Interval Fundamentals

You can train our mind, body, and craft. In recent posts on training your body, we skimmed the surface of training endurance (podcast, blog) and economy (podcast, blog). As I think about you developing your long range plan, I imagine that you’ve developed a plan for building your endurance because you extended your easy running and planned the right balance of stimulus and recovery to adapt sufficiently to be able to go the distance, or for the time, required for your goal(s). Maybe you’ve incorporated some heavy resistance training in the early parts of your training season, transitioned that to plyometric activities and incorporated sprints all to work on developing durability and running economy.

If that’s all you do, run a lot of miles, often on race-like terrain and run all out for short bursts a few times per week, you’re doing the majority of the work that will improve your body’s capacities. Maintain that consistently, increasing the weekly mileage or time slowly, until you find your upper limits for your lifestyle, goals, and overall health and you’ll do very well over time.

To further extend stamina. You may want to run somewhere between. Where it’s not so comfortable that you could keep going for many hours but not so hard that you fatigue in seconds. You could simply warm up, then hit a pace that feels hard, sustain that until you can’t hold the pace any longer, then cool down and call it a workout. Let’s say, depending on the exact effort, you might be able to sustain that hard section for a few minutes to nearly an hour. Do that once per week or once every other week and you’ll have a big training stimulus. Over time, that may be hard to continue doing, either due to the fatigue factor - it will take several days to recover from what is effectively an all-out race for whatever time or distance you cover - or due to the drain on your motivation - such efforts can be very taxing psychologically, especially when they aren’t real races. But, that’s an option.

More than a century ago, it was recognized that you can accumulate more time running at a relatively fast pace if you took brief rest intervals to recover, then got back to the faster effort. Instead of lasting just, for example, 20 minutes in a continuous effort, you could accumulate 30, 40, or even 60 minutes at that same speed if you did it a few minutes on and a few minutes off. Due to the rest intervals that broke up what would otherwise be a continuous run, this style of training eventually came to be called interval training. And, although, the intervals were originally conceived as the recovery periods between bouts, we now more typically call the hard bouts intervals.

It’s important to appreciate that the notion of accumulating more time at a given pace or effort as a superior training stimulus to necessarily shorter, but continuous bouts is not definitively and clearly demonstrated. It’s very logical and there are many reasons to think this should be true. There are, however, no clear answers from science. While many studies have been completed over the years, the variability in study designs, lack of sufficient control groups, and complications intrinsic in trying to make only one feature different among training interventions leave us only being able to say if you do exactly X, then it’s likely to be better than exactly Z for measurement Y. We’re not able to really extrapolate that to the sorts of answers I know listeners of this show would want.

What if, instead of running 6 x 3 minutes hard one day per week, you ran it once, just 3 minutes hard every day during your run six days per week? Proponents of interval training would argue that there’s something intrinsic to the repetition, to the repeating of the intensity in one session that’s important but the reality is the we don’t know for sure. What about running 6 x 3 minutes vs 3 x 6 minutes at the same intensity, such as one that you could sustain in a continuous bout for 20 minutes. Is there a benefit of one over the other vs just running continuously for 18 minutes. Your perception of effort will be different for each of those three workouts as would some of your physiological responses. From that we could propose that you’d have different adaptations, assuming there’s validity in the model of thinking that states: ’stress this measurement then everything that produces that value will adapt’ . But, in reality, none of that is as clearcut as many would have you believe and certainly not as clear as we’d all like. It’s an unfortunate reality that a lot, I’d argue even the majority, of sports science research is insufficiently applicable and/or poorly designed.

So, what do we do about it? For now, we’ll recommend intervals for harder work efforts and compressed into one or two session per week because our understanding of physiology and our appreciation of the realities of psychology, motivation and enjoyment in training long term conspire to make that the best candidate approach as a general recommendation for most runners.

Intervals allow the runner to accumulate more time at a given pace, effort, or work rate in one session than they could sustain if trying to maintain it in one long, continuous bout. The expectation is that accumulating more time at a work rate than is sustainable in one go will lead to greater adaptations and, perhaps, require less recovery time.

How fast or hard should these intervals be? To answer that question, we have to review some of the bioenergetics at various intensities of running.

First, a reminder about motor unit recruitment, or a primer if you didn’t listen to my discussion with Dr. Roger Enoka in episode 59. As you progress from running easy to running faster and faster, you engage more and more of the muscle cells (often called muscle fibers) in the muscles you use and you do so in groups of muscle fibers called motor units (that’s called motor unit recruitment). So, running easy - some motor units used, running faster - more motor units recruited. And there’s complementary increases in breathing depth and rate, in heart rate and stroke volume (and therefore cardiac output), among others. When you run an ultra-marathon, you aren’t recruiting all your muscle motor units because you’re not running very hard or fast. But, as you fatigue, to maintain your pace, you need to recruit more motor units. The motor units you recruit for very easy contractions are pretty economical but the ones you recruit for faster paces, greater force, or to maintain pace in endurance are progressively less economical. To make those motor units recruited when you're fatigued in an ultra marathon more economical and less fatiguable, you have to recruit them in training. There are two ways to do this. Option 1 is to run at slower paces until you require them due to fatigue - that’s exhausting and hard to recover from. Or you can run fast, which requires them to engage.

Second, the amount of oxygen that your mitochondria convert to water at a given running pace is a measure of your economy. All the evidence we have also supports the notion that the rate of oxygen consumption is linked to the mechanisms of stamina, or more specifically to the mechanisms of fatigue. This is hugely important. In all sorts of studies, focused on seemingly disparate aspects of physiological capacities during exercise, we always find that features of oxygen consumption are inextricably linked to fatigue processes. And before you say that should be obvious, there are situations in which it wouldn’t be, yet it remains so.

Now imagine we’ve started you running at a very easy pace while we’re measuring the amount of oxygen you are consuming. There’s a ratio of the oxygen you consume to the work you are doing, though in running studies it’s often simplified (sometimes problematically) to the pace at which you’re running. As you speed up, your rate of oxygen consumption increases. The ratio remains the same, to a point. There is a straight line relation between the amount of oxygen you consume and the work you are doing even as we increase work rate. There is the same increase in oxygen consumption for each X step increase in pace or work rate. Expressed another way, the inverse, each increment in work rate produces the same increment in oxygen consumption. What this means is that economy is the same across the easier work rates.

That relation holds true until you get to what is commonly called the aerobic threshold. I don’t like that term because it implies an underlying physiology that isn’t true. There’s always some anaerobic metabolism going on in your body, even when you’re asleep. So, there’s no switch between aerobic and anaerobic metabolism at that supposed threshold. If we get deeper into the weeds of this topic, we can parse features of bioenergetics that make the term a little more palatable for colloquial use, but it fails due to the ease with which it’s misinterpreted as a term. In fact, in the late 1970s and through the 1980s some groups called it the anaerobic threshold. If you read those older papers, and even some more recent ones, you may get the wrong message as what is commonly called the aerobic threshold today was often called the anaerobic threshold.

The term I prefer for this intensity where you begin requiring more oxygen per unit increment in work than you did at lower work rates, that is the point we deviate from linearity, is a term that I’ve coined and that’s the economy threshold. I prefer that term not just because I came up with it but because it specifically an accurately states what we’re talking about.

Knowing what we do about the mechanisms, it’s probably true to also call it the efficiency threshold (at a cellular level) but that requires an entire discussion of its own. Above this economy threshold you consume relatively more oxygen for each increment in work rate - you become less economical. And yes, this is roughly the same point at which we would see a sustained increase in lactate levels of the blood above resting values.

In our scenario, we’re having you progressively increase your work rate. Imagine we were to instead have you go from a very easy pace - or even just walking - immediately up to a set work rate. It would take you 1-3 minutes for your metabolic machinery to get going before you reach a steady state of oxygen consumption to meet the demands of that work rate. You know this from experience, if you speed from walking to a pace you could sustain for hours, it still takes you a little bit of time - maybe a couple of minutes - to adjust. For work rates higher than the economy threshold, it actually takes longer than 3 minutes because at three minutes, give or take a very small range, your body begins to develop a second delayed increase in oxygen consumption that slows the attainment of a stead state. This additional oxygen consumption, beyond what’s predicted from the linear relation between oxygen consumption and work rate for easy paces, those below the economy threshold, is called the slow component because it delays - slows - the attainment of a steady state. The bottom line is that your running economy is reduced above the economy threshold by an additional energetic demand that doesn’t begin until three minutes into an interval bout. In subsequent intervals, after the first, some of that demand will manifest earlier than three minutes. This will become important in a moment.

Now, if we continue to have you increment your work rate by running faster or increasing the incline over which you’re running, then the increase in your oxygen consumption continues to outpace the increase in your work rate as you become progressively less economical at higher and higher outputs. But, if you stay at that work rate for several minutes, 3-8 minutes, you’ll still reach a steady state. Your economy is not so good, but you’ll reach a stead state within 8 minutes. That’s true until…you knew there’d be an until didn’t you…until, you reach a work rate that we call the fatigue threshold. The fatigue threshold is sometimes equated to the anaerobic threshold. In this case, the term actually is much more on point. The fatigue threshold is your critical velocity or critical power. Just below this work rate is your rmaximal steady state. Running below this value, you will reach a steady state and you can maintain that pace for a long time. Marathons are run just below this pace and fatigue ensues due to factors like glycogen depletion and demands of thermoregulation.

Above this work rate the picture changes entirely. A steady state is not possible. Cellular oxidative and metabolic homeostasis is no longer achievable. Your oxygen consumption will rise until it reaches V̇O2max. Critical velocity can be maintained for about 40-60 minutes. But note that we can never measure an exact value for CV precisely as it does shift slightly from day to day and, given that it is a complex tipping point around unpredictable vs predictable fatigue, there must be soft edges of a few percent to any attempt at specific measure.

If you’ve kept track, you’ll notice that we’ve defined two thresholds and three domains of metabolic rates that are inextricably linked to fatigue processes because they are anchored on demonstrable changes in the kinetics of oxygen consumption. There’s easy and moderate below the economy threshold (again, often called the aerobic threshold) where your economy is at the same value for all the work rates, then there’s a heavy intensity domain where your economy declines because you require more oxygen - and in fact, more energy - for each increment in work rate but where a steady state is achievable. Then we encounter the fatigue threshold, beyond which a steady state is impossible and the work reserve is small and finite.

So, we’ve observed three domains of metabolic rate that bioenergeticists call moderate, heavy, and severe with the economy and fatigue threshold defining the boundaries. Economy declines after the first domain while a steady state is achievable through the first two.

What does this have to do with running intervals? Well I’m glad you asked.

Any intervals in the heavy and severe domains of effort are going to manifest features of reduced economy. But, only intervals in the severe domain are going to engage all of the structures and processes that produce a predictable limit to your sustainable capacity. You need only be slightly into the severe domain to achieve that training stress. Intervals in the heavy domain do stress some of the motor units that contribute to fatigue in the severe domain, but you're leaving others inactive, and therefore unstressed. All of this is to draw attention to the fact that intervals in the heavy domain are stressful and only require some of the less economical neuromusculature.

Intervals aimed at enhancing stamina should be done at a work rate just into the severe domain, above maximal steady state, critical velocity and power because they will engage all of the musculature required to deplete your work reserve (for those of you into the critical power literature, we’re talking about W’ for power or D’ for running). There is no reason to perform intervals much above this intensity because there would be no additional stress on the oxygen transport and utilization systems given that V̇O2max will be achieved if the bout is continued long enough; basically, you’ve now maxed out engagement of your aerobic machinery at the whole body level. Working harder or faster only fatigues you quicker without stressing aerobic metabolism anywhere in your body any more.

How do you identify this work rate for yourself? It’s actually pretty easy to get a good, though rough, idea. You can run the 3-minute test where you sprint all-out for 3 minutes and the average pace over the last minute is a good approximation of your critical velocity. You can also run a 1-hr time trial where you try to cover as much ground as you can in 1 hour and the average pace over the entire hour is you an estimate of your CV though maybe slightly underestimating it. Both of these tests are highly dependent on your motivation and the value has some day-to-day variability that is based both in motivation and in small difference in actual physiologic capacity between days. But, it will be a reasonable datum point. Run your intervals a bit faster than that pace.

Another way to gauge a good pace for intervals is to use your 10 km race pace, assuming you can run 10 km (or 6.1 miles) in less than an hour. Finally, if you use a Stryd running power meter, they’ve just incorporated an auto-calculation that uses your recent training to estimate your critical power. As long as you’ve run enough across the range of intensities, it seems to provide a good enough estimate to use in running intervals.

If you run intervals around this pace, effort, or power, you’re going to get the majority of the rest of your training effect for stamina.

So, what about all the other paces like at a steady state pace, 5 km or 1 mile race pace? For variety and fun, I think these are all great. But, I have no basis from which to tell you that doing so is going to make an appreciable difference unless that difference comes from the mysterious world of placebo and mind-body connection.

I do think that some long runs that are often called tempos, which are for approximately 20 minutes at a comfortably hard effort between 10K and half-marathon race pace, can be very useful once in a while, both mentally (because they’re hard) and physically (because they are sustained). But, that’s not interval training and not part of today’s topic, though some people will do two or even three of them in one session.

We don’t have studies that have specifically addressed the extent to which short intervals at, say, 1 mile or 5 km pace affect stamina at paces relevant to ultra-marathons vs longer intervals. It’s my hypothesis that there may be differences but that those differences depend on the athletes genetic basis (e.g., speed vs endurance predispositions) and the nature of prior and current training.

Note that those differences may also depend on the effect of the training on your psyche. If you believe in the training program, and you trust that it is well-matched to your physiology, and you’re having fun, then your outcomes will be significantly better than if you were in a program that you question or don't trust.

A study from Wake Forest found that psychosocial scores were stronger predictors for running injury than flexibility, quadriceps angle, arch height, rear-foot motion, strength, footwear, weekly mileage, running surface, gait, and previous injury. Other measures were partially predictive but none as much as these psychosocial measures. I suspect there’s a lot of this at play in responses to training programs as well.

Now how many intervals, how long, how often, and how much rest between? With the caveat that we don’t really know, despite a lot of very good and well reasoned theories, the number and duration should combine so that you accumulate about 20-40 minutes per workout with a total of about 30 minutes per week for less well-trained runners and up to 60 minutes per week for well-trained runners. That’s because the pace your running at is one that would fatigue you in one hour if you’re at the 1-hr pace or in ~40 minutes if that’s your 10 km race PR and you’re using that pace. The idea is to accumulate an amount of time that’s roughly 50-100% of the time in which you’d fatigue if you ran that pace continuously, either in one or two sessions every 7-10 days. On the lower end for people new to running, returning from time off, or new to intervals and the higher end for strong, healthy, experienced runners.

There are many ideas on the durations that are best for these intervals. However, on scrutiny, the defense of any specific duration ultimately regresses to a theory rather than a value with rigorous or consistent experimental evidence. Bouts of 3-5 minutes duration seem to be more effective at raising V̇O2max in unskilled or moderately trained runners for example, but that value will be developed through training anyway and training it specifically isn’t a good use of your training time in my opinion. Moreover, there’s no reason for an ultra-marathon runner with at least a year of consistent running foundation to actually reach V̇O2max during an interval session. The primary goal in your non-sprint interval training is to accumulate time in the window of intensities that would lead to V̇O2max if you continued them long enough but not to actually get there so that you build stamina at your sustainable (i.e., easier) paces without so much breakdown that you wear down after a few weeks and require a full recovery week.

There’s an old idea that time spent at V̇O2max might be an effective approach to building stamina. The idea was highlighted in Astrand and Rodahl’s book titled Textbook of Work Physiology (affiliate link) where they wrote in the 2003 edition “the importance of the time spent at V̇O2max remains theoretical”. The same is true now, in 2019. And, the work of multiple groups supports the hypothesis that accumulating time near, but not at V̇O2max - roughly 90% of V̇O2max - is optimal for endurance athletes in conjunction with lots of easy to moderate training. As I’ve been emphasizing, the best way to do that is to complete bouts of several minutes each at roughly 10K race pace.

Intervals of shorter duration are more tolerable even when of slightly higher intensity than longer bouts for most runners which may be of benefit. Moreover, it’s possible that overcoming the metabolic inertia at the beginning of bouts and recovering after may play a role in the adaptive stimulus, in which case more bouts (of necessarily shorter duration) might be useful. Finally, the factors that drive the anaerobic basis for fatigue in supra-CV exercise do manifest even in the first minute of a bout after the first one or two intervals, which is likely to be meaningful for stimulating relevant adaptations. To put that in planer English, short bouts of 2-4 minutes are likely to be useful.

Longer bouts, such as 4-8 minutes, give sufficient time for full manifestation of the slow component I spoke of earlier depending on the precise intensity of effort. The first bout manifests no slow component if it’s three minutes or less. However, after the first bout, subsequent bouts start off engaging some of these fatigue factors regardless of the bout duration.

Don’t worry if that’s more than you ever wanted to hear. Basically, it's just explaining the physiology for the scientific findings like those of my guest in episode 71, Dr. Stephen Seiler, and the empirical observations of some coaches and scientists that bout durations of 5-10 minutes at hard sub-maximal efforts can be superior to bout durations of 2-4 minutes in improving some indices of endurance. https://www.ncbi.nlm.nih.gov/m/pubmed/23037620/ and https://www.ncbi.nlm.nih.gov/m/pubmed/21812820/.

As I prepared for this episode, I reviewed several of my episodes with specific guests, I e-mailed with a few scientists, I read and re-read a lot of papers, and I spoke with quite a few ultra-marathon coaches. Leveraging all of that, and folding it into my own observations as a coach, it’s the general consensus that regularly performing long strenuous intervals is sustainable over a season only by exceptional and highly motivated athletes. That doing a lot of long, hard intervals ultimately leads to burnout and injury for most runners. Perhaps this is why some who do this only do so in blocks and still often have athletes experience marginal gains, plateaus, and even succumb to injury. All of this leaves me presently recommending that you mix it up and complete interval training bouts with durations of 1-10 minutes, which can be the same within a given workout or a mixture even within that one workout and do what you find fun on that day. Be careful on your shorter bouts that you don’t fall into the trap of making them much faster or harder, which is very easy to do. Save that intensity for your true sprint interval days. One of the benefits of running longer bouts is that you won’t make the mistake of running them too fast more than once.

Take good notes about what you did and how you feel during, after, and on following days so that you can review those notes for trends that help you optimize what’s working best for you. Again, accumulating about 30-40 minutes per session in 10-20% of your running sessions is a good starting point for most trained runners; and that’s consistent with the research on many hundreds of endurance athletes.

The notion that recovery duration needs to be carefully prescribed and some ratio of the bout duration is a common belief without evidentiary support. For high intensity intervals near sprint paces, lasting seconds, the work:recovery ratio does matter a lot. But, the evidence actually shows that the length of the recovery interval matters little when the bouts are minutes long and when the recovery interval is also on the order of minutes. In fact, it can take 45 minutes for metabolic kinetics to recover fully from just one 6-min bout in the heavy domain, even if you feel totally recovered after just a couple of minutes.

In trail running, the heterogeneous elevation changes make a specific prescription, such as 3 x 8 min or 6 x 4 min or 12 x 2 min anywhere from awkward to impractical without running repeats over a given section. Because most trail runners love the exploration aspects of training, repeats over the same terrain is often unpalatable and detracts from the joy of the process. If you’re fine with repeats on the same section of terrain, go for it. If that’s not you, then choosing instead to accumulate 30 min on inclines one day and 30 min on flats another day can be more fun for many runners. The necessarily varied bout and recovery interval durations will not compromise the training stimulus as far as we know. It’s generally easy for a trail runner to find a route where the trail will present a new section suitable for a new bout soon enough. But, again, be sure to pay close attention to the effort output on each bout to be sure it’s where you want it. It can be easy to turn a workout like this into a maximal grind on each bout, and that’s not what we want.

If you tuned out, refocus your attention now. One of the biggest mistakes I have seen in planning interval training over a season is the tendency to do so in block style and to abandon high speed and sprint intervals when longer intervals are incorporated. The case for focusing on one intensity for a period of time can sound good but all the information we have, as I’ve consulted the science, researchers, and top coaches, indicates that doing so leads to poorer performances, slower development, and greater likelihood of plateau and injury. So, continue to do some speed work, even if it’s just 5-6 strides twice per week when you add longer intervals.

Research has shown that some of this higher intensity interval training improves measures of endurance capacity and performance once there’s a meaningful foundation of base mileage (or time) and, importantly, that significantly more is not usually beneficial. Averaging 1-2 sessions for every 10 training sessions seems to be the optimal range to work with as you dial in what’s most effective for you. You’ll have to figure out empirically the frequency of these sessions that works best for you, providing sufficient stimulus for development without overreaching.

How can you test for efficacy? One of the methods I described for determining the pace or effort for these intervals is perfect. A 10 km race, a 1-hr time trial, or the 3-minute sprint test are all good choices for monitoring improvements over time. If you have a Stryd device, it will auto-calculate and notify you when there is a change, which is nice and requires no thought or specific testing on your part. Again, I’m not getting into the uncertainties of such a device, just noting it’s an option, and I have no financial affiliation.

When should you do these longer intervals in a season. There are many approaches. Again, there’s no clear consensus on an optimal approach. I can say that national level endurance athletes in many sports tend to do them almost year round with a slightly greater amount around mid-season, before getting squarely into competition phase. I think you can do these year round, but then I also don’t believe much in the more complicated periodization contrivances of many programs. When I look at top performing endurance athletes, I see very consistent year-round training with only subtle shifts in modalities and intensities. If you begin to plateau, take 1-3 weeks off from these workouts and then build back into them.

A final note. What I’ve presented is a mixture of my experience over the years as a coach of athletes at all levels, published studies, and deep discussion with other expert scientists and coaches. As such, other people will disagree on some points. Through this series, I’m presenting my best, educated and experienced current understanding of solid and reliable training approaches for endurance runners. One of my goals is to cut through the mess of theory and dogma that I encounter every day when I read books, blogs, and listen to interviews to bring you at least some semblance of a guide that you can use with confidence.

Ten, fifteen, twenty years ago, I had different ideas. Some features have held steady but others have evolved considerably. I expect, even hope, that in the future, I’ll have new and improved approaches. So, take what I’ve presented for what it is. Try it if it resonates. If it doesn’t, then you do you. What I’ve presented will work very well for most people most of the time, and extremely well for some people some of the time. It will also do poorly for some people all of the time, and all people some of the time.

Are you dizzy? After you catch your cognitive balance from that twisting of words, just know that it’s a good start if you need a starting point and it’s a good reference to hold up to what you are currently doing in your quest to improve your training.

Take detailed and thorough notes in your training diary. Then take the time to review those notes once in a while with an open mind, allowing patterns to reveal themselves. Then test subtle shifts in your training. Be sure to allow plenty of time to really test out adjustments in your approach and avoid making big changes too fast or too often.

Consistency rules. Not only in the context of training regularly, 5-6 days per week, but also over the long term. Following the workouts-of-the-month from your favorite magazine, blog, or YouTuber is a surefire way to stifle progress.

Remember the haiku from my guest in episode 18, Dr. Michael Joyner: Run a lot of miles, some of them above race pace, rest once in a while. As ultra runners, nearly all of our training may be above race pace, especially if you run 50-100 mile races or longer. So, I’d tweak the haiku and say: Run a lot of miles, some of those miles hard and fast, rest once in a while.

At this point we’re really beginning to make finer adjustments to the plan for training your body. Remember that how you eat, sleep, think, and move all play a role in helping you become your ultra best.