Runners: 3 mistakes you’re making in your interval sessions
Interval sessions are a key part of most good training programs — they build the strength and power you need for racing, without the overall physiological strain of maintaining race pace continually. They also yield improvements in the all-important VO2 max and in running economy at race pace. You are probably doing intervals already, and you should be, but you are probably doing them wrong. Here are 3 reasons why:
Mistake 1: you’re doing intervals of constant duration
There is an elegant simplicity to session structures like “8 * 200m, 2min rest,” and that kind of effort for sure has its value, but there is no fundamental reason to assume that a linear structure optimally induces training stimulus. This study by Vaccari et al found that a series of intervals with decreasing duration allows an athlete to spend more total time within the range of 90% to 100% of their maximum oxygen uptake.
Intuitively, this makes sense; in a square structure, for the first few intervals your body will be accessing stored energy resources (ATP mainly) and not maximally processing oxygen, and only maxing out oxygen usage once those resources are depleted.
In other words; if your intervals are all the same length and intensity, the early intervals will be easier than the later ones. Either the early ones will be too easy or the later ones will be too hard. You can only consistently hit the ideal intensity if you decrease the duration of successive intervals.
The protocol used in the Vaccari study is a perfectly fine structure, but not optimized; it’s a sequence of 3min (2min), 2min (80s), 1min (40s), 40s (27s), 30s (20s) repeated until exhaustion. In the above, the first number is a duration at above 90% of VO2 max effort and the second is the duration of easy intensity rest in between. The participants in that trial were of moderate fitness, and better-trained athletes could certainly tackle a more challenging version, perhaps reducing duration in smaller increments.
Mistake 2: you’re standing still in your rest periods
Strava clout is such a prized commodity that American soldiers have compromised national security for those precious kudos. The price of Strava fame is far lower for us everyday athletes and weekend warriors, so it can be said without contestation that how athletes train is influenced by how they are going to look on Strava.
This is ruinous to interval training; in the past, the mark of a serious athlete doing intervals was not so much how FAST they ran their intervals, but rather HOW they went about them. Serious athletes would have brief active rests between intervals, jogging while while their heart rate dropped back into survivable ranges. Casuals, on the other hand, would treat an interval session as a series of well-rested sprints, pampering the ego and letting the cardio-pulmonary system off easy.
The value of jogging your rest breaks is enormous and multi-faceted. Firstly, the idea that you can still run (or at least jog) even when feeling awful provides psychological reassurance when it comes to events like a marathon, where the main task is precisely that of running (or at least jogging) while feeling awful. The biggest advantage is more fundamental, however; keeping the cardio-pulmonary system in the active zone for the entire duration of the session provides training stimulus to the full range of energy systems, not just the glycolytic system that is stimulated during sprints. There seems to be something magical about being able to simply drop a couple of gears to recover, rather than needing to stop completely, and it establishes that recovery pace as a sort of baseline speed that your body can tolerate even under considerable stress.
The incentive of doing intervals “properly” so that you can go on to win races has been supplanted by an incentive of taking lengthy rests and standing still so that Strava doesn’t add them to your moving time. The solution is to just suck it up and focus on elapsed pace rather than moving pace, and endure the shame of your interval sessions showing a pace you can actually run.
Mistake 3: you’re only doing only one set
Mike Mentzer devotees aside, scarcely anyone doing strength or power training would suggest doing a single set and calling it a day. The reason for that is quite simple — the body operates using several different energy systems, which kick in to different degrees at different intensities, and sometimes the limiting energy system (the one that runs out of steam and makes you quit because you can’t maintain the intensity without it) isn’t the only one you want to train.
In the case of interval training, generally what brings you to a stop is that your blood has reached the maximum concentration of lactate, a by-product of the anaerobic glycolytic energy system, that the body will tolerate. After a period of time, that lactate will be cleared (primarily by the liver, assisted by some other mitochondria-rich tissues), and the body will be willing to resume effort. Failure-by-lactate isn’t generally the objective of interval training; instead, interval sessions are aimed at adapting the body to operating at or close to maximum oxygen uptake*.
For this reason, interval sessions tend to be structured so as to operate close to maximal oxygen uptake but with breaks to allow lactate to clear. As mentioned before, it’s best to keep moving during these breaks, for a range of reasons but mainly so that oxygen continues to be used in the muscles during the rest periods, since that’s the entire point of the session. So-called ‘oxygen debt’ means that jogging or easy running between intervals can extend the period of maximized oxygen uptake.
By the end of a set of intervals, a range of different types of muscle fatigue will have set in, breaking that cyclical equilibrium between exertion and rest. However, most people will discover, if they attempt it, that if they spend ten minutes or so recovering after a set of intervals, they are able to do another set of intervals, albeit sometimes a slightly less substantial set than the first one.
This significantly increases training stimulus and, for anyone not currently doing this or something similar, tends to yield rapid and significant increases in fitness, efficiency and endurance.
I like to do a ‘recovery ladder’ for that rest break. This consists of a progression of short jogs with 30 second walking breaks between them, starting with 30s of jogging and increasing each subsequent jog by 30s until you get to 2 minutes. This process keeps the legs perfused with blood while not stressing oxygen uptake, allowing recovery, and brings the heart rate down to a manageable level. Having completed that ladder, I find I can generally repeat the set of intervals, though often I’ll find that intensity declines by a shade.
Conclusion: my personal favourite interval session
My favourite interval session ties all these principles together and adds in a secret ingredient: hills.
I use a hill that is about 500m long with gradient around 6% (these are entirely flexible and I do sometimes use something shorter and steeper) which happens to have 11 lamp-posts along its length. For the first hill interval I will run past all 11 of them on the way up but then, when jogging back down, I turn around at the 10th before heading back to the top. I repeat this, shaving off one lamppost at the bottom each time.
This results in a structure similar to that used in the Vaccari study, in that each interval is progressively shorter and the rests are always shorter than the intervals (this is achieved by shaving the lampposts on the way down rather than on the way up). Once I’ve run out of lamp-posts, I will do a recovery ladder and then repeat it all from scratch.
This session yields amazing and rapid fitness improvements for me, and it’s got a lot of room for customization. Try it out, keeping your eye on elapsed pace and avoiding Strava’s moving pace trap.
*as to why lactate, a product of anaerobic metabolism, is produced at effort levels below maximum oxygen utilization, that is a fiddly matter of chemical driving forces which isn’t intuitive to everyone. Perhaps the simplest way to sum it up would be to say that transfer of oxygen is driven by concentration differences between blood and muscle, and that transfer rate is only maximized when oxygen levels in muscle tissue dip well below normal.
