“….the next time you hear about someone’s epic interval session, be sure to find out what the rest interval looked like if you want to gain a true understanding and appreciation for what type of session it was.“
Interval workouts have long been considered a useful training modality in the runner’s toolkit. Often, runners and coaches will share their favorite workouts, or hardest workouts, etc. Whether it is a mind-numbing repetition of the same duration like 20x400m, or ladders so complicated that the runner can hardly keep the workout straight, the discussion typically centers around the distances and number of repetitions. The factor that I want to talk about today is, to steal a phrase from Malcolm Gladwell, overlooked and misunderstood: The rest interval.
Most people assume that the longer the rest interval, the easier the workout. While a longer rest does in fact allow the athlete greater recovery, that increased recovery time also allows the runner to complete the subsequent repetition at a higher intensity. In other words, the shortening the rest interval actually requires the intensity of the workout to be lower…though not necessarily subjectively easier. The duration of the rest interval, in other words, is critical in determining what intensity you will be able to sustain, and thus, what type of training adaptations you may expect from the workout.
There are no hard and fast rules here, but I am going to use the 400m repeat example to illustrate how altering the rest interval creates vastly different training stimuli. None of these is the “best workout” or even the hardest workout – that may vary depending on individual and the execution of the workout.
Workout #1: Let’s start with probably the most common type of 400m workout that distance runners would be familiar with, where the pace is 3k or maybe 5k pace, and the number of reps is in the 10-15 range. This falls into the category of what people call a “VO2max pace” workout, in the parlance popularized by Coach Jack Daniels. The rest for this type of workout would typically be a minute or a bit longer, perhaps an equal time to the repetition time. Some people may jog a 200 rather than standing around. Generally, the rest feels like quite a bit of time early in the workout, but the recovery is not complete, and thus the rest interval begins to feel more and more parsimonious as the workout continues. This is a workout that is going to allow the individual to run at a fairly high quality throughout, but not to sprint. Heart rate will be up near it’s max range, and by the end of the workout, it will still be quite elevated at the start of the reps due to the cumulative fatigue.
Workout #2: Now let’s increase the duration of that rest period. We could call it 3 minutes, give or take. This is going to allow us to run substantially faster than we did in the previous workout, and still be recovered for the next repetition. These types of workouts are commonly done by mile/1500 runners, often targeting their race pace. Each rep is pretty hard, but the longer rest allows the maintenance of that higher intensity. Likely 8-10 reps of this intensity will be enough, but you can see already that higher amount of rest changes things dramatically. The VO2max pace workout will dip a bit into anaerobic territory, but this one lives there, with such a high energy demand that it’s no longer a primarily aerobic workout. Rather, it will teach the athlete to produce the high speeds needed to race the mile, and the stimulate the physiology to deal with the dramatic consequences of the energy demand that is needed to do so.
Workout #3: Finally, let’s cool things off a bit, with an entirely different 400m rep workout. This one may be a higher volume (20+ reps), lower intensity workout typically at about a threshold effort. Threshold (or tempo) effort is described as moderately (or comfortably) hard, so you would think 400s at this pace would be a piece of cake. And you would be right, except that the rest interval in this type of workout is restricted to something like 15-20 seconds, barely enough time to recover at all. Indeed, there are times in these workouts where it feels like more work to slow down and accelerate again than it would cost to just keep going. As you may imagine, a miscalculation of your pace can be disastrous in this type of workout. Because of the short rest, the physiological adaptations are similar to those you might expect from a longer, continuous effort, but breaking it up into intervals makes it an entirely different psychological experience.
So which one is harder – the short rest and lower intensity, or the long rest at a higher intensity? That will depend on your training, your physiology, and how you execute the workout. Which is better? Again, they all have a purpose, so the best workout depends on the effect that you are aiming for with the workout. So the next time you hear about someone’s epic interval session, be sure to find out what the rest interval looked like if you want to gain a true understanding and appreciation for what type of session it was.
Chris Lundstrom is the head coach of Minnesota Distance Elite – formerly Team USA Minnesota – which includes some of the top distance runners in the USA, including Annie Frisbie, Dakota Lindwurm and Joel Reichow.
