The Tank Model of Fitness

CrossFit’s definition of fitness is as measurable, observable, and repeatable as any you will find. Eschewing the typical vagaries and silly bullshit of the fitness industry, CrossFit does not seek to hide behind words that have little meaning. Rather, CrossFit rigidly defines and captures the concepts that matter most to the science of improving human performance. That’s what we are interested in: an effective means of creating and sustaining fitness in the human body. No nonsense, no words like infrastructure and synergy that can mean anything, no “muscle confusion,” just science that advances human performance, expressed in scientific terms.

However, most scientific theories can also be expressed in ways that are simple, understandable, and accessible to the non-scientific populace. Einstein’s exposition of relativity by using the analogy of moving trains is but one great example. In this article I intend to present an analogy of CrossFit’s definition of fitness. In no way do I intend to redefine or even supplement CrossFit’s model of fitness, work capacity, and general physical preparedness. Thinking of an existing idea in a non-typical way often forces us to reconsider assumptions. This advances our depth of understanding, provides a forum for debate, and equips us to present the idea to others.


Think of your body as a physical tank of fuel. The tank is cylindrical, defined by a radius and a height. The fuel inside the tank is adenosine triphosphate (ATP). While the word is long and scary, let’s just call it ATP and think of it as the fuel that powers your muscles. You spend your fuel through physical activity—exercise. You replace the fuel you spend through the three metabolic pathways: phosphagen, glycolytic, and oxidative. This creates a basic system that’s very intuitive: a holding tank of energy with inflows and outflows.

Figure 1. Tank Model Overview

Now, what are the relevant metrics of this system and what do they mean?

Tank Size

First, tank capacity just screams for definition. The volume of a right circular cylinder (V) is simply given by

What’s the practical meaning of radius, height, and the resulting volume of the tank? I propose that the tank’s radius represents general physical preparedness (GPP) and its height the skill specific capability currently in use, which I shall hereafter refer to simply as “skill.” In this context, GPP represents capacity across all ten general physical skills. Skill is an athlete’s capability at any given tested skill. Volume, the resulting product of the two, is overall work capacity in any given task. This will be explored in detail later. But for now, note three important nuances of this model:

  • Radius is squared and contributes to overall work capacity far more than height.
  • Radius is constant across all skills.
  • pi is just a constant that is the same for everyone, so it can be ignored.

ATP Inflow

ATP inflow is provided by the three metabolic pathways: phosphagen, glycolytic, and oxidative. Think of each pathway as a faucet that provides ATP to your tank. The oxidative pathway is the only pathway that creates ATP in the presence of oxygen (aerobically). It is constantly operational, even as you sit at a desk and read this. However, it provides relatively little ATP compared to its siblings. Therefore, the oxidative pathway is illustrated as a faucet providing a constant, but low volume flow of ATP. The glycolytic pathway is next, providing larger bursts of energy, but not at a constant rate like the oxidative pathway. The glycolytic pathway provides a mediocre amount of energy in prolonged bursts lasting from 30 seconds to a few minutes, but it cannot sustain this flow indefinitely and eventually exhausts its volume, forcing a rest before rebuilding enough ATP to begin flowing again. Therefore, the glycolytic pathway is illustrated by a medium volume, medium duration flow. The phosphagen pathway is last, providing very large but short bursts of power. It lasts less than ten seconds before quickly running out of ATP, and must rest before providing more energy. Therefore, the phosphagen pathway is illustrated with a very large volume and very short duration flow.

Figure 2. Metabolic Pathways of ATP Production. Illustration by James Pratt

ATP Outflow

ATP outflow comes in the form of any physical activity. To contract your muscles you must spend ATP. We are interested in the ATP outflow of exercise.


The analogy of tank volume to work capacity and the definitions of height and radius require some examination. First, is it reasonable that GPP across all ten general physical skills plays an exponential role in determining work capacity while skill specific capacity plays only a linear role? I contend that GPP trumps skill in the type of movement most important to humans and CrossFit alike: functional movement. This is an important caveat, because the analogy does not hold for very specific tasks. Regardless of Tiger Woods’ work capacity, a top CrossFitter is unlikely to best him at the assignment of imparting energy to a golf ball with a golf club—performing work in a very specific way that greatly emphasizes one or two physical skills almost to the point of completely excluding the others in comparison. However, the functional movement of shoveling is a supporting example. In a race to dig a hole, I contend that a well-trained shoveler with little GPP will be easily beaten by a CrossFitter with a high work capacity that has never held a shovel. Therefore, I contend the analogy holds for functional movement: safe, natural, human movement characterized by its ability to move large loads, long distances, quickly.

Figure 3. Tangible Definitions of Tank Size

Next, is it reasonable to have a single measure of GPP, symbolized by the radius of the tank? Yes. Can GPP really be summarized into a single measure? No. Theories are only useful because they aren’t completely accurate. They allow us to understand immensely complex ideas within an understandable framework that we can grasp and work with. Of course GPP can’t be summarized as one variable, constant across all physical systems of any given human at any given time. However, using it in this context is quite instructive, and that’s reason enough to forge ahead.


What does all this mean to advancing human performance? Let’s examine how we can “improve” our system. We can improve our system, i.e. create greater work capacity, via three basic methods

  • Enlarge our tank
  • Fill our tank faster
  • Drain our tank slower

Enlarge Our Tank

We enlarge our tanks through the perpetual cycle of shock and adaptation that Selye described more than 70 years ago as the General Adaptation Syndrome. More simply, we enlarge our tank through exercise and rest—CrossFit programming. We can enlarge our tank used for any given task by either expanding its radius, our GPP, or its height, our skill at that task. Expanding GPP is more difficult and time-consuming than improving skill at a task, but yields a greater return in work capacity. Expanding the height of our tank is less demanding, but increases our work capacity only linearly and only in a single skill. An increase in tank radius affects all skills. Coincidentally, were you to try expanding a real, metal tank, you would find it much easier to connect an extension to the top of the tank to expand its height rather than cutting the tank open to expand its radius. The analogy is only coincidental, but true.

Figure 4. Unbalanced Athlete Favoring GPP

Also note that a tank woefully unbalanced in either direction is quite odd.  Figures 4 and 5 show contrasting examples. Figure 4 is an example of high GPP and low skill, such as an elite CrossFitter with one crippling weakness.

Figure 5 is an example of low GPP and high skill, such as an elite endurance runner with little capacity elsewhere. A real tank like the one depicted in Figure 5 would be quite unstable compared to Figure 4. A significant force applied to the top of the tank could cause it to rupture and crash. While somewhat coincidental, this analogy illustrates that GPP is more important than skill to overall work capacity in functional movement.

Figure 5. Unbalanced Athlete Favoring Skill

Fill Our Tank Faster

The three metabolic pathways continuously fill our tank. How can we get them to fill it faster? ATP production efficiency is a complex topic of biochemistry, but we can take a couple macro steps to aid the process: Paleo/Zone nutrition and training. Providing quality fuel is the most important step we can take to ensure our bodies produce ATP efficiently. Your body needs phosphocreatine, glycogen, and oxygen to produce ATP. We must provide two of those three fuels through our diet. We store phosphocreatine in our bodies after production in the liver from amino acids. We also intake some phosphocreatine through eating meat. Glycogen is produced from glucose that we derive from carbohydrates. A diet rich in lean meat, fruits and vegetables, and nuts and seeds is the best way to supply these nutrients. My experience is that a diet comprised of near-Zone protein/carb ratios coupled with an emphasis on Paleo ingredients provides the best results for my body. I neither religiously follow 40/30/30 carb/protein/fat proportions nor do I religiously eschew all non-Paleo foods, but I have my own mixture of both. Use those two widely-researched constructs and find what works for you. Finally, nothing will remind your body that you need to increase ATP production better than high-intensity training. Provide the stimulus of exercise and your body will provide the response of increasing ATP production.

Drain Our Tank More Slowly

How can you get more miles per gallon from your existing ATP stores? In the context of functional human movement, the best method I know is practice. Want to become more efficient at the snatch? Practice the snatch. The same can be said for any exercise or maneuver. As you forge neuro-muscular pathways between your brain and muscles, your body will become capable of performing work more efficiently. Think of the last time you learned a new skill quickly in the gym. You didn’t get stronger over the course of an hour, but you might have become more efficient at muscle-ups. But we don’t necessarily practice just for practice’s sake. Becoming more efficient at many different functional movements not only prepares us better for everyday life, but will allow us to perform more work in less time during training. This increases our power output, furthering the all-important GPP, enlarging the radius of our tank. And of course, practicing individual skills also enlarges the height of our tank for that skill.

Limitations and Conclusions

This model isn’t meant to be rigorously applied such that variables are assigned scalar values and your work capacity is calculated as the volume of the tank. This is just a framework for understanding the relationship between several important variables in fitness: general physical preparedness, skill-specific capacity, metabolic pathways of ATP production, and ATP consumption through exercise. Furthermore, this model can be viewed as a single tank that represents your entire body and applies to the range of skills in use at any one time, or you can look at your body as being comprised of many different tanks, each contributing to your performance in a specific task. Either way, this framework adds a new twist when I tell one of my clients to “not leave anything in the tank” at the end of his or her workout.

5 thoughts on “The Tank Model of Fitness

  1. Completely nit-picky comment here: in Figure 2 wouldn’t the phophagen pathway ‘drops’ need to be shorter than the glycolytic ‘drops’ for the image to scale to what you are describing in the pathway delivering ATP? Again, very nit-picky, but the image gives a visual example that contradicts the text that accompanies it.

    Something else I thought about while reading this, how much ATP can each metabolic pathway provide in one ‘burst’? This applies to the glycolytic and phophagen pathways, but I’m curious as to the limits of each in regards to the volume of ATP each can provide in one ‘burst’ and if the duration of each can be extended somewhat? Can the volume of ATP delivered be increased?

  2. Russell, you are absolutely right. I updated figure 2. Thanks.

    I think the amount of energy provided per “burst” is proportional to the amount of fuel available. More glycogen reserves in your muscles means more fuel for making ATP via the glycolytic pathway. The training we do promotes our muscles to develop larger fuel stores and spend those reserves more efficiently. That’s my .02. I may be wrong.

  3. “Volume, the resulting product of the two, is overall work capacity in any given task”

    OK, let’s take Oly Lifting for example. Oly Lifting Capacity= Pi*(GPP^2)*Sport Specific Skill

    So your equation will tell us that the best way to maximize your oly lifts is to maximize GPP since it’s the only variable that’s squared. Not making sense to me.

    Furthermore if you were to graph your work capacity against the time domain, you would come up with an infinite number of cylinders. Doable? Maybe, but it doesn’t look quite as pretty as a normal line graph. So why the deviation from the standard?

  4. Joe, my idea says that the best way to maximize your ability at an oly lift would be to maximize both GPP and skill. Having a high GPP would certainly improve your ability at an Olympic lift. However, skill is still a component even thought it’s only linearly related to the resulting work capacity. Perhaps in oly lifting skill is much more pronounced than in some other tasks. You should also notice that I pointed out the theory doesn’t hold for tasks that require a single physical skill almost to the exclusion of all the others. Oly lifting doesn’t exactly fit that mold, but it’s on that end of the spectrum since it’s speed and coordination intensive. My theory doesn’t fit everything perfectly, and I explained that. Theories are only useful because they’re not perfect representations, but simplified ones.

    Why the deviation from the standard? I enjoy critically thinking about interesting topics. That’s reason enough. Also, fear of deviating from a standard is not how science works. Science constructs hypotheses and tests them. My hypothesis may be incomplete, or inadequate compared to the material from which it draws, but I am certainly glad I didn’t fear to deviate from some so-called “standard.” Greg Glassman has said that if we find a better model of fitness then we’ll use it. I don’t think this model is better (it’s a derivative work of Glassman’s model), but we’re unlikely to find a better model if we’re all hesitant to look at the topic differently.

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