, you then need to go out and practice sustaining that power. So, if you got to the end of those 4×8 and let’s say you are holding in those eight-minute intervals 300 watts, I’m going to tell that athlete to go out and do a 20-30-minute effort, maybe repeat two 20-minute efforts trying to do that same wattage to keep it around that 290-300, learn how to sustain it. As I mentioned before, this is also a good base season interval because A, it’s not too hard, if you are absolutely killing yourself in all four, you’re probably going too hard. The first few should feel like I could have gone a little bit harder, it’s only that last one where you go okay that was a bit of a struggle to get through, but you should never be dying in these, so I like that and the base. The other reason this is a good base interval is that it takes that long time to see the gains, 8 to 14 weeks, if you’re doing that in the middle of the season, your season might be over before you’re getting all the gains. So, a good thing to start in say December or January and be seeing those full gains by the time the race season starts.
Trevor Connor 13:12
Again, we’ll post the fit file so you can download this, put it on your bike computer, put it on your trainer, and give these intervals a try yourself. So, now you know how to execute 4×8 minute intervals. Love to hear what you think about them, go to forums.fasttalklabs.com to tell us about your experience, share screenshots of your intervals, ask us your questions about them, we really want to hear what you think.
