Why cadence matters.

A significant debate in the running world today concerns cadence. The question is: At which cadence should a person run? Some argue that the minimum cadence should be 180 steps per minute (spm), on the grounds that it is far more efficient than slower cadences.

Several important counterarguments have been made to this claim. One is that high cadences occur more often in elite runners, and then only during races (and that these same elite runners run at very low cadences during their warm-ups).

In this sense, nobody has ever run at one cadence—and indeed, there simply cannot be a “minimum” cadence: every run that anyone has ever run started out at a cadence of zero (when they were standing still) and their cadence slowly or quickly climbed to the cadence that they adopt habitually at a cruising speed. So, in “reality,” everyone has an infinite number of cadences at which they run: They start from a cold zero steps per minute, and pass through 0.0001 spm, 0.0002 spm, and so on, as they make their way past their habitual “cruise” cadence, up to their personal maximum.

The people who first prescribed a “running” cadence, when pressed on the issue of whether there is “one” running cadence, would almost certainly agree that people go through an infinite progression of cadences during either acceleration or warm-up. They would probably say that they didn’t mean that 180 spm was the sole cadence at which people should run (which is clearly impossible), but rather that 180 spm is the paradigmatic cadence of the human body—the cadence that these elite athletes warm up to (or should, if they don’t), in order to get the most out of their run.

(To be honest, I don’t know the rationale for 180 spm in particular as the cadence of choice—instead of say, 182 or 178 spm. I haven’t read anything about muscular dynamics that suggest that 180 spm is the optimum (or why it is). My belief is that the optimum would be somewhat dependent on the individual’s dimensions. I But it’s very clear that across individuals, 180 spm is a much more efficient cadence than 150 spm, for example.)

By this argument, why do high cadences show up the most in races? Because that’s when efficiency matters the most.

Think of this in the same way we describe “being awake.” We understand it to include a certain degree of alertness. We go through a spectrum of wakefulness from the point that we initially open our eyes and brush off the cobwebs to the point where we can be at the top of our game in a networking event.

It behooves us to define “full wakefulness” not at the point where we are not asleep, but rather, at the point where all the possible systems that contribute to alertness and cognitive function are up and running. If you can “get awake” but can’t brush off the cobwebs—implying that you can’t bring critical cognitive systems into play (or into play enough)—you’ve got a real problem.

Running works similarly. The main argument is that because these physiological systems create a higher degree of efficiency by producing a high cadence, it behooves us to understand “running” as including a high degree of activity of these physiological systems. (In these terms, “running-like movements” can occur at all cadences, but “running” occurs only at the full activation of these systems.)

Cadence increases efficiency because of its impact in a crucial neuromuscular process known as the Stretch-Shortening Cycle (SSC). When the foot lands, muscles all across the body are passively stretched. Then the muscles contract (or shorten) almost immediately, releasing the energy stored during stretching. This helps the leg recoil and be recycled into the next step.

The longer the interval between the initial stretching and the subsequent shortening, the more energy becomes dissipated in the form of heat. The longer the wait, the less mechanical energy available in the muscles and tendons at the moment of shortening.

At a low cadence, the interval between the stretch and the shortening is very long, meaning that a lot of energy is lost as heat (and efficiency drops). But as cadence increases, the interval shortens to the point that very little energy is lost (and efficiency rises).

I often write about how a new capability gives you twice the benefits you expect: For example, because of the improvement in efficiency that comes with a higher cadence, someone that runs a given distance more quickly is not only faster, but it takes them less energy to run the same distance. So the physiological improvements of proper training contribute to produce a much wider set of advantages.

The above shows yet more benefits: The energy that goes into stretching a muscle has to go somewhere: it can either get returned as elastic energy, or it can dissipate as heat. See the problem?

Even though I’m not aware of a lot of research on the conversion of elastic energy to heat, we can say this: the person with a longer stretch-shortening interval—who loses more stored mechanical energy as heat—has two problems, not one: As we discussed above, they have a lower energy return. But also, the additional heat creates a greater thermoregulatory load on the body.

So the runner with the faster cadence (usually the fitter and more skilled one) will not only be more efficient than the runner with the slower cadence, but they’ll also stay cooler. (And to top it all off, the fitter one is probably also the one with more developed cooling capabilities).

Just to be clear: if you’re fit and skillful, you’re also faster, more efficient, stay cooler and can cool down better, but if you’re less fit and unskilled, you’re also slower, less efficient, you get hotter and you’re not as good at getting rid of that heat.

Of course, none of this changes the fact that there is a curve that shows that people do in fact run at lower cadences at lower speeds, and at higher cadences at higher speeds. And it makes sense why they would: despite its benefits to efficiency, you don’t need a high cadence at a low speed. However, sticking to this descriptive reality of the world isn’t very helpful: the problem is that cadence has been shown to correlate more with absolute speed than with relative speed. This generally means that a relatively slow runner going close to their maximum speed will have a much slower cadence than a relatively fast runner going close to their maximum speed.

If we just go by the observed speed-cadence relationship (and let that iterate itself in every runner), the faster runner will always be more efficient. In other words, slower runners won’t get the chance to be efficient.

Good coaches try to get slower runners to run at a fast cadence to allow them to achieve a greater degree of efficiency (although the faster runner may have more overall efficiency due to other advantages). And by forgetting about speed (at least initially) and focusing on increasing cadence, it’s possible to accomplish exactly that.

In defense of the endurance running hypothesis, part 1: how we think about evolution.

The endurance running hypothesis is the idea that humans evolved primarily as endurance runners. The argument goes that the human physique evolved and took its shape and function from the primary adaptive pressure of persistence huntingthat of chasing down our prey until its body shuts down.

However, this hypothesis is not without its detractors. A significant amount of scientists provide an array of counterevidence to the endurance running hypothesis. (And the debate continues.)

Take for example the case of the human gluteus maximus (butt muscle). Lieberman et. al. (2006) claim that the human gluteus maximus evolved its shape and size due to endurance running.

However, another article in the Journal of Comparative Human Biology finds that the gluteus maximus grows much more in high-force sports (weightlifting) and high-impact sports (such as soccer), than it does in endurance running. In fact, they also show that the butt muscle in endurance runners is no larger than in the non-athlete population.

What I disagree with is their conclusion, which is paraphrased in the “What does this mean?” section in the image below:

“The human gluteus maximus likely did NOT evolve through endurance running, but through varied explosive and forceful activities.”


My disagreements with the article (and the image) are primarily about how and why we interpret the science to mean a certain thing.

At first blush, the fact that endurance running doesn’t enlarge the gluteus maximus as much as other sports seems to detract from the idea that the muscle takes its shape from endurance running. But I think it actually adds to it.

By my analysis, these findings show that the basic, untrained shape and size of the gluteus maximus—it’s “factory specifications,” if you will—assume that it’s going to do the amounts of cutting, jumping, weightlifting, and sprinting that a habitual endurance runner might need to do. But it requires aftermarket modification to meet the (literally) outsize power and stability requirements of soccer or weightlifting.

Let’s say that a muscle evolved under a particular adaptive pressure. This means that its shape and size literally evolved to do that thing. If you take a muscle that usually doesn’t do a thing for which it evolved to do, and you ask it to do that thing, you are asking it to do something that it has prepared to do for millions of years of evolution.

In order to fit a function that it has been designed to do, the changes in shape and size that the muscle should have to undergo should be smaller, not larger. You would expect a muscle to change far more if you ask it to do something that is less aligned with its evolutionary job description.

Let’s illustrate this by looking at the arm and hand.

We probably all agree that one of the things that specifically sets us apart from our hominid cousins is the ability to coordinate the thumb with the rest of the fingers in order to grasp and manipulate objects to a high degree of dexterity. In its simplest form, this is the capability to oppose the thumb and the fingers—to make an “OK” sign with the thumb and each of the fingers of each hand.

Now let’s take a snapshot of the people who take this unique human ability to its very pinnacle: string musicians, graphic artists, etc. Their livelihood depends on the degree to which they can explore the potential of one of the major evolutionary functions of the human hand.

Compare the forearm muscles of a violinist or painter with that of a weightlifter. The weightlifter’s arms, hands, and shoulders will be much larger and more powerful. (I trust I need not cite a scientific, randomly-controlled study on the matter.) Why? Quite simple: weightlifters engage in activities that develop the body to phenomenal proportions.

But if we go by the conclusions of the article, the fact that the arm and hand get bigger through weightlifting would mean that it didn’t evolve for the kind of fine motor control that you produce in the arts. (Or that lifting heavy objects is its primary evolutionary role). A particularly ambitious version of this argument would be to suggest that one of the core functions of opposition is to become better able to lift heavy objects. But all these suppositions break down when you realize that our primate cousins were not only quite able to grasp branches and use them ably, but that opposition emerges at the same time that hominid arms were becoming smaller (and less powerful), not larger (and more powerful).

Of course, the human hand (and upper extremity in general) still needs to be able to grow and develop in order to be able to lift heavy objects—and can indeed grow to a huge degree to exhibit that function. But its core evolutionary function is to produce the unparalleled dexterity of the human being.

Furthermore, the fact that the non-painter’s hand remains relatively unchanged in size compared to the painter’s hand means that the non-painter’s hand is already relatively set up to perform that kind of dextrous function—because that’s what it presumably evolved to do. This should serve as evidence (not counterevidence) that the hand is primarily for painting (and other fine motor tasks), not for weightlifting.

We should think the same of the gluteus maximus.

Let me conclude by saying that nothing I’ve written here means that the gluteus maximus evolved exclusively for endurance running. Indeed, there is ample evidence suggesting that the architecture of the gluteus maximus is uniquely multifunction as far as muscles go. (In future posts, I’ll delve more into the nuanced view of the gluteus maximus that I proposed above: that it owes its shape and size to the fact that it is a muscle designed for the kinds of “varied explosive and forceful activities” that a bipedal, primarily endurance running animal expects to have to do.)

But what we can say is that the fact that the gluteus maximus gets bigger through a particular stimulus has no bearing on its core evolutionary role, (or on the evolutionary story of the organism as a whole).

Marathon Training, Part 1: Basic Requirements

When people want to know how to train for a marathon, they usually ask you for a training plan. This typically typically center around the following:

  • What kinds of workouts you’re supposed do.
  • How long those workouts should be.
  • How long you have to train before you’re ready.

Answering these questions is very difficult (if not impossible). Everyone is different, and begins their training at a different point. 

These questions are far too vague (or depending how you look at it, far too specific). It’s only a question that applies to you in particular. So instead of providing a training plan, I like to arrive at the issue from a different direction. The question I ask is:

How do you know that a body is ready for a marathon?

This question is much more useful. Why? Because being ready for a marathon is the same for every human.

The catch is that how to get there might be wildly different from one person to the next. For one particular person, your basic marathon training plan might be exactly what they need. Someone else may need to train for much longer, or with less intensity (or both). For yet another person, it might not include a crucial element that particular person needs—an element with which the training plan might work perfectly.

You’ll find that when you genuinely ask the above question—and truly inquire as to what it takes for a body to be physically and physiologically ready to run a marathon—you’ll inevitably conclude that ninety-five percent of the people who do cross the finish line of a marathon were not prepared to run the race.

I believe that one of the most important reasons that injury and illness is so rampant in the marathon is NOT because the marathon is inherently injurious, but rather because it is so physically and physiologically demandingand the vast majority of people who run it have not achieved the capability of meeting those demands.

A major goal of mine in life is that people do NOT get injured running a marathon (or any other race). And I believe that a first step in that direction is to help people understand what “being ready for a marathon” really means from a physical and physiological standpoint—beginning with the idea that there is such a thing as being “marathon-ready.” Only then can we genuinely expect ourselves—the individuals who constitute a modern athletic culture—to face a marathon with every expectation of success.

 I answer the question of marathon readiness in the following ways:


In order to run at peak efficiency, you must be able to sustain a cadence in the ballpark of 180 steps per minute (spm). This is important because the critical systems necessary for maximizing running economy only become activated at around that cadence. For an array of biomechanic and metabolic reasons, it’s important that our definition of “running” includes the activation of these critical systems. The above means that to run a marathon:


It is said that 99% of the energy that you use to run a marathon comes from the aerobic system. This means that you must be able to run the race at an overwhelmingly aerobic intensity. How fast?

Putting the two together

The above two requirements, when put together, give us a third, “master” requirement:

  • You must be able to produce a cadence in the ballpark of 180 spm while running at a pace that is 15 sec/mile faster than your speed at aerobic threshold, and maintain it for the duration of the marathon.

A word on training load

There’s another way to look at this issue: how much someone needs to be able to sustainably train in a given week to be reasonably certain that they can run the race.

Sustainably means that there is no increase in stress, no nagging pains, and every reason to believe that the body can continue to train at that rate without injury.

So, a marathoner’s easy week should look like:

  • A volume of twice the race distance (50-53 miles).
  • An intensity that is exclusively aerobic (under the aerobic threshold).

*A good way to estimate the aerobic threshold without the need for a laboratory is by using Dr. Phil Maffetone’s 180-Formula. The 180-Formula produces the MAF HR, or Maximum Aerobic Function Heart Rate.

Sample easy week

All training is under the MAF HR, and cadence remains relatively close to 180 spm.

  • Mon    7 mi
  • Tue     9 mi
  • Wed    7 mi
  • Thu     9 mi
  • Fri       7 mi
  • Sat       12 mi
  • Sun     REST


There are no guarantees in life. But if you can run an easy week like this, I can be reasonably sure that you’re ready (or almost ready) to run a marathon. How to work up to this, and how to navigate the many pitfalls and angles of the journey, is the hard part.

Part of why I rarely give training plans or talk about these requirements—popular demand has essentially forced me to—is because you can’t really meet them if you haven’t ironed out all of the physiological, biomechanic, and neuromuscular issues your body may have.

(And again: that’s the hard part—and it’s the part that you can’t really address with a training plan.)

And even if the prospect of running a marathon has never been in your sights, once you do iron out enough of your body’s athletic issues, you’ll find that going on 25-odd mile, easy long runs every month has become a fact of life. You’ve become familiar with the distance—and the idea of running it a little faster with a lot of other people seems as simple as that.

(This post is about being ready for a marathon. How to become competitive at the marathon is, of course, a different question.)

A training logic in 4 basic steps.

In recent posts I’ve outlined some of the difficulties that runners face when training—a phenomenon I call the runner’s catch-22: people want to start running, but they either don’t get fast, or they become overtrained and their health deteriorates.

This is because running is relatively physiologically demanding: the minimum requirement for being able to run at all is far more rigorous than (say) for cycling. Most of the time, the reason people experience the Runner’s Catch-22 is because they’re physiologically not ready to train for their chosen sport. They need to develop more fitness on multiple levels before they’ll be genuinely ready to begin running.

In this post, I provide the basic concepts I use to develop a training plan. This is not just for runners, but for anyone that hopes to increase fitness in a safe, structured, and predictable way. My goal for this article is not just to provide a bird’s eye view of the “how-to,” but also to give the reader a framework to understand why it might not be a good idea to run some race or get into some other sport until certain requirements have been met. To do so, I divide this process into 4 basic steps: Training for (1) the person, (2) the sport, (3) the event, and (4) competition.

At the end of each step, I provide several questions whose answer will help you figure out the duration, frequency, and type of exercise that is best suited to helping you develop towards your athletic goals. (Keep in mind that in practice, these steps are far less discrete than I make them out to be.)

If you skip one step, you’ll have a very difficult time meeting the next. And the problem isn’t that you’re flaky, or that you’re not an athletic person, or that you’re not determined. No amount of determination will be enough to overcome the fundamental problem: That you skipped a step.

 Step 1. Training for the person:

 Even before you pick a sport to train for, it’s crucial to consider your overall situation: physical, physiological, psychological, nutritional, etc. If you’ve been sedentary all your life, hoping to suddenly be able to run and lift things over your shoulders will be damaging at best and impossible at worst.

Take a long, hard look at your particular body: all the muscle imbalances, digestion problems, moods, energy levels. Typically, any body is well-suited for its present activity levels: what, how long, and with what intensity you do whatever it is that you do. But the less activity you do (or that any part of your body does), the harder it is to change.

The best strategy is NOT, for example, to become a runner despite insulin resistance or a severe muscle imbalance. You’ll just hurt yourself in obvious and non-obvious ways. Instead, any training program should first address the constraint—muscle imbalance, insulin resistance, etc.—(and eliminate it) in order to bring the body back to a relative baseline of physical and physiological competency. What does that baseline look like? In a basic sense, when you go searching for odd pains, sorenesses, various symptoms of sickness, and you just can’t find any.

Keep in mind that while the process of doing so might include some “running” (for example), the fact that you’re “running” doesn’t mean that you’re actively training the running movement, or that you’re explicitly training for the running sport.

Ask these questions about yourself, and train according to the answers:

  1. At present, how (and how much) are you physiologically able to train?
  2. In the simplest terms, what is the biggest barrier to growth?
  3. Considering the answer to question (1), how can you train to remove it?

Note how question #3 is about training yourself out of the constraint, rather than mitigating the constraint through other means. NOT training yourself out of the need for orthotics (to the extent possible), means that it will be more difficult to get faster and perform more consistently. In systems terms:

“Any long-term solution must strengthen the ability of the system to shoulder its own burdens.”

This is how I start.

Step 2. Training for the sport:

 When I say sport in this context, I mean “the specific movement or movements required for participation in the sport.”

There are minimum basic requirements that must be met to even be able to participate in any given sport. (Training for proficiency at a sport comes later.) Any conceivable sport has minimum participation requirements in at least 5 domains of human motor expression: mobility, stability, skill, power, and endurance. However, for all sports, one or two key requirements reign above all others. For example:

  1. Deadlifting: The most salient requirement for deadlifting is more transparently understood as a mobility requirement: to perform a clean toe-touch. While standing upright with feet together and knees straight, to be able to reach down and tap your toes with the tips of your fingers without having to strain (read: while breathing continuously). If you can do this, it’s a good bet that you’re going to be able to consistently grow and develop in the deadlift.
  2. Running: The requirement for running is more transparently understood as a power requirement: To be able to accelerate into a cadence in the ballpark of 180 steps per minute (spm). This ensures that the critical neuromuscular processes necessary to efficiently maintain the running movement are developed enough to carry your weight.

(I say that a “salient requirement” is “more transparently understood as X” because if you really pick apart the toe touch or the ability to hit 180 spm, you’re going to find mobility, stability, skill, power, and endurance components for each.)

For some people, a cadence as low as 175 spm works just fine. I’ve yet to meet the person who hits peak efficiency below 170 spm. Keep in mind that a cadence of 180 spm is brisk as hell.

In order to meet that requirement, your joint stacking (the alignment of your ankles, knees, hips, and shoulders) has to be excellent—and has to stay excellent for the minimum amount of steps that it takes to accelerate into 180 spm. (And that’s just for starters. Maintaining a cadence of 180 spm for any kind of distance is much more difficult).

If you don’t have the requisite mobility in a given area (say, you have a hip restriction), movement becomes more awkward. That means you probably can’t produce stability: your abs can’t keep your upper body steady, making it difficult to control the arcs of motion of your arms and legs. So you can’t develop a high level of skill (the ability for your entire body to move in the best possible way given its structure and capabilities).

This means that it takes a lot more power to accelerate into a cadence of 180 spm. So, training for just about any event (short or long) becomes inordinately difficult—and as a result, you might just end up coming to the (wrong, wrong, wrong) conclusion that you’re “not athletic.”

A few guiding questions:

  1. What are the minimum requirements for your chosen sport (mobility, stability, skill, power, and endurance)?
  2. How (and how much) do you need to train to meet them?

 Step 3. Training for the event:

 I define event as: “the minimum planned volume of sports-specific activity.”

If the deadlifting competition starts at 100 lbs, then you better be able to meet the minimum requirement for deadlifting when loaded with a weight of 100 lbs. What does this mean? That you have to be able to perform the equivalent of a clean toe-touch—no straining—with 100 lbs on you.

It’s similar for running. If you want to run 100 yards, you have to be physiologically capable of accelerating into a cadence in the ballpark of 180 spm for 100 yards. If you want to run a marathon, you have to keep a cadence of 180 spm for the entire marathon.

This is why training for the event is s Step 3 in my list (and not Step 1). I’m well aware that a lot of people would like to pick from a menu and “choose” to run a marathon instead of a 5k because they “like” the marathon better. It doesn’t work that way. That would be like a novice “picking” to enter a deadlifting competition that starts at 250 lbs instead of 150 lbs, because they “like” 250 lbs more. For obvious reasons, you don’t do it.

What we don’t realize is that distance must be earned as surely as weight. Weight, is volume. Distance, is volume. They may not be the same kind of volume, but they’re both volume. They both deserve the same respect: they’ll both break you (in different ways) if you don’t train accordingly.

If you haven’t earned a certain distance (read: if you can’t physiologically meet the sports-specific requirement for the entire duration), pick a shorter distance. Here’s 2 questions to help you in this process: 

  1. What are the sports-specific requirement at the planned volume (duration, weight, speed, etc.)?
  2. How (and how much) do you need to train to meet them?

Step 4. Training for competition:

I define competitiveness or competence as “being able to exceed the sports-specific requirement for a particular event.”

It has nothing to do with being particularly good (that would be “elite-” or “semi-elite competitiveness.” It’s just about being better than the minimal physical and physiological requirements the event requires.

Training for competition, then, occurs when you can already meet the sports-specific requirement for the event, and now you want to exceed it. This is also a great way to gauge whether you’re ready for a more demanding event. Once you can hit 190 spm for 100 yards, you’re pretty sure you can train for 200 yards at 180 spm (and expect to make good gains). Same with deadlifting: if you are able to do 2 reps at 100 lbs, you can probably start training (say) for 1 rep at 150.

An important caveat: None of this means that the best, or the only way to train is to increase reps first, or increase power first (or whatever). Training is always strategic and multileveled, and you always approach it from as many angles as there are people in the world. The above only means that exceeding the sports-specific requirements at a given event is a decent gauge of whether you’re ready to train for a more challenging event.

  1. Can you exceed the event-specific requirements?
  2. How (and how much) do you need to train to exceed them for . . .
    • Greater competitiveness at the same event?
    • Participation in a more challenging event?

Final thoughts:

In future posts, I’ll break down these steps further and provide concrete examples of what they look like in training. I’ll discuss how to use the 4 steps together to design a more comprehensive training plan.

Runners: “Aerobic training” is not the same as “Endurance training.”

It’s common that training which develops the aerobic system is equated with training that increases the body’s endurance. It’s understandable: the aerobic system burns fats in the presence of oxygen in order to provide long-term energy for the body—exactly what it needs for endurance. But the problem is that a powerful aerobic system isn’t the only thing necessary for increase endurance.

The most important difference between “aerobic training” and “endurance training” is this: the former trains a critical supersystem of the human body (the aerobic system), while the latter improves the product of the successful interaction between the aerobic system and many other parts and functions of the body (endurance performance).

What runs isn’t the aerobic system—it’s the entire body. While the aerobic system can be powerful, it can’t perform on its own. Whenever we talk about “performance,” even when the subject is endurance performance, we’re talking about how (and how well) the body uses its aerobic power to create one particular kind of athletic movement.

Roughly, endurance means: “how long the body can produce a particular movement or action without falling below a minimum threshold of performance.”

Another way to say this is that the aerobic power is general, and endurance is specific. Geoffrey Mutai (elite marathoner) and Alberto Contador (Tour de France cyclist) both have extraordinary aerobic systems. In both athletes, all the parts that enable their muscles to be fueled for long periods of time are extremely developed.

It should be noted that in both athletes, we are talking about developing essentially the same parts, developed to comparable levels and talking to each other in very similar ways. Both these athletes also obtain fundamentally the same general physiological benefits—a greater ability to recover, better health, longer careers—all despite competing in wildly different sports.

However, their endurance in specific sports varies wildly. We can expect Mutai to be a proficient cyclist, and Contador to be an able runner, but we can expect neither to have world-class endurance in the other’s field. In other words, Mutai’s endurance is specific to running, and Contador’s is specific to cycling. This is because:

  • Both sports use different sets of muscles: runners use a larger set of muscles for stability than cyclists, since the latter have so many more points of support. Cyclists have the handlebars, pedals, and seat, whereas runners have at most 1 foot on the ground.
  • They load joints in different ways, and use very different ranges of motion: cyclists keep their waist and hips relatively flexed, while runners keep the same joints extended.
  • They use different neuromuscular mechanisms to facilitate endurance: running economy depends on a powerful stretch-shortening cycle, while cycling economy does not.

In my opinion, the stretch-shortening cycle is the most important piece of the running puzzle (and also one of the most overlooked). Running shares a lot of pieces with just about every sport—and developing them is very important if you want to become a good runner. But without an increasingly powerful stretch-shortening cycle, all the power that you develop in any other system (cardiovascular, respiratory, etc.) doesn’t translate into actual running performance increases.

As discussed above, the aerobic system is responsible for sustaining endurance. The best way to exclusively train the aerobic system is by running at a physiologically intensity (below the aerobic threshold).

This is a problem for less aerobically-developed runners: it takes a lot of juice to run the stretch-shortening cycle effectively. In previous posts I discussed how the minimum requirement for running properly is to be able to produce a (very fast) cadence of around 180 steps per minute (spm). This is because the muscles’ stretch-shortening cycle hits peak efficiency around that cadence.

So, these runners often need to run at a higher intensity: they’ll use the maximum output of the aerobic system at max and engage some of the anaerobic system in order to produce a cadence of 180 and properly activate their stretch-shortening cycle. If they fall below their aerobic threshold with the goal of doing “aerobic training,” their cadence falls and the stretch-shortening cycle will largely deactivate.

When I talk about hitting 180, I mean hitting 180 at an average step length: It’s possible for a weaker runner to shorten their stride to artificially increase their cadence without going above the aerobic threshold. But I consider this a rather useless hack, since in my experience it doesn’t really get runners the performance benefits expected of reaching “the magic 180 mark.” (More on this in a future post.)

For a workout to be “running performance training” (endurance or otherwise), it needs to train the key pieces necessary to improve running performance. So whenever you’re not actively training the stretch-shortening cycle, you’re not really doing “running performance training” in my book. “Running endurance training” would be about teaching the body how to run for longer, at a lower intensity, while maintaining a reasonable cadence.

So, whenever an aerobically weak runner trains under the aerobic threshold, I consider it to be quality aerobic training but NOT “running performance training.”

It’s not that their running performance won’t increase—it will. Let me illustrate with a rather extreme example: If playing checkers is the only active thing someone does, playing checkers is better for their running performance than not doing so. But because it doesn’t train the critical systems for running, I don’t think of it as “running performance training.”

Of course, running at a low cadence shares a lot more with running at a high cadence than playing checkers does. But the idea here is to set the highest possible bar for what “running performance training” should mean: training the key systems that running performance rests on. And running without substantially activating the stretch-shortening cycle really doesn’t meet that criteria.

(We can say that running without the stretch-shortening cycle still helps you to improve your running—to a point. But you can’t hope to maximize your performance gains without it.)

For a competent runner (someone who can engage their stretch-shortening cycle at low physiological intensity), “aerobic training” and “running endurance training” become identical: just about all of their training provides all the benefits they need to maximize their running endurance.

What is a less-powerful runner to do with all this information? If I could say only one thing:

Jump rope! Jumping rope (on both feet, alternating feet, on one foot, spinning around, crossing the rope, etc.) is training primarily the stretch-shortening cycle up and down the body, almost identically to the way it’s used in running. IMO, if a runner does only one other thing besides running, it should be to explore and master the jump rope to its fullest potential.

UPDATE Nov 18, 2016: Another (great!) article on the mechanics of running, also touting the potential of jumping rope.

But there’s a lot more than this. Now that I’ve covered all the theoretical ground I absolutely need to cover for my following posts to have any real substance, I can begin to discuss concrete strategies that the runner can use.

Addendum (for the curious): Why do I focus so much on fleshing out the principles (and, more importantly, taking so long to get to the processes)?

Because the idea, of course, isn’t to “balance” aerobic training with performance training. (That’ll only increase endurance.) The idea is to potentiate aerobic training with performance training. (That’ll maximize endurance.) And to turn balance into potentiation, it’s necessary to already have understood the “why.”

The Runner’s Catch-22, Part 2: Power Facilitates Endurance.

In my first post of this series, I discussed a very common training problem plaguing the beginner runner: that it takes a certain amount of power to habitually produce an efficient running cadence (in the ballpark of 180 steps per minute, or spm), and it takes incrementally more power to produce it over longer and longer periods of time.

Enter the beginner, relatively untrained runner, who aspires to run longer races such as marathons. While it’s quite possible to run at 100% of maximum power output for 100 yards, it’s necessary to run longer distances at a decreasing percentage of the body’s total power output: in order to sustain activity for the long periods of time in which it takes to run a marathon, a runner must be working at around 55-65% of their maximum power output.

The problem is that producing an efficient cadence takes power. What happens if it takes 85 or 90% of your total power output to produce an efficient cadence? You won’t be able to sustain that cadence for a mile, let alone a marathon.

(This is a bigger problem than it seems.)

Think about deadlifting a 250 lb barbell. It’s not just about being able to lift the damn thing. At that weight, you should be able to (say) maintain the shape of the lower back, relax the shoulders, and produce a proper hip flexion and extension through the entire movement. The point is that it’s not just nice to be able to meet the minimum power and mobility requirements for the deadlift. You have to, or you’re flirting with injury.

Same thing for the marathon—it’s about being powerful enough to sustain a cadence in the ballpark of 180 spm for the duration of the entire race (for starters). This means that you need to be a good bit more powerful to run a marathon than to run a 5k.

In order to produce a certain cadence for a long period of time, you must be more powerful than to produce that same cadence over shorter periods.

 Over the course of this series, we’ll keep coming back to the same issue: in order to run well, the muscles need to be powerful enough to produce that cadence. If they’re not, they’re less efficient. Let me be completely clear: a powerful runner who can hit 180 spm habitually is more efficient than one who can’t. Let me reiterate this: if you are powerful, you get an added efficiency bonus that a less powerful runner doesn’t have. One last time: if you’re weak, you’re slow and inefficient, but if you’re powerful, you’re fast and efficient.

There is a crazy tangle of ironies to be exposed here: when the muscles are too weak to produce a cadence of 180, it takes a lot more muscle power to be able to run at the same speed. But because your muscles are weak, the speed you are able to run at is much, much slower than you’d expect if you supposed that both the fast and the slow runner were equally efficient.

If you’re powerful enough to produce a cadence of 180 for 50 or 60 miles (in other words, really powerful) you get massive dividends in energy savings.

(This is related to why the “correct” running form—not just for sprinting, but for all running speeds—is the one aligns the body in such a way to help it produce the most power.)

Thanks to this, runners like Jim Walmsley are able to sustain blazing speeds for very long periods of time. Gear Junkie reports that Walmsley recently crushed Rob Krar’s Grand Canyon rim-to-rim-to-rim record, running 42 miles with over 40 thousand feet of elevation gain (and another 40,000 of elevation loss) in just over 5 hours and 55 minutes.

Power is necessary for endurance for very specific reasons. In order to produce endurance—a.k.a. to stay in activity for long periods of time—you need to be burning fuel for long periods of time. But the body’s fuels (fat and sugar) aren’t created equal. The body burns less fats and more sugar as it works at a higher percentage of its total power output—a problem because even a very lean body stores about 100 times more calories in fats than it does in sugars.

Let’s say you’re trying to run at an efficient cadence. The less powerful you are, the more sugars you’ll have to be burning to sustain that cadence. Even if you’re burning 40% sugar to sustain an efficient cadence, you’ll run out of sugars that much more quickly than a more powerful athlete—who might only need to burn, say, 15% sugar to sustain the same cadence.

At some point, you’ll be left with 2 choices: (1) stop running, (2) reduce your cadence (and speed) to the point that you’re burning almost only fats.

Notice how stark these choices are: number one means that you just can’t run as far as the more powerful athlete. And number two means that now that you’ve bonked/hit The Wall—yes, this is what “hitting The Wall” means—you need to run the rest of the distance less efficiently than you’ve been doing so far. Got it? Now that you’re exhausted, you need to spend more energy per mile for the rest of the run.

We’re getting at what it really means to be “ready” to run a marathon—or any other race. It isn’t just about being capable of finishing itin the sense that your body didn’t fall apart before you got to the end. You need to be able to run the whole thing above a minimum threshold of performance. (Now you tell me what that is.)

The Runner’s Catch-22, Part 1

I’m calling this series of posts “The Runner’s Catch-22” to address a very common problem in the running world. A lot of beginner runners—let’s face it—want to run long. Very long. But in attempting to do that, they get ill, injured, or overtrained. And their hopes of running long (and doing so consistently) get quashed.

Running isn’t just about running (as every injured runner knows). It’s about how to run well. But in all sports—in fact, in all movement—there’s a minimum power requirement that must be met: if you want to stand (correctly), your legs, along with your core and spine, have to be able to move into a standing position and be strong enough to support you. If you want to walk (well), your leg joints have to be able to flex and extend to a certain degree, and one leg has to be able to support more than your bodyweight while the other travels through the air. And if you want to run (properly) you have to be able to meet an even more demanding set of requirements. And this is where the story of the “Runner’s Catch-22” really begins.

A lot of things have to be working well for a runner to be powerful—form and movement are vital, for example. Having proper form feeds into your ability to produce power (in the same way that it would work for a weightlifter or a baseball player). So with poor form, you might never be able to meet the power requirement—or go significantly beyond it. So, what is this power requirement?

The body must be able to produce a habitual cadence in the ballpark of 180 steps per minute (spm). 

The body is most efficient at around 180 spm: this is the cadence that best engages the tendons’ elastic component, maximizing the amount of energy that can be taken from the previous step put into the next one. (This is a concept also known as energy return).

UPDATE: For people who are new to running (particularly those who only started being active as adults), meeting that power requirement usually requires a lot of power training, which is a problem for beginners. Experienced runners often are able to produce a cadence of 180 spm easily and habitually, for runs of any distance. (In fact, hitting 180 easily is how I would define “experienced.”) If that’s you, most of this post won’t apply to you.

Power training uses and develops the body’s anaerobic system, which is very powerful, but also produces negative by-products that, in large quantities, are ruinous to the body’s tissues. The anaerobic system is counterbalanced by the aerobic system, which disposes of those harmful by-products and allows the body to remain in activity for long periods of time.

So if you want to be able to train without trashing your body, you need a powerful aerobic system to support the anaerobic system. Just one little problem: while the anaerobic (powerful but dirty) system grows extremely quickly, the aerobic (less powerful but clean) system grows veeery sloooowly.

This is the runner’s Catch-22: Until you have a well-trained aerobic system, it is almost impossible to safely do large amounts of anaerobic training. Trying usually means burnout, illness, injury, or overtraining. But if you can’t do a lot of anaerobic training, you can’t develop power to the point that you can produce an efficient cadence (of 180 spm) at the kinds of low intensities where you can develop the aerobic system.

The wrong move—the one that so many runners take—is to lower their cadence to run more distance. Why? Because they’re set on running, or because they don’t know that there’s better ways to train the aerobic system when you’re not powerful enough to ballpark 180 spm:

  • Cycling/Spinning
  • Walking
  • Rowing

(I’d add bodyweight circuit training to this list, but it’s typically far more aerobically demanding than running would be.)

It’s important to realize that the other option—running at an inefficient cadence while the aerobic system develops—is NOT a neutral, “eh, screw it,” kind of option. It’s not very bad—the aerobic system will probably still develop in time—but it’s not the fastest way to train, and certainly not the best way to guarantee you’ll achieve your goal.

(There’s ways to produce a cadence of 180 at slower speeds, such as shortening your stride. But that opens another can of worms—to be featured in another post of this series.)

Learning a movement pattern the wrong-slash-less powerful way—yes, they really are the same thing—is the best (and probably least-discussed) way to prevent you from performing at a high level. If you learn how to throw a ball by releasing it far forward of your body instead of at ear level, you’ll very quickly plateau in terms of how much force you are able to put into it (meaning that you’ll never throw at 60 mph, let alone 90).

Your body develops through movement. If you don’t move, you don’t use your muscles, which means that your metabolism doesn’t develop.  If you can’t throw a ball faster than 60 mph (because of poor mechanics), your muscles won’t be able to grow in strength beyond what it takes to throw the ball at 60 mph. So your metabolism (aerobic or anaerobic) will never need to grow beyond that.

It’s impossible for your metabolism to grow to be able to produce an energy expenditure that you don’t have the biomechanic possibilities to harness.

Slow or low-cadence running isn’t a death sentence. Slow runners with relatively few biomechanical problems or muscle imbalances do increase their cadence and low-level strength by slow running . . . in time. So it’s often the case that people do end up running much faster and at a much higher cadence after a few months (or years) of slow running. But your power (and your cadence) won’t improve with slow running as fast as it could with actual power and cadence training.

How to get around the Catch-22? Below is the short answer. (The long answer will take a few posts).

  • An overwhelming amount of aerobic training (in sports where you can meet the power requirement).
  • A small amount of running-specific power training (mostly plyometrics).
  • A small amount of running at a cadence in the ballpark of 180 spm.
  • Monitor metrics including HRV (heart rate variability) and MAF (Maximum Aerobic Function) Test to determine your short- and long-term physiological readiness for power training.

Endurance: the ultimate test of physiology.

For a beginner runner to get into running because in 6-12 months they want to run an ultra-endurance race (or even a marathon) is—to put it mildly—folly.

There’s a reason the marathon is the final event in the Olympics: It’s by far the hardest. A recent The New Yorker article reports on one athlete trying to describe the experience of running a 2:10 marathon: “You feel like you will die. No, actually die.”

There are fundamental differences between the endurance sports and the power sports. Oftentimes, when discussing these differences, people think about what gives an athlete a competitive edge: for power sports, it’s higher concentrations of Type I muscle fibers. For endurance sports, it’s more mitochondria, and a greater oxygen carrying capacity.

This is important, but it’s not what I’m talking about. I’m talking about understanding the endurance sports by attempting to discuss what “endurance” is—not human endurance, or endurance at sports, but rather what “endurance” means in a fundamental sense. And for that, I find it best to discuss extremes.

Take a power sport: the 100 meter sprints, for example. Usain Bolt is a phenomenal athlete. There’s no question about it. And there’s no question that there’s a certain glory to be had in being the fastest human being on the planet—glory that is simply not available to the marathoner. Let’s set that aside. What would have to happen for Bolt to be unable to continue competing?  In other words, what would “catastrophic system failure” mean for Bolt?

My answer is: an ACL injury, or a torn hamstring, probably. In other words, something breaks.

Now let’s look at the marathon. Rarely does something break in that way in the endurance sports. There’s plenty of microdamage—achilles tendinitis, stress fractures, chronic fatigue, etc. But when something breaks, truly breaks to a point where the person cannot compete (in the “catastrophic system failure” sense discussed above), what does that look like? It’s typically the entire system that fails. Take a gander at a list (compiled off the top of my head) of the quintessential ailments you see in a marathon:

  • Extreme dehydration
  • EAH/EAHE (Exercise Associated Hyponatremia/Hyponatremic Encephalopathy)
  • Heart attack
  • Kidney failure
  • Heatstroke
  • Respiratory infections

What these issues all have in common is that they’re systemic failures—they’re what happens when the body as a whole, rather than a specific part (say, the hamstring) can’t cope with the event. In other words, they’re what you get when the body starts to come apart at the seams.

The best way to think of this difference is that when you bust a hamstring (or even break your spine in certain places) you can still use your body as a whole except for the part you broke. But when you get any of the illnesses that typically occur during a marathon, it’s the entire body that is put out of commission—sometimes permanently.

To put it simply, we can think of speed and power as a question of how powerful the body is. And while speed and power have tons of importance in the endurance sports, we can say that endurance is primarily a question of how good the body is at holding itself together. In other words, endurance is a test of the body’s fundamental integrity: of how much stress can you subject it to for how long without any substantial collapses in any critical processes.

And this is the main difference between the endurance and the power sports. In the power sports, the body has to be very, well, powerful, but it doesn’t have to be all that good at holding itself together—at least not in ways that relate to the ailments described above. After all, the power sports only ask the body to perform for a few moments: it stops before it becomes dehydrated, or before enough lactate builds up that the kidneys fail, or before the lungs become stressed enough that they become susceptible to infection (etcetera, etcetera).

But that’s not the case in the endurance sports. The body is going to be in activity for a very long time. If any of its systems (respiratory system, cardiovascular system, etc.) are working at different rates, some of those systems are going to get tired first. This is a problem: those systems were only active in the first place is because they were providing a critical service to the body’s endurance performance.

When one of those systems fails, some critical process associated with it also stops. If the body continues activity in this state, critical processes start falling like dominoes. And the body starts coming apart at the seams.

It’s not that endurance sport are “better” or “more of a sport” than power sports. But it is the case that being highly successful at an endurance sport takes much more time, much more consistency, and much more athletic maturity than to be highly successful at a power sport. This is why, for example, it is not uncommon to see 19 and 20 year old athletes competing in power sports at the Olympic level—the 400m, the 1500m, etc.

It’s usually those very same athletes who, 10 or 15 years later, are running marathons. Once their athletic career was already taking off, it took their body an additional 10 to 15 years to be physiologically organized and cohesive enough to run a marathon.

On the other hand, any athlete who is seriously contending for a medal at an endurance sport at 20 years of age, is a unicorn. Either they’re already so athletically mature that they’ll have a wildly successful career ahead of them, or they have already pushed themselves so far, so fast, that decades of chronic illness and overtraining are already on the horizon.

It’s almost impossible to do an “easy workout” when you’re stressed.

A while ago I read an excellent article titled Why heart rate always matters. It goes into great detail on a topic I’ve previously discussed here on running in systems: why the heart rate is always going to be an excellent representation of what is happening with the body’s stress response and energy metabolism. I think that some of the topics it discusses, as well as the excellent debate in the comments, are worth expanding on. Here’s an excerpt from it:

“Our fight-or-flight system often activates without any actual demand. When we get ‘stressed out’–engaged in a heated argument, mulling over a burdensome worry, or simply sitting in traffic–seldom is any physical task being undertaken. But the body is being activated. The engine is revving higher and tremendous sugar–the preferred fuel of fight-or-flight responses–is burned when under psychological stress, which is a major factor in ‘stress eating!’ We function as if we’re fighting an intense battle.

Stressed out and going for a run? Your body will perceive the cost of that run as higher (because it is already dealing with your life stress) and will activate a more intense energy system to cover all the demands. More energy cost!”

There was a particular comment in the article that I wanted to address:

“Very well written article and I agree with most of it.
However, I think you overstate the impact of activation level on energy expenditure…

…In my understanding, the energy demand dictates the energy production. And the energy demand is mainly dictated by the mechanical work of the muscles and all the side processes needed for that level of power output.
I agree, that the excitation level directly impacts the chosen energy supply system but as long as this system doesn’t actively provide energy, it’s [maintaining] cost will be relatively low.
Yes, a higher activation will have a higher energy demand but I don’t believe it’ll come anywhere close to exceeded mechanical [energy] demands.”

I agree with the commenter in that I, also, believe that the author was overstating the impact of activation level on energy expenditure. However, I think the author’s overstatement makes it difficult to observe 2 key implications of this discussion:

  1. Activation level  (a.k.a. stress) changes the type of energy metabolism, which means that it changes the ratios of fuel (fat and sugar) that it uses.
  2. Training stimulus is inextricably tied to activation level and energy metabolism. This means that the ratios of fuel usage have a much bigger say in how the body perceives the workout (as low-intensity vs. high-intensity) than the rates of fuel usage.

The point is that while the author does overstate the energy cost of the stressors he mentions, it doesn’t really matter—there’s things the athlete just can’t get out of training if their body is taxed in the ways the article mentions.

A lot of people think that low-intensity means “slow,” “easy,” or “consuming little energy.” It doesn’t. Low-intensity is when the workout is easy on the body—specifically, when the body is burning a majority of fats for fuel, and the sugar that is being utilized is burned wholly aerobically  (in the presence of oxygen). In other words, there is no substantive anaerobic work. Highly-trained endurance athletes, who burn fats at much greater rates than the rest of us, can run at very high speeds while remaining in a completely aerobic state. Such an athlete may be running blazing times in a workout that is for them, metabolically speaking, a low-intensity workout.

Now let’s look at higher intensities: In order to produce the energy necessary to approach your top speed, a lot of changes have to happen within the body. One of these is that the body has to go from burning a greater percentage of fats (which burn relatively slowly and so provide energy at a relatively lower rate), to burning a greater percentage of sugars (which burn relatively more quickly and so provide energy at a much faster rate). So, in order to get closer to your top speed, a greater percentage of your energy has to come from sugar.

In order to release more sugar to the bloodstream (to be utilized by the muscles), the body releases hormones called glucocorticoids—glucose (a.k.a sugar) releasing hormones. The main glucocorticoid is cortisol, which many will recognize as the main stress hormone. Another hormone that is release during the stress response is insulin, which helps muscle cells avail themselves on the sugar that cortisol released into the bloodstream. Cortisol and insulin, then, work synergistically to produce (and increase) sugar metabolism.

To recap: want to run closer to your top speed? You need to release more sugar. How do you do that? By getting more stressed. But because of some of the body’s more complex molecular mechanics—fodder for another post—the body can’t release a bunch of sugar and still be releasing fats. What would happen is that you’d just flood the bloodstream with unhealthy concentrations of both fuels. So, when insulin is released or when anaerobic function (which is dependent on sugar) increases, fat-burning drops. If sugar-burning goes up, fat-burning goes down (and vice versa).

This works the other way around too. If you get more stressed because, say, you had a rough day at work, or you got into an argument, you’ve got more cortisol and insulin running through your body. But it’s not like the body can decide to release (and use) sugar only when the reason for cortisol and insulin release is because of increased athletic demand (a.k.a. athletic stress). For any other stress (work stress, etc.), cortisol and insulin become released, and increase carbohydrate metabolism. Research on the metabolic effects of social stress in fish supports this idea.

This, incidentally, is why people get tired after a stressful day at work or an argument that stretches for too long. They didn’t use up all their fat-stores at work, obviously. But because the stress put them in sugar-burning gear, enough of their sugar ran out that they start feeling tired. It’s not that they ran out of fuel, but rather that they ran out of the fuel they’ve been stuck using.

It also takes a relatively long time for the cortisol to get out of your system—and when it does, it’s not like you can just pop back into action and go for a run. The adrenal glands, which put out cortisol (not to mention various other mediators of the stress response) have been used up. They’re tired, and will resist further activity. And since you use all the glands in the body to one (significant) degree or another during training, it’s not a good idea to train with exhausted or depleted glands.

Asking your body to work out when you’re already out of a major fuel and your stress glands are tired is an even worse idea: the “same” workout is relatively much harder for a tired gland that’s nearly out of adrenaline and cortisol than for a rested gland. Training after a period of stress is, in physiological terms, almost exactly like doing back-to-back training sessions. Effectively, you’re extending the period of stress.

And if on top of that, your blood sugar is low (as usually happens after a period of stress), you’ll be asking those tired glands to produce even more cortisol and adrenaline than they would usually have to: in their already tired state, it’s not enough to simply produce enough cortisol to maintain blood sugar levels—they have to make up for the lack of sugar in the bloodstream.

If on top of that, you’re “stuck” in sugar-burning mode because you still have all that errant cortisol and insulin flowing through your system (since you’re still stressed), you’ll be depending on sugar—which you’ve substantively burned through—for the duration of your training session. Because the body is inhibited from fueling itself with fats (due to the insulin in your system), it has to rev up those exhausted adrenals even more to provide the requisite cortisol.

Insofar as your body is stressed, it will respond to what is normally an “easy” workout as if it were a “mini high-intensity workout.” In other words, you can’t really have a “low-intensity training session” when you’re stressed (and expect to accomplish your goals in any sort of way). 

This is why doing MAF training—exercising under the aerobic threshold—under stress (or after a period of stress) produces such a dramatic drop in speed/power output at the same heart rate. When you’re under stress, exercising at a rate that looks anything like the aerobic training you do when unstressed would mean elevating your heart rate far beyond your aerobic threshold. Because aerobic work output is so reduced in a stressed state, it’s a much better idea—and a much simpler fix to the problem—to simply rest for the day and do your “easy” training session tomorrow.

What the hell is the aerobic system? Part 1

Frequent readers of my blog know just how much I like to use car metaphors to describe the human body’s function. So here’s another one: the aerobic system is the body’s main powertrain.

(The powertrain is the chain of systems within the car that power gets channeled through: from the engine, through the gearbox, down the main drive shaft, across the differential, and into the wheels. The drivetrain on the other hand is typically understood as the powertrain minus the engine.)

When most people think of increasing aerobic function, they think of increasing the capabilities of the body’s aerobic, Type I muscle fibers (also known as slow-twitch fibers). While muscle fibers are hugely important—they are the main power producers of the body—they are one subsystem of many that need to be working synergistically and at similar rates for the aerobic system as a whole to be able to express any kind of power.

It’s important for us to realize that when we are talking about developing the aerobic system, we are talking about much, much more than just the aerobic muscle fibers themselves. Quite literally, the whole powertrain from beginning (lungs) to end (muscle fibers) needs to work and develop together for it to be of any use.

The body, unlike the car, stays on all its life. The car can shut off if it runs out of fuel. But if the same thing happens to the body, it dies. So any system that is going to take on the responsibility of being the body’s main powertrain has to be able to provide a stable flow of energy over a very long term.

The best way to accomplish this is by burning a cheap, safe, light, efficient, and plentiful fuel source: fats. (As I’ve discussed before, burning carbs/sugar comes with a lot of strings attached: it’s dirty, heavy, scarce, inefficient, addictive, and dangerous. The only real advantage it has—and it is a BIG advantage—is that it produces energy at a much greater rate than fats.)

Being the system that provides stable, long-term energy means that you need to burn the stable, long-term fuel. Because of this, the aerobic system has to burn fats in particular as its primary fuel.

In other words, I use Phil Maffetone’s rendition of what the aerobic system is. This means that while I like statistics such as VO2Max (maximum volume of oxygen utilization per minute) as measures of aerobic power, I don’t believe they are a measure of the functionality of the aerobic system. Why? Because you can consume far more oxygen when you’re burning sugars than when you’re burning fats. And besides, we’ve defined the aerobic system as providing energy over the long-term. Therefore, aerobic functionality has to do far more with fat-burning, which occurs in a big way at moderate percentages (55-65%) of VO2Max, than with sugar-burning (which occurs in a big way at 65-100% of VO2Max).

(Note that very highly-trained endurance athletes are often an exception to these percentages. Why that’s the case is for another post.)

One of the reasons this system has the name “aerobic” is because fats cannot be burned outside of the presence of oxygen. So, bringing oxygen into the body and enabling its efficient transport throughout is absolutely essential to our capability to use fat as fuel. In fact, this is one of the most important differences between fat and carbs: carbs unlike fats, can be burned both aerobically and anaerobically.

This may seem like an advantage, but it’s somewhat of a disadvantage—in the same way that the disadvantage of cocaine is how powerful it is. You’re far better off experiencing the feeling of reward in the less powerful, sustainable, and old-fashioned way.

This is not to say that sugar has no place in our utilization of energy: at any given point in time when we’re at rest or doing light activity, we’re burning a small percentage of carbs. But when sugar stops being your auxiliary fuel (and becomes your go-to fuel), you’re in trouble.

By primarily using a fuel that is very powerful, it’s much easier to use only that fuel. Why would you use the other, less powerful fuel? (Sure, because it’s lighter, cleaner, safer, and more efficient.) But there’s also this: since carbs burn way quicker, the body can get lazy and forget about maintaining its fat breakdown and transpo systems, with little short-term negatives—but huuge long-term drawbacks.

By the time that the downsides of relying primarily on sugar begin to roll around, the body is hooked and the systems that burn and transport fat are in utter disrepair. The body can only store about 2,000 calories of carbs at a time (compared with some 120,000 calories of fats on the low end). When it prefers sugar over fats, it has to be eating all the time.

In layman’s terms, this is known as a “metabolic SNAFU.” (That acronym fits particularly well here, just because of how ubiquitous and “normal” this situation is.)

So what are these systems? Let’s trace the flows of oxygen and fats to find out.

Fats have to be broken down from fatty tissue, transported through the blood vessels, and burned by the mitochondria—the cell’s aerobic motors.

Oxygen comes into the body through the respiratory system, then gets transferred to the circulatory system, and finally permeates into the muscle cells where it is used as a reactant to convert the fats into energy.

But if we’re going to talk about flows of materials (oxygen and fats), it’s not enough to just discuss the parts that they flow through (and the systems that convert them into energy). We have to talk about the parts that regulate those flows, for the simple reason that if those regulatory systems stop working, chaos ensues. So these regulatory systems are as critical to the function of the aerobic system as, say, the car’s computer is to the function of its powertrain. It’s a part of it, pure and simple.

Let’s look at oxygen.

As any asthmatic or person with hay fever will tell you, those regulatory systems make a difference. The reason a lot of people start wheezing when they run too hard for too long is because the part of the nervous system responsible for increasing the body’s activity levels (known as the sympathetic nervous system, or SNS) gets too tired, and its function collapses. A crucial part of increasing activity levels is to open up, or dilate, all of the body’s ducts (a.k.a the airways and blood vessels) so that more stuff can flow through, at a faster rate. So, when the SNS becomes exhausted, its ability to keep the airways dilated goes away. Its opposing system—the parasympathetic nervous system, or PNS, whose job it is to shut the body down—takes over. One of its jobs is to constrict the airways—and so they close up (hence the wheezing).

Regulation of fat-burning functions in a very similar way. The system most directly responsible for regulating fats is the endocrine (a.k.a. hormonal) system, affecting primarily (and IMO most critically) whether or not, and at what rate, fats are broken down. This process is known as lipolysis: lipo = lipid (fat); lysis = breakdown.

Lipolysis is accomplished partially thanks to a hormone called leptin. In healthy humans, leptin exists in the bloodstream in a big way only when the body is at a reasonably low level of stress. So, one of the reasons that fat-burning starts going down at an exercise intensity even slightly over moderate—which is known in the biz as the AerT or aerobic threshold (go figure)—is because the increase in exercise intensity puts out stress hormones that inhibit the activity of leptin. As exercise intensity increases beyond the aerobic threshold, lipolysis begins to slow down.

So it doesn’t really matter if the muscles have a whole bunch of mitochondria that were developed by training at a high intensity (remember: a.k.a. stress), and burning lots of sugar in an aerobic way. If the body’s lipolytic systems haven’t been trained, it’s going to burn very, very few fats during exercise. So it doesn’t really matter what’s going on in the muscles. Muscles (even aerobic muscles) get really big really fast and their ability to consume fuel increases very quickly—but the rate of lipolysis takes much longer to improve.

The rate at which the body is capable of breaking down fats (rather than the rate at which it can burn them, or the rate at which oxygen can be supplied) is typically the bottleneck. And that’s why “fit” people all too often manage nary a shuffle when they start running under their aerobic threshold: they’re sugar-burning beasts, but under the AerT the hormones are optimized for burning fats, not sugar. Those powerful muscles they have? They’re being fed fats with a teaspoon.

And one of the reasons it feels so slow is because they’re exercising at a relatively small percentage of their oxygen intake and transpo capacity. Why? Because they’ve trained it primarily in concert with their sugar-burning system. Their fat-breakdown system needs to become waaay stronger before it’s going to break down fats at a rate that is challenging or even meaningful to their present oxygen transpo capabilities.

Understanding which systems comprise the aerobic system is far less important than grasping the point that the aerobic system really is the entire powertrain. It’s far less critical to know whether your car has a carburetor or a fuel injection system, than to know that you should consider how the entire powertrain (and the car as a whole) is going to behave when you decide to upgrade some particular component.

If you’re going to swap your L4 engine block from with a V8, you also have to swap out the fuel pump and a host of other systems (not to mention the entire chassis)—or you’re going to end up with a V8 engine getting fuel at a rate meant for an L4. The understanding that you need to go look at the whole picture, instead of just at the muscle fibers (or whatever)—will inevitably take your search in the right direction.

There’s a bunch of other parts of the aerobic system left to cover. In my next post I’ll talk more about how the fat-burning process goes down and why it’s impossible to burn more fats when the rate of sugar-burning goes up. I’ll also get more into why the body is wired to rely more on sugar as stress levels go up (hint: because sugar burns faster). In later entries I’ll talk about how the various other parts of the aerobic system interact with each other, and why aerobic function can really only be developed and optimized at relatively low levels of exercise intensity.

Biological, psychological, and social systems affect our development of speed, power and endurance. Let's discuss them candidly.