The Definition of Fitness

In the world of exercise and health, the definition of fitness has been incredibly vague over the years. Most health magazines measure fitness in inches lost, looking “toned,” or having a visible six-pack, all of which are external factors that have little to do with the actual health or capabilities of one’s body. Even Merriam-Webster dictionary doesn’t have a clear description, defining fitness as:

1 : the quality or state of being fit (not much help here)

2 : the capacity of an organism to survive and transmit its genotype to reproductive offspring as compared to competing organisms

By this definition, we could say that anybody currently alive and with offspring is the most fit, which isn’t necessarily true. 

Let’s dig a little deeper into the dictionary, to see if we can find any more clarification. The word “fit” has various definitions, including “sound physically and mentally” (Again, fairly arbitrary). The description that most applies to physical fitness is “adapted to the environment so as to be capable of surviving.” In other words, the fittest beings are those that can survive a broad range of challenges. We do see this in nature: organisms that can adapt to challenges and changes are those that survive and flourish, while the organisms with limited adaptations decline. This is the closest we have to a definition of physical fitness, but even so, it’s far from a measurable standard. It’s crazy that fitness, something that most people are trying to attain, has such a murky definition! 

Greg Glassman, the founder of CrossFit, saw this problem, and aimed to solve it. He is a scholar, the son of a scientist, and as such wanted to bring actual evidence-based data to the fitness world. How can we define or measure anything if we don’t have a standard for it? You can read previous blog posts to learn more about what CrossFit is, and why it works. The main takeaway is that CrossFit defines fitness as work capacity over broad time and modal domains. 

This aligns with the dictionary’s idea that to be fit, an organism must adapt to challenges and changes in the environment. CrossFit proposes that the more work we can do in all sorts of scenarios (broad domains), the fitter we are. Many people consider endurance athletes or sports stars as the pinnacle of fitness, but for the most part specialized athletes train very specific skill sets, and therefore have chinks in their fitness armor. Much like the specialized organisms in nature, if they come up against a new challenge, they will have a hard time adapting. But because CrossFitters train for nearly every type of physical challenge, we are generally fitter overall than those that train in just one modality. 

CrossFit uses four different models to explain and support their definition of fitness: The Ten General Physical Skills Model, the Hopper Model, the Metabolic Pathways Model, and the Sickness-Fitness-Wellness Continuum. I could write many pages about each of these models, but for the sake of this article I will try to keep things short, and only give a brief overview of each. 

Ten General Physical Skills: 

The ten general physical skills are a list of athletic traits: Cardiovascular/respiratory Endurance, Strength, Stamina, Flexibility, Power, Speed, Coordination, Agility, Balance, and Accuracy. He or she is fittest who is most balanced across all these skills. You can imagine that if an athlete is skilled in strength, but has no coordination, or if they have speed but no flexibility, they will be very limited. By training and practicing each of these skills, we become better-rounded athletes.

The Hopper Model:

The second model draws on the “unknown” nature of fitness. Think of a bingo or lottery hopper. Inside of the hopper we put every imaginable physical challenge: anything from climbing a tree to running a marathon to a 1-rep max deadlift. If we pull a dozen random challenges out of the hopper, he or she that is able to complete all of them and come out on top is the fittest. It’s important to note that the overall winner may not actually win any single event, but if they have a broad level of fitness, they will likely “outrun the lifters and outlift the runners,” ultimately outperforming those with a very specialized skill set. 

Metabolic Pathways Model:

The third model is all about metabolic pathways. There are three main metabolic pathways or systems that our body uses to produce energy. The first is the phosphagen pathway. This energy system is used for extremely short, powerful bursts, lasting no more than ten seconds (example: 1RM squat or 100-meter sprint). The second pathway is the glycolytic pathway, which lasts from several seconds to a few minutes, at around 70% of our maximum effort (example: Fran or a 2k row). The final system is the oxidative (or aerobic) pathway, which kicks in during longer, lower power efforts, anything over several minutes (5k run, Filthy Fifty, Murph, etc.) He or she is fittest who is able to use all of these energy systems efficiently and switch between them easily. Do you ever feel a “slump” at a certain time in a workout? Are you always dismayed when you see a long chipper WOD or a short sprint programmed for the day? This might be hinting at which of your energy systems needs more work! 

Sickness-Wellness-Fitness Continuum: 

The final model in CrossFit’s view of fitness is called the sickness-wellness-fitness continuum. This model posits that typical health markers can be graphed on a continuum from sickness to wellness to fitness, with the fittest people having markers surpassing “healthy” levels. For example, a blood pressure of 195/115 is considered unhealthy or diseased, 120/70 is healthy, while an athlete might have an even better blood pressure of 105/50. We see this trend with all the other measures of wellness, including bone density, body fat percentage, cholesterol, triglycerides, and more. By this logic, fitness is actually “super-wellness,” and a hedge against illness and disease. If you have built up a good hedge, then even if you lose some health or fitness as you grow older, you will just slide down into the wellness category, which is still much better than being “sick!” CrossFit’s view is that health and fitness are one and the same. CrossFit and its lifestyle have helped thousands of people (myself included!) protect against, improve, or reverse chronic illnesses. 

wellness continum.jpg

Each of these four models help define fitness in concrete and measurable ways. It is not the inches around our waist, nor is it a number on the scale. It is measured in what we can DO. By training for increased work capacity over broad time and modal domains, we have not only defined fitness in empirical terms, but set forth a formula for a healthy life. The muscles are just an awesome side effect. 

-Coach Sonja


Bowler, K., McDonald, C., and Shatila, N. CrossFit Level One Certification Course, presented at CrossFit Belltown, Seattle, WA. 20-21 August 2016. 

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CrossFit Pierce County, “CrossFit Certification Seminar Notes.” 10-13 February 2006. Pages 3-5. (Online) Available from: (Accessed May 2019.) 

CrossFit Training. “What is Fitness? (Part 1)” Level 1 Training Guide,Version 3.1. CrossFit Inc.,2016, pp. 17-23.

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Glassman, G. “Understanding CrossFit.” The CrossFit Journal, Issue 56, 1 April 2007. (Online) Available from: (Accessed May 2019.)

Glassman, G. “What is Crossfit?” The CrossFit Journal, 28 November 2009. (Online) Available from: (Accessed May 2019.)

McKee, C. “Adapt or Die: Inspiration Comes in Different Ways.” Medium Corporation, 17 April 2017. (Online) Available from: (Accessed June 2019.)

Merriam-Webster Available from: (Accessed June 2019).

Sam, G. “Bite-Sized CrossFit Concepts: The Hopper Model.” CrossFit Virtuosity, 18 June 2014. (Online) Available from: (Accessed June 2019).