My eye started twitching a little this morning when my training partner suggested we aim for nine repeats up the hill in front of us. I’ve long said that if you see a training log full of workouts with odd numbers like nine and 13, you know the athlete has a problem dialing in the right intensity and finishing what they start. Fortunately, I came up with a mental solution: it’s three sets of three, I told myself. That’s a perfectly rational and intentional workout.

I’ve been thinking a lot about the details of how we prescribe interval workouts because of a massive doorstop of a book called Science and Application of High-Intensity Interval Training that I got recently. (To say that it’s 664 pages long undersells its heft. Amazon lists its shipping weight at 4.6 pounds.) It’s edited by sports scientists Paul Laursen and Martin Buchheit, and includes contributions from more than 30 experts from Olympic teams, pro sports leagues, and universities around the world. It goes into a lot of detail.

I’m not going to engage in the hopeless task of trying to summarize a book that has separate chapters (numbers 28 and 29) on how to apply interval training for rugby union versus rugby sevens. For that, I suggest you pick up the book, check out Laursen and Buchheit’s website, and perhaps sign up for their online course. But there are three high-level principles in their framework that are interesting to think about as you plan your own training.

The first is to consider what you’re hoping to get out of your training session. Laursen and Buchheit identify three key physiological responses that interval training can trigger: aerobic (pushing your VO2max, say), anaerobic (high lactate levels), and neuromuscular (muscle recruitment patterns, soreness).

Before you can decide what the appropriate workout is, you have to decide which of those three elements you want to trigger. It’s not an all-or-nothing choice: you can do workouts that target any two or even all three elements. But the goal, as Laursen explained to me, is to target your workouts with “Navy Seal precision” rather than the usual WMD approach that wipes you out in every respect. If you’re a soccer player doing mid-season conditioning, you don’t want a workout that leaves you with a bunch of neuromuscular fatigue that will compromise your performance in a subsequent game, so (as we’ll see below) you should stick to short intervals.

With that in mind, Laursen and Buchheit subdivide interval workouts into five basic types:

Long intervals , typically lasting 2 to 5 minutes with 1 to 3 minutes recovery.

, typically lasting 2 to 5 minutes with 1 to 3 minutes recovery. Short intervals , lasting 10 to 60 seconds with less than a minute recovery.

, lasting 10 to 60 seconds with less than a minute recovery. Short sprints (“repeated sprint training”), lasting 3 to 10 seconds with less than 45 seconds recovery.

(“repeated sprint training”), lasting 3 to 10 seconds with less than 45 seconds recovery. Long sprints (“sprint interval training”), lasting 20 to 30 seconds all-out with 1 to 4 minutes passive rest.

(“sprint interval training”), lasting 20 to 30 seconds all-out with 1 to 4 minutes passive rest. Game-based high-intensity interval training, which in team sports typically involves small-sided games interspersed with rest.

Each of those workouts has different characteristics, which can be further tweaked depending on your goals. Long intervals, for example, will give you an aerobic and anaerobic stimulus, and can also trigger neuromuscular fatigue if you do enough intervals. Long sprints, on the other hand, will predominantly hit the anaerobic and neuromuscular systems.

So how do you hit those various systems? Laursen and Buchheit have a nice flowchart that takes you through the decision:

Got it?

Okay, don’t worry if it doesn’t make sense immediately. Basically it’s a decision tree: you decide if you want aerobic, anaerobic, and/or neuromuscular strain, and it tells you what types of workouts are appropriate. Notice that it’s not a single unique answer: for example, short intervals (shown on the chart as HIIT Short) can give you aerobic, aerobic plus neuromuscular, aerobic plus anaerobic, or aerobic plus anaerobic plus neuromuscular stimuli.

That, as it turns out, leads to the third big principle: each workout type is fully customizable. The indefatigable Laursen and Buchheit identify 12 variables you can tailor to make a workout suit your needs. They have another flowchart illustrating these factors, but the gist is as follows:

For each given interval, you can choose how hard (1) and how long (2) it is.

For the recovery period between each interval, you can choose how hard (3) and how long (4).

How many intervals in each set (5), and how many sets in total (6).

How long (7) and how hard (8) the recovery between sets is.

The total training volume of the workout (9).

The type of workout, like run versus bike, road versus trail, etc (10).

Environmental factors like heat and altitude (11).

Your nutritional status, such as whether you’re fully fueled or fasted (12).

Astute readers may notice that the ninth variable, total volume, is actually dictated by the first eight variables. It’s basically an overall estimate of the duration and intensity of the workout. The reason they list it separately is that they argue that it’s an important number to keep track of for the big picture of your training progression. In simple terms, if you’re used to doing 12 x 400 meter reps, and decide that you want to try some 1,600 meter reps, being aware of your total volume suggests that you’ll be able to safely handle about three reps, or perhaps four since the intensity will likely be lower.

So how do you put all this information into practice? That’s the 664-page question, of course. My workout this morning, which was three sets of three hills that took me a little over a minute to climb and two minutes to jog down, doesn’t fit neatly into any of Laursen and Buchheit’s categories: it’s somewhere between long and short intervals. In their framework, I suspect I got good anaerobic and neuromuscular jolts, but a weaker aerobic stimulus. With an 8K race coming up soon, that will inform my choices in my next workout: I’ll do a more classic long interval session. And I’ll definitely do an even number of reps.

My new book, Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance, with a foreword by Malcolm Gladwell, is now available. For more, join me on Twitter and Facebook, and sign up for the Sweat Science email newsletter.