Have you heard this before…keep your easy days easy and your hard days hard? Well, is there a physiologic basis for not spending much time training in the middle? Is there a downside to training in the middle? Tempo runs and threshold runs are supposed to be important, aren’t they, and they’re in the middle?
Exercise bio-energeticists, such as myself, describe three zones or domains of exercise intensity based on the ratio of oxygen consumption to work rate and the sustainability of the intensity. These three domains are termed: moderate, heavy, and severe. In this episode, you’ll learn some of the physiologic bases for why you should train mostly in the moderate domain, some in the severe domain and little in the middle (the heavy domain).
Moderate intensities are the easiest. Within this moderate domain, the ratio of oxygen consumption to work output is linear and predictable…as we increase the work rate (by going faster or going up a steeper incline, for example) oxygen consumption increases by the same amount for each unit of increase in our output. This linear relationship indicates a maintained exercise economy…you’re using the same amount of fuel, indirectly quantified as oxygen consumption, per unit of output.
As the intensity increases, you reach a point where these relationships get more complex. This is roughly the point where lactate levels rise above resting values (first lactate threshold), where you begin to produce more carbon dioxide per unit of oxygen consumed (gas exchange threshold), where you begin to breathe more relative to your oxygen consumption (first ventilatory threshold), and what you may hear called the aerobic threshold. None of these thresholds occur at exactly the same point as any other because they each measure something slightly different. But, they do all occur in roughly the same tight range of exercise intensities and they mark the division between moderate intensity exercise and heavy intensity exercise. What is particularly interesting, and exciting to exercise bio-energeticists, is that the rate of oxygen consumption in this heavy zone is disproportionally high compared with the values in moderate intensity. In other words, you consume more oxygen per unit of output…you become less economical. Though we’ve been studying this fact for decades, and it was the basis of my PhD work 20 years ago, we’re still not exactly sure what causes the extra oxygen consumption. However, we do now have a decent grasp of what’s going on. It seems that it’s a feature of muscle cells recruited at heavy intensities resulting from accumulating di-protonated phosphate inside those cells, causing both a greater oxygen cost of generating ATP, the cells energy molecule, and a decrease in the energy derived from each molecule of ATP. In other words, the muscle cells recruited to perform at heavy exercise intensities become less economical and less efficient than those recruited only in moderate intensities.
Exercise in the moderate domain is the most economical and it is predictably so. Exercise in the heavy domain, therefore, is sustainably inefficient. Good marathon runners run their races in this domain because the poor economy is sustainable long enough for the duration of the marathon and the poor economy is affordable for the trade-off of a good race time. But when you look at the training of the best marathon runners in the world, they don't spend much time training in this domain. Ultra-marathons are performed at a lower overall intensity, somewhere near the break between the moderate and heavy domains.
As we increase exercise intensity further, we reach the range of output that includes the second lactate threshold (most athletes and coaches using the term lactate threshold are talking about this one), the respiratory compensation threshold because you now begin hyperventilating even disproportionately to the acids being produced, the second ventilatory threshold, and the anaerobic threshold. All terms for roughly the same exercise intensity for a given person. Exercise above this second set of breakpoints or so-called thresholds is in the severe domain. In the severe domain, oxygen consumption will not reach a steady state, but instead will continue to rise until fatigue. This is near the maximal lactate steady state, near tempo effort, and a little lower than critical power (which may be called critical velocity for runners). The amount of work you can perform above critical power (or the distance you can cover above critical velocity) is an exact value; you can run a little faster than critical velocity and reach that distance slower or you can run much faster and get there sooner. But, the distance is an absolute value.
What does all this mean for your training? There’s a notion in the world of athletics training that training at a particular intensity makes you more economical or efficient for performing at that intensity. While there’s evidence that this is the case in untrained people, it isn’t clear that this is necessarily the case for trained individuals. Specifically, we know that you don’t have to train at intensities where you are inefficient in order to become more efficient at those intensities; in fact, training at moderate effort may be the most effective way to become more efficient at heavy intensities as I’ll explain a little later. Moreover, training in the heavy intensity domain creates a higher recovery burden for the amount of adaptive stress created. In other words, you’re not only less efficient in the workout itself when exercising in that domain of intensities, the stimulus to improve is also less efficient because you require more recovery per unit of adaptation for improvement. You’re better off training mostly in the moderate domain, where you get the majority of your stimulus for endurance gains, and some in the severe domain, where you get stimuli for optimizing neuromuscular recruitment, further cardiac adaptations, connective tissue stress, and perhaps some other useful adaptations. This is the bio-energetic basis for the common phrase keep your easy days easy and your hard days hard. Doing much training in the middle is inefficient use of your time and your body’s adaptive capacity. Tempo runs and lactate threshold runs are popular but are at the upper end of this range of inefficient training domain. Those runs may provide a small psychological benefit, though you can get that in better ways, and may be useful for dialing in race effort for events of 1-2 hours duration. But, for ultra endurance athletes you’re better off dedicating that training time to complementary workouts like heavy weight lifting or simply recovering until you're in final preparation for competition.
How do you know where these domains are for you? Outside of specific lab testing where your exhaled gasses or blood values of metabolites are measured, there are more practical ways to estimate whether you are in the right range. The moderate domain ends when you’re no longer comfortable carrying on a conversation of short but full sentences. Another test is to count out loud comfortably (quickly but not as fast as possible), you should get beyond 20. On a 1-10 scale, your rating of effort (or rating of perceived exertion, RPE) should be 5-6. If you’re in doubt, err on going easier. As for the transition out of the heavy domain and into the severe domain, laboratory testing is a better tool than a talk test. However, I recommend doing your hard training a little higher still, above critical velocity, which is RPE 8-9 out of 10. If you want to get better at neuromuscular control, muscular recruitment patterns and the like, then you have to engage most of the mass of the relevant muscles; that is, you have to recruit most of the muscle some times in order to learn how to recruit it more economically at any intensity.
You can determine your critical velocity using a 3-min all-out sprint test. The procedure is to warm up fully, then run an all-out non-paced sprint and continue pushing 100% for 3 minutes. Yes, that is just as hard as it sounds. If you can give 100% start to finish, then your average pace over the last 30 seconds is your critical velocity and the distance you covered beyond what you would have covered if you had paced at your critical velocity during the 3 minutes is your D’, which is the extra distance you can cover no matter how fast you try to run above critical velocity. Whether you run just barely above critical velocity or much faster, that is the extra distance that you can cover before you will fatigue.
Let’s have an example: In 2017, Eliud Kippchoge had a critical velocity of 6.04 m/s and a D’ of 250 m. That means he could run faster than 6.04 m/s, but it doesn’t matter if he runs at 6.1 m/s or at 6.5 m/s, he will stop from fatigue as soon as he covers 250 m more than he would if he ran exactly at is critical velocity of 6.04 m/s for that period of time. Doing the math, he can run 0.5 m/s faster for 500 s, 1 m/s faster for 250 s or 2 m/s faster for 125 s. But, he has the capacity (the D') for covering 250 additional meters for any pace above his critical velocity. It’s a fascinating, true threshold, and has received the appropriate alternative name of the fatigue threshold because the amount of work you can do above critical power, or the distance you can cover above critical velocity, is fixed no matter how slowly or quickly you go. It is trainable, but that’s a potential topic of a future episode though probably not especially important for ultra-endurance athletes because the distances are small relative to your event distances.
You can sustain a continuous effort at critical velocity for about 20-30 minutes. Your hard interval workouts should comprise repeats at this or a slightly faster pace and you should run in intervals to accumulate about 30 minutes at that rate depending on your level of experience and fitness to recover.
You might question whether you should focus hard intervals on efforts a little above or a lot above critical power (if you use a power meter) or critical velocity (if you prefer to think in terms of pace). Let’s answer that question. When I started my PhD program, I had the great pleasure of spending a day with Per Olaf Åstrand, one of the founders of human performance science and author of the classic book titled Textbook Of Work Physiology. In that book, he (along with his co-author Rodahl) wrote: “It is an important but unsolved question which type of training is most effective: to maintain a level representing 90% of the maximal oxygen uptake for 40 min, or to tax 100% of the oxygen uptake capacity for 16 min’’. Both of those intensities are above the critical power/velocity threshold but Åstrand and Rodahl questioned which was better. Indeed Dr. Veronique Billat is known for establishing interval training protocols that maximize the amount of time you’re at V̇O2max during a single session. And that’s sound physiologic information…but, does maximizing time at maximal capacity actually give us the best adaptions and improvements. It appears not. It appears that we’re better off dialing back a little and spending more time a bit below this level…it appears that training around 90% max for longer is better than training at 100% for shorter time. Because fatigue is so predictable, and the capacity for accumulated work above critical velocity is exact, no matter how it’s used, we would generate the hypothesis that the same fatigue mechanisms are stressed regardless of running a little or a lot higher. By extension, we’d then suggest that accumulating as much time above that value, not at maximum, and then properly recovering between workouts, should be the goal. And the science supports that notion. We do now have an answers to the question posed by Åstrand and Rodahl.
Dr. Stephen Seiler’s research group, based in Norway…a country known for producing exceptional extreme endurance athletes…trained subjects using intervals comprising 4 bouts at 16, 8, or 4 minutes duration twice per week. The instructions were to complete each interval session with the highest average intensity possible for the entire session. The result was that each workout was a maximal effort over that duration of time, but the relative intensity had to be lower for the 4x16 minute bouts, a little higher for the 4x8 minute bouts, and highest for the 4x4 minute bouts. The 8-minute repeats were significantly superior to 16 and 4 minute repeats at raising V̇O2peak, power at V̇O2peak, and power at 4mM lactate (sometimes approximated as the maximal lactate steady state). The authors concluded, “Interval training intensity and accumulated duration interact to influence the adaptive response.” Specifically, the workouts at 90% HRmax that accumulated 32 minutes of interval time were superior to both lower and higher intensities. The same group repeated these results in cross-country skiers and came to the same conclusions as they found 2 interval sessions per week at about 90% HRmax accumulating 40-45 minutes in intervals of 5-10 minutes duration each was significantly superior to 2-4 min intervals at higher effort. 90% of max is hard for sure, but quite tolerably hard…it’s not collapse dizzy, slurring words, tasting metal hard. How can you judge whether your hard day session was of the correct overall effort? There’s a validated post-workout RPE scale called the session RPE…these hard training days would rate a 7 out of 10 when you reflect on the entire workout in the hours after your session.
It’s also critical to maintain your base of endurance workouts as you add high intensity training. It’s tempting to back off of the lower intensity training when you bring in high intensity workouts. That would be a major training mistake.
Once you’ve built a solid training base of easy running, add high intensity training. A study from the lab of legendary performance exercise physiologist Dr. Jans Bangsbo demonstrated, in cyclists, that adding high intensity interval training with 30 second sprints for 7 weeks but with a reduction in overall training volume by reducing the amount of easy training resulted in higher lactate production when cycling at 85% V̇O2max. When we think about training at a given intensity, we should be aware that we’re training what we’re using for that effort - the mind, heart, neural control, specific muscle fibers recruited and so on. We get adaptation in the cells and pathways where we disrupt homeostasis and we get enhancement of capacity in functions that are central to that intensity. So, it isn’t surprising that reducing the volume of low intensity endurance exercise (which is lactate scavenging) while increasing high intensity training (which is lactate producing) results in higher lactate production at 85% of V̇O2max. I’ve heard people suggest that training at intensities where you produce appreciable lactic acid (like threshold and tempo runs) should make you better at handling or clearing the lactate; but that misunderstands the underlying physiology and adaptive motive. It’s important to recognize that lactate isn’t a signal of disrupted function or homeostasis at that intensity, it’s actually what those cells do to produce some of the energy needed. If you want to get better at endurance, you have to focus on endurance intensities.
There’s little utility in spending much time training in the heavy domain (such as tempo intensity runs)…that is, between conversational and critical velocity. A 2010 training study by Guellich and Seiler concluded “Training at <2 mM blood lactate appears to play an important role in improving the power output to blood lactate relationship. Excessive training near threshold intensity (3-6 mM blood lactate) may negatively impact lactate threshold development. “ I’ve made the point on previous podcast episodes and blog posts that we don’t really care much about lactate or lactic acid, as such, when it comes to ultra-endurance sports. In these examples, we’re using it as one marker of metabolic state and an indirect proxy for endurance related capacities. Your hard days should be hard, not tasting metal hard, but very hard; your easy days should be truly easy. Can they be too easy? No, as long as you’re maintaining good running form. It’s easy to get sloppy about running form when you’re running slow and easy, so pay attention to it on those days.
I’m about to make an extreme simplification. As such, it has shortcomings. But, the spirit is valid. Moderate domain exercise develops endurance capacity, heavy domain exercise develops tolerance of inefficient movement, severe domain exercise develops movement economy as long as you don’t do too much of it. In summary, keep 80% of your training conversational…where you can still hold a conversation or maybe count comfortably to just past 20. It’s a very rough estimate, but this equates to approximately 70% of maximum heart rate for many athletes. Run 20% of your workouts as hard interval days where you get your HR to 90% of maximum. The goal is to accumulate time where your HR is at 90% of max during the work part of the intervals. If you don’t have a HR monitor or don’t like following HR, use an effort that’s 8-9 out of 10. Vary the work times in the 5-10 minute range with 3-5 minutes rest intervals while accumulating 30-40 minutes of total work time for the session (15-20 minutes if you’re just getting started).
These are guidelines and will require trial and evaluation on your part. You absolutely will respond differently than the person next to you, and progression will have to be dictated by how fatigued you feel during and after the workouts as well as how quickly you’re recovering…always start on the low and more cautious end of the ranges until you know how you respond. Progression should focus on accumulating more time at that intensity rather than making the work periods harder. One hard day per week for most runners and anyone new to running is enough. Well-trained runners can tolerate two of these sessions per week. Everyone should keep detailed notes in a training diary so that you’re able to leverage your own data to create evidence-based prescriptions in the future.
I'll post a script of this episode along with references for further reading on the show page at www.scienceofultra.com/podcasts/68
With that, we'll bring this episode to a close and remind you to eat well, sleep well, and move well....happy training!