Tag Archives: aerobic system

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.”

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.

Defining the “long run” for better endurance training.

The long run is touted by many to be the centerpiece of training for marathoners and other endurance runners.

Most people think of the long run as a protracted effort that causes their body to produce the mental and physical adaptations needed for endurance races. But the ways in which people prepare, fuel, and run during these long efforts are often not the most optimal. And the reason is because long runs aren’t about running long per se—they are about training the particular systems of the body that enable us to run long.

This isn’t just wordplay: I often see well-intended runners filling their hydration belts with sugary foods and energy gels in preparation for a long run.

That’s a problem.

Let’s consider which of the body’s systems are designed to help us run a long distance. We need a very abundant fuel source, as well as an engine that can burn that kind of fuel for a long time. Sugars (a.k.a. carbohydrates) won’t be a good primary fuel source: they exist in relatively small quantities inside the body compared to fats. Furthermore, the Type II (fast-twitch) muscle fibers that utilize them fatigue quickly.

So we need to rely heavily on a more plentiful fuel: fats. In order to burn fats, we’ll need to use several systems: the hormones that help break down and transport fat, and the Type I (slow-twitch) muscle fibers that can burn them (as well as the lungs, heart, and blood vessels, which together allow oxygen to get to the muscle fibers and enable fat utilization).

Running for a long time is all about burning fats. But when a runner depends on sugar to fuel their long runs, as far as the metabolism is concerned, it’s not a long run.

Using sugars to fuel the long run means that (1) not only is the quickly-fatiguing sugar-burning engine being used for much longer than it’s designed for, but (2) it’s only being relied upon because the engine that is supposed to do the job isn’t powerful enough to produce the required activity levels.

The body is getting tired and worn down at an absurd rate. But that’s also only happening because it was already not capable enough to run that fast for that long.

As the body gets tired, it gets stressed. As it gets stressed, it use of oxygen declines, and it starts being forced to consume sugar anaerobically—without the presence of oxygen. This compounds the problem: the main by-product of anaerobic activity—lactate—suppresses the body’s ability to use fat for fuel.

What does this do to our definition of a “long run”?

I like to define the “long run” as a run that occurs (1) below a threshold of stress that allows for burning fat at a very high level, and (2) long enough that the various systems necessary for burning those fats (and for supporting and moving the body for the duration) become challenged enough to develop.

In my opinion, the ratio of fat to sugar utilization necessary for a run to qualify as a “long run” is 42% fats and 58% sugar, a Respiratory Quotient (RQ) of .87. This measure correlates with the aerobic threshold—the highest level of activity at which virtually all of the body’s energy is being processed in the presence of oxygen.

While the percentage of fat utilization at this point is already declining, after an RQ of .87 it begins to drop much more quickly. Since the lactate produced by anaerobic activity inhibits fat usage, the percentage of sugar used increases dramatically.

You can get an RQ test at any exercise lab, or even some doctor’s offices. But my favorite way of finding a ballpark measure of the aerobic threshold is Phil Maffetone’s 180-Formula. The 180-Formula gives you the heart rate at which you reach your aerobic threshold, which makes it very easy to keep track of your fat utilization while running.

Using sugars to support ourselves through a long-run is a self-defeating endeavor. We won’t create the adaptations we hope for. Because the body hasn’t adapted, we’re subjecting it to stresses it can’t really handle. It’s not going to grow that well, or that quickly, or in the direction we want it to, and it might break down on us a few times along the way.

Let’s keep our long runs easy enough.

UPDATE (10:46 AM, 12/14/15) : I’d previously written that total fat utilization was at its peak at an RQ of .87. A reader pointed out to me that this wasn’t the case.

UPDATE (11:35 AM, 12/14/15): I should mention that the criteria I discuss in this article are perhaps necessary but not sufficient to call something a “long run”: Commenter “Van” suggested that a better definition for “long run” is that which occurs at the heart rate which corresponds with the maximum rate of fat oxidation (rather than the maximum rate of oxygen use at which there is no anaerobic function). I’m not convinced at this point, but I’ll be sure to update again—or maybe write a follow-up post—if that changes.

QUESTION FROM A READER: Why does my heart rate spike at the start of a run?

A lot of people who run with heart rate monitors often see their heart rate spike at the beginning of a run, only to subside after a mile or two. This kind of spike only happens if you didn’t warm up long enough.

  • If the body isn’t warmed up, there is little blood flow to the muscles (and therefore little oxygen).
  • For a short period of time, the muscles have to work anaerobically, increasing the heart rate.
  • The body rushes to shunt blood away from the organs and towards the muscles. This is a major stressor.
  • The spike in heart rate subsides when increased blood flow (and the oxygen that comes with it) allows the muscles to work aerobically.
  • Therefore, the spike itself is an indicator that your heart rate was inadequate.
  • It takes 12-15 minutes for the body to warm up properly.

Before starting a bout of exercise, our body’s internal machinery is largely inactive. The metabolism is working at a very low level, the big muscles are relatively quiet, and blood is moving largely within the core—cycling through the various organs, and back to the heart and lungs.

Muscles are fed by vast networks of capillaries—tiny blood vessels existing within the muscles themselves—which ensure that blood goes to and from every muscle cell. During rest, the majority of these capillaries are constricted. Very little blood goes in or out of the muscles.

This eases the demand on the heart during rest: constriction of the capillaries and peripheral blood vessels means that the overall volume of the cardiovascular system is greatly reduced. The heart doesn’t need to pump very hard to maintain blood pressure, which keeps the heart rate relatively low.

During exercise, muscles demand a huge volume of blood flow, and so the capillaries dilate to accommodate it. But the body isn’t designed in such a way that the capillaries can expand pre-emptively. They expand due to exercise itself. Asking the body is asked to exert itself from a cold start can be a major stressor: it has to drain blood from major organs abruptly and it has to shove them into muscles whose capillaries haven’t dilated yet—a process that can send the body into shock.

Because of this, a proper warm-up—a period of very low-intensity activity—is important for all exercise, but is critical for running: Every step we run, our legs have to break our fall. It takes a big use of the muscles to make this happen.

Without proper blood flow, the muscles are on their own. If the capillary networks haven’t yet expanded, very little blood is getting to the muscles for those first few minutes. This is a problem because blood carries oxygen. No blood, no oxygen. But even without oxygen, the muscles still need to find a way to perform the required activity. In this situation, the only way to accomplish this is by working anaerobically.

I’ve written before how anaerobic work is intricately tied to the stress response: when the body is under stress, it raises the heart rate and kicks up the functioning of the anaerobic system—which is able to provide energy at a massive rate—in order to deal with a presumed threat to its existence. The connection between stress, anaerobic activity, and a high heart rate runs deep: if any of the 3 increases, the other two will follow.

The observed spike in heart rate is a direct indicator of increased anaerobic activity.

It subsides after a mile or two is because it typically takes the aerobic system 12-15 minutes to activate completely. Blood finally pervades the muscles, bringing oxygen and allowing the aerobic muscle fibers to do their thing.

Heart rate drops to a manageable level once the aerobic system is in play—and to the degree that it comes into play.

Here’s the important part: A spike in heart rate doesn’t just tell us that our aerobic system wasn’t fully on yet. It also tells us that our warm-up was inadequate. The spike in anaerobic activity means that blood and oxygen was largely absent from the muscles. The body was forced to rush to bring blood to the muscles. Blood was hastily drained from the organs, and unceremoniously shoved through capillaries that hadn’t yet expanded to accommodate it.

That amounts to a lot of unnecessary stress.

The solution: warm-up for longer.

The Maffetone Method, training the aerobic system, and answers to common frustrations.

For the past few months, I’ve been working in various capacities with Phil Maffetone, who has made many important contributions to exercise science and the endurance sports. He is a proponent that aerobic function—the ability of the aerobic system to utilize fat and oxygen to power the body—is the foundation for all health and athletic achievement.

In a recent article, I discussed this view from an evolutionary perspective: the aerobic system is in charge of the long-term upkeep of the body. Conversely, the functioning of the anaerobic system (which burns sugar in the absence of oxygen) is tied to the autonomic stress response, and necessarily coincides with a high heart rate. The organism primarily uses the anaerobic system to survive an imminent threat to its existence, or (in the case of predators) to capitalize on an opportunity for its survival.

When the anaerobic system stays on for too long—or becomes responsible for the body’s upkeep—chaos ensues. The Maffetone Method (also known as MAF) is all about bringing order to this chaos, and therefore facilitating the body to develop correctly.

A majority of people who try out Phil’s recommendations for the first time—(train at a heart rate that guarantees aerobic function while excluding all anaerobic function)—find that this means running very, very slow. And furthermore, a number of people don’t observe changes to their “aerobic fitness” for some time.

The problem isn’t that the method “doesn’t work.” It’s just that some of our bodies (and in particular, our aerobic engines) are in a state of utter disrepair—and the body is an extremely smart investor. The body will sometimes use the fledgling aerobic system to patch itself up and fill in the cracks before using its newfound potential for anything else.

I often hear that the aerobic system develops slowly. I believe that it develops astonishingly quickly. But while our attention is on the “fitness” we so desperately want—which we want so much that we rarely bother to define it—we miss the fact that the aerobic system is diligently working to achieve it.

Often, the body’s last priority is increasing athletic ability—as it should be. Think about it: if we are succumbing to infections because our aerobic system is struggling to power our immune response, or our bones have insufficient density due to increased acidity (which the aerobic system potently counteracts), then the last thing that we want is to be subjecting this engine to more stress.

This is car engine whose piston rings are rotten. Its valve springs are rusting off and its fuel injection system is all clogged up. Not only do we have no business racing this engine, but the very last thing we should do to it is add a turbocharger. That’s not what this engine needs. But the systems of the human body are so opaque to us, and the cultural narratives around athleticism so damaging, that this is exactly the position that we find ourselves maneuvered into—and outright believing.

The human aerobic engine comes from an even better brand. But we need to look under the hood to notice.

Before the body is ready to be challenged with anaerobic exertion, the aerobic system must have achieved 3 benchmarks of competence: (1) as mentioned above, it must provide the overwhelming majority of the energy for the body’s basic upkeep, (2) it must be powerful enough to sustain a high level of brain function—while the muscles are hybrid engines, the brain is exclusively an aerobic animal—and (3) it must be able to adequately absorb the stresses incurred from present lifestyle.

When an underdeveloped aerobic system is being trained, any gains that are made will go towards securing the body’s basic upkeep: if there were chronic issues—such as carbohydrate intolerance, infections, etc—all gains will go first to combat those, and to make sure that they do not reappear. Speed will not increase.

Once that step is complete, any gains in aerobic function will go towards maintaining a high level of stable cognitive function throughout the day: if you had low or fluctuating energy levels, any gains will go towards stabilizing those. Speed will not increase.

And there’s the issue of present anaerobic function: if your your lifestyle or work demands a heightened level of focus, (or hell, you run two blocks with a backpack to catch the bus every day), your aerobic system will have to be that much more robust before it will be able to start contributing anything to your athletic output. Speed will not increase.

Phil Maffetone’s approach to health and athletic achievement does not just require us to develop the aerobic system. When discussing why our aerobic system is so underdeveloped, the Maffetone Method helps us realize that the present fitness culture (and the assumptions and beliefs that surround it) need a major overhaul.

Two people—one with a hugely powerful aerobic system and one without—will find that they have a very different “training response.” One will be able to tolerate a magnificent training volume, and one won’t. Present exercise science—and our own fitness instructors—will often tell us that the issue is genetic, or that we’re not good athletes. But a lot of times, that simply isn’t the case.