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Intervals 101

Intervals are a fundamental component of training. They enable athletes to add intensity in a controlled manner, and target specific energy systems.

Intervals are a critical training component for any athlete looking to generate adaptations and improve fitness and performance. In this workshop, Coaches Trevor Connor and Ryan Kohler explain the importance of intervals.

There are three major attributes of intervals that athletes can’t get from training races. First, intervals allow an athlete to add intensity in a controlled manner. Second, the efforts target specific energy systems. Third, your heart rate response allows you to better understand what is really going on inside.

For a more detailed analysis of intervals, watch A Deep Dive on Interval Execution.

Video Transcript

Ryan Kohler  00:00

Hey everybody, it’s Coach Ryan, here with Coach Trevor, and today we’re going to talk to you about intervals.


Intro  00:14

Welcome to Fast Talk Laboratories, your source for the science of endurance performance.


Trevor Connor  00:25

So, Ryan, let’s start with the real basic question, what are intervals?


What Are Intervals?

Ryan Kohler  00:29

Well, let’s look at some intervals. So, as you know I’m pretty partial to my VO2 max intervals, but for your sake today, we’re going to draw your famous five-by-fives with one-minute rest.


Trevor Connor  00:38

I appreciate that.


Five-by-Five Intervals

Ryan Kohler  00:40

So, this is our interval we’re going to go up here to threshold, take it out roughly five- minutes, bring it down for a short minute, and then bring it back out. Ideally, like we said, this would be a five-by-five, so we would repeat this a few more times and that would extend out. So, let’s talk about the structure of these intervals. Now that we know generally what this looks like, what are the components of the interval? So, one is we mentioned this earlier is the intensity. So, we’re at 100% this is our intensity, how hard are we going? The next piece is the interval length. The other component down here is our rest period, how much time are we taking in between intervals? Then how many repetitions total? So, we said this is your five-by-five, so we would have one, two, three, four, five to continue out. That’d be the other component of building an interval set. And then finally, how frequently do we do this? So how many days per week?


Trevor Connor  01:37

Great. So, that’s what intervals look like, that’s kind of the basic. So, Ryan, is there another way to get this sort of intensity?


Unstructured Intervals

Ryan Kohler  01:45

There is. So, let’s go over here and talk about unstructured intervals, and we’ll use the Zwift race as the example. Now, if we start off here in a race, and we ramp up that power, we’re going to end up somewhere around threshold or above, and then we’re going to spend the rest of the time up and down all over the place until the end. So, that’s what this would look like.


Trevor Connor  02:05

That really, really hurts.


Ryan Kohler  02:06

It does.


Trevor Connor  02:08

So, are there advantages to doing this?



Ryan Kohler  02:12

There are. So, the first one, if we think about racing, well, racing is fun. So, that would be our first advantage. This is a fun way to achieve that intensity. The other piece is that it’s hard. So, if we look at this, we spend a lot of time around threshold, that’s going to be really, really hard compared to a structured interval set where we go hard to some degree, we rest, and then we repeat that in a very structured way. This is just we might be going hard for longer than we thought, we might be going easier for less time than we’ve planned on, but overall, we can go really, really hard during these. So, we have advantages for unstructured racing. What about structure? What are the benefits of doing structured intervals?


Trevor Connor  02:54

Yeah, so let me just give a little bit of an Asterix here. I actually love doing this. So, as much as I love to give my athletes intervals, this is, as you said, it’s fun. I know a lot of athletes who say I really like to do this, because of this element right here, that they just can’t go that hard when they do intervals. But there are benefits to intervals, and the biggest one is that you can target specific energy systems.


Overload Principle

Trevor Connor  03:18

Let me show you why that is important, and we’re going to go right back to this is the fundamental principle of exercise physiology of training adaptations, which is the overload principle. So, right here, we have your overall performance level. So, let’s say this dotted line is where you’re currently at. Here’s what the overload principle looks like, you are kind of sitting at your current level, you do a whole lot of hard training, that actually lowers your level, because you’re doing some damage. If you do enough damage, your body goes, boy, I don’t like that, and then when you recover, your body’s going to repair but it’s going to super compensate to say, I can make you bigger and better and stronger so the next time you do that to me, it doesn’t hurt so much. That right there is the overload principle, that’s how it’s drawn. There are other ways that this can take shape. One is you do a little bit of damage, so not nearly as much your body goes, yeah, I have to repair from that it hurt me a little bit, but I can handle that. So, your body’s just gonna go right back up to where it was, and not improve. Another way is you do a ton of damage that your body can repair from, but it’s so much that you’re just going to kind of come back up to the same level again, and then the worst-case scenario this is your overtraining syndrome is you do so much damage that your body can’t really repair from it, you don’t do enough recovery, and you actually come up to a lower level. That’s what you don’t want. So, the ideal is this one right here. Now, the way I love to explain this to athletes is to think about this as units of damage. So, I’m going to use a little red dots here to say, there’s one unit of damage, there’s two, there’s three, four, five, six, seven, eight, nine. So, the way to think of this is, if you do about that five units of damage, that’s when you’re going to get that nice super-compensation, if you only do a couple units of damage, you can kind of stay the same level, you get down to that nine, you’re in trouble. So, my issue with this, where this can lead to is, you could do a whole lot of damage, it could take you down lower to that eight, nine, ten if you do a ton of that. And certainly, you talk to guys who go and do a lot of racing, they’re just struggling to recover, they’re just struggling to get back to their other level. Here, what I like about interval work is if you target a specific energy system, you can really do all the damage to that one energy system. So, you can say, I’m doing threshold work, right now, I’m really trying to hit that anaerobic threshold, improve it, maybe I’m trying to improve my lactate clearance, so I’m going to do an interval work that’s going to give me four or five units of damage just in that one energy system, and then you’re going to get that super-compensation. So, at any given point in your training, you’re just hitting one or two energy systems, so you could really hit them, but the overall damage is something that your body can handle. Again, going back to here, you’re hitting every energy system at watts. So, there are two choices. One is, you go do a 30-minute training race, I see this a lot, you do a little bit of damage to every energy system, so the problem is, while you’re maybe doing that total five units of damage that your body can handle, it’s just one unit of damage to each energy system, and those energy systems are not going to super compensate. So, I see this a lot with people they do a ton of these 30-minutes Zwift races, they feel great, but you go are you getting stronger, it’s, I seem to be kind of the same level. The other way to do it, look, sometimes you have to do this I was a race, raced the professional races go and do these six, seven days stage races, you hit you do enough damage every energy system, but the total damage to your body is huge. That’s where you’re getting down to that eight, nine, ten units of damage, then you’ll literally have to take a week off the bike just to be able to recover from that. So, it’s really hard with this to do enough, you’d have the damage on a specific energy system to really hit it hard.


Ryan Kohler  07:26

Yeah, so that really makes the case for structured intervals. Are there more benefits to structured intervals?


Structured Intervals

Trevor Connor  07:32

Yes, actually there is it. This is almost saying this as a coach, but it could help athletes a lot as you can learn this, which is you can really see what’s going on, and it’s going to help you to figure out how to best structure that work. And so, I’m the heart rate guy, I admit to that I love heart rate, heart rate is always read, so let me grab my red marker here. I’m going to give you one example of the way that you could really see what’s going on with an athlete using interval work that he couldn’t see over here, because frankly, over here, heart rate is gonna be like power, it’s just gonna be all over the map, and you can look at that and go, wow, you had hard, that’s about all I can tell an athlete when I look at that. Over here, let me show you these two examples. So, you’re doing the intervals, your heart rates down here, you get to the start of the interval, you see this slow rise in heart rate that kind of looks like this, comes down a little bit, then you have that slow rise again. So, there’s one way you can see that the response to these intervals. Here’s another one, you get to the intervals, heart rate comes up really rapidly kind of levels off, get to the end of the interval drops down a lot, you do the next interval, comes right back up levels out, drops down.


Ryan Kohler  08:58

Yeah, these are vastly different looking. So, what does one tell you versus the other one? What can you take away from both of these types of heart rate profiles?


Heart Rate Profiles

Trevor Connor  09:07

So, what you’re seeing here with this first athlete, where it’s a slow rise, that tells me they have an underdeveloped aerobic energy system, they’re really relying on anaerobic energy. So, there’s actually a term in exercise physiology, if you filled this in, this is what’s called oxygen deficit.


Oxygen Deficit

Trevor Connor  09:27

So, when you start going hard oxygen deficit is that like the time it takes for the VO2 to come up basically for the aerobic system to respond. So, with this athlete, you’re seeing that oxygen deficit is very large. With the other athletes, it’s actually quite small. So, that first athlete relying on anaerobic energy metabolism, their aerobic system is underdeveloped. So, this other athlete you can see there’s oxygen deficit is a lot smaller, what you’re seeing as they are really relying on their aerobic energy system, so this tells me, this athlete has a well-developed aerobic system, this athlete doesn’t. So, this particular athlete, I would say, this is a really good interval workout for them to do really get that aerobic system revving, build that up because it seems to be a weakness for them. That’s just one example, but this is the sort of thing that you can see with interval work that you can’t see in this giant kind of mess over here. So, Ryan, this is actually been a really kind of short, quick, here’s what intervals are about and why you should do them. Is there anything else that we should know about?


Ryan Kohler  10:34

Yes, so we’ve only talked so far about one type of interval here, and we’ve talked about the different variables that go into building the interval. So, this example, we use a five-by-five threshold. So, what tends to happen is that at times, people would only focus on say, the intensity portion.


Importance of Rest Period

Ryan Kohler  10:51

So, let’s look at two intervals and show how important the rest period can be for affecting the outcome. So, I’m gonna go back to my favorite types of intervals now.


Trevor Connor  10:58

All right.


Ryan Kohler  10:59

And we’re gonna go shorter. So, let’s look at a Tabata style where we’re doing 20- seconds on 10-seconds off, and these would be very short, very highly compressed intervals, and this would carry on for a few sets. So, this is our 20-second work period, here’s our short 10-second rest period in between, but let’s compare that to a sprint workout where we go here and we do the same 20-second sprint, but then we might take five, six, seven minutes of rest in between, and then do another 20-second sprint. Very different workouts, but this is one example of how the rest periods can dramatically affect how that workout actually feels.


Trevor Connor  11:35

Right. In terms of intensity, they both go as hard as you can.


Ryan Kohler  11:38



Trevor Connor  11:39

But it’s got to be very, very different intensity because of that rest.


Ryan Kohler  11:43



Ryan Kohler  11:44

Thanks for joining us today. Hope you learned a bit about intervals. Remember, this was only the tip of the iceberg, there’s a lot more to learn. So, if you’d like to dive deeper, check out our intervals pathway at Thanks for joining us.