“It is rigid dogma that destroys truth; and, please notice, my emphasis is not on the dogma, but on the rigidity. When men say of any question, ‘This is all there is to be known or said of the subject; investigation ends here,’ that is death.” —Alfred North Whitehead in “Dialogues of Alfred North Whitehead,” as recorded by Lucien Price.

Every fall semester, I teach a course in exercise metabolism that is geared toward students in our strength-and-conditioning and clinical exercise physiology master’s degree programs. Students entering these programs are required to have bachelor’s degrees in exercise science, and as such they have completed extensive undergraduate coursework in the sciences and usually have some background in athletics. It seems reasonable to expect these already-educated students to be able to answer a simple question: What is fitness?

Initially, students seem to think that my question is some sort of icebreaker to get the class talking about something we can all understand before we address more complex course material.

However, they soon realize that the discussion of the definition of fitness is the agenda for the day. Indeed, I dedicate an entire class to defining fitness because it is of such fundamental importance to the understanding of exercise metabolism that it is pointless to delve into the details of the physiological responses to exercise without understanding what fitness is. All biochemical and physiological responses to physical work, I explain, are governed by the relationship between the work’s demands and an individual’s fitness.

In these discussions, I’ve heard many proposed definitions that have to equal degrees amused and frightened me. Students often laugh at themselves when they are unable to provide an adequate definition or provide one I simply refuse to accept. But it’s also powerfully frightening to realize that none of these students are able to provide a useful and scientifically rigorous definition of fitness.

Why can’t these students define fitness? After all, they are academically qualified and highly motivated. They are capable of high-order critical thinking, analysis of complex concepts, integration of ideas and intelligent conversation about all topics in the exercise sciences. Many of these students perform sophisticated experiments using state-of-the-art methods and publish research in leading academic journals. Why, then, are they unable to provide a working definition of something so fundamental to the discipline?

I have concluded that my students’ deficiency is no fault of their own. It is a shortcoming of our field. Students don’t know the definition because they are not provided with one in their formal education. The academy has failed them.

I am not the first to search for a scientific definition of fitness and come up empty.

In “What is Fitness?” CrossFit Founder Greg Glassman gave a similar anecdote in which he explained that even long-established strength-and-conditioning organizations had no definition of fitness.


Published in 2002, “What Is Fitness?” by Greg Glassman provided the world’s first scientifically rigorous definition of fitness. (CrossFit)

That classic article was published in October 2002 in the CrossFit Journal. In characteristic academic fashion, I was quite late to the party in 2015, when I first had the thought of having the what-is-fitness discussion with my classes after starting my CrossFit journey in January of that year.

In searching the internet—especially the scientific literature—my assessment of the situation was essentially the same as what Coach Glassman had encountered about 13 years earlier. I am disappointed to report that academic exercise physiologists made essentially no progress on a useful definition of fitness during that time.

However, I came across the abundance of resources provided by CrossFit Inc.—namely CrossFit.com, the CrossFit Journal and the “CrossFit Level 1 Training Guide.” I will never forget the voracity with which I read “What is Fitness?” and especially “What is Fitness?—Part 2” in the “Training Guide.” One Glassman quote in “Part 2” bears repeating:

“By tracking the difference in time between workout attempts, we are looking at changes in power. We did not have to study this much longer to come to this understanding that your collection of workout data points represented your work capacity across broad time and modal domains. This is your fitness.”

It is interesting that I had to venture out of the academic realm to find something so profoundly scientific. Here we have, at long last, a definition founded on something measurable, repeatable and evidence based—the ability to perform work per unit time; i.e., the ability to produce and sustain power.

I hope the average person understands the importance of this conceptual advance. Before Coach Glassman articulated this idea in these terms, there was no scientifically defensible definition of fitness. It did not exist. But now, with fitness defined as work capacity across broad time and modal domains, we have a framework for understanding physical function, health, well-being, physiology, biochemistry and molecular biology in a way that was not previously possible. Overall, I would argue that presenting a useful definition of fitness is one of the great contributions to the human health sciences in the current era—probably in history.

As is often the case for major scientific breakthroughs, Coach Glassman’s definition of fitness is elegant in its simplicity: Work done per unit time is power. When that work is done by a person, we are discussing human power output; i.e., human performance. Therefore, it is logical to define an individual’s capacity to do physical work in a given amount of time as his or her fitness for that specified time domain. A broad and generally inclusive definition of fitness must include all possible exercise durations and modalities. Thus, it is perfectly tenable to define fitness as work capacity over broad time and modal domains. The definition is rigorous and scientifically sound.

Returning to the definitions available from the exercise-science literature, we see no such rigor. Instead, we see the term “fitness” used most frequently without having first been defined, and when we do see a definition, it is fraught with fallacy and therefore not particularly useful.

Fallacies of Previous Definitions of Fitness

When I ask my students to define fitness, they commit some of the same intellectual errors as our field’s leaders: They provide a circular definition, often using the word “fit”—“fitness is the state of being fit.”

Let’s take a closer look at the shortcomings of other definitions of fitness provided by the academic community. Spoiler alert: All definitions are fallacious in some way. Moreover—and as Glassman also pointed out in “What Is Fitness?”—the word “fitness” is at times used in academic sources but is never defined.

In “ACSM’s Guidelines for Exercise Testing and Prescription,” the American College of Sports Medicine (ACSM) defines fitness as “a set of attributes or characteristics that people have or achieve that relates to the ability to perform physical activity.”

In another of the ACSM’s widely cited sources, “Quantity and Quality of Exercise for Developing and Maintaining Cardiorespiratory, Musculoskeletal, and Neuromotor Fitness in Apparently Healthy Adults: Guidance for Prescribing Exercise,” physical fitness is defined as “the ability to carry out daily tasks with vigor and alertness, without undue fatigue and with ample energy to enjoy [leisure] pursuits and to meet unforeseen emergencies.’’

Several fallacies are apparent. First, the same authoritative body provides two definitions for the same word, which is confusing. Which definition is better? Does one apply to certain situations or examples more so than the other? I would suggest that having two definitions for the same word is absurd.


Figure 1— Many students define fitness as a “a state of being physically fit.” This is a logical fallacy known as a “circular definition.” (Staff/CrossFit Journal)

With regard to the first definition, let’s look at the terms “attributes” and “characteristics.” Which attributes and characteristics are desirable for fitness? Are they measurable? What is the difference between an attribute and a characteristic? And what is “physical activity”? Is this not a term that requires its own definition before it can be used effectively to define another word? With these terms undefined, the definition of fitness can certainly be described as obscure. Alas, we have a definition that commits the fallacy of obscurity.

In the ACSM’s second definition of fitness, the terms “vigor” and “alertness” are especially problematic. Like the attributes and characteristics we encountered in the first definition, these terms need to be contextualized and defined. What is vigor, and what is alertness? What are examples of tasks that are performed with or without vigor and with or without alertness? Why are vigor and alertness required for helping us understand what constitutes a state of fitness? And what is a “daily task”? What about non-daily tasks? Shouldn’t I be prepared for those if I am to be considered fit?

When discussing fitness in academic circles, it does not usually take more than a few minutes for the conversation to veer to cardiorespiratory fitness; namely, maximal oxygen uptake or VO2 max. It is true that the VO2 max is a physiological measure of our aerobic capacity. It is my opinion that the academic community is mistaken in fixating on VO2 max as a measure of fitness for reasons discussed in the Journal previously. All maximum-effort physical challenges require not only the aerobic but also the anaerobic energy systems. A definition of fitness that does not consider the capacity of the anaerobic energy supply systems (phosphocreatine and anaerobic glycolysis) is an incomplete and useless definition. Moreover, with CrossFit’s articulation of the 10 domains of physical fitness, we can appreciate that VO2 max—which most closely correlates with cardiovascular/respiratory endurance—is but one parameter under the fitness umbrella. It is inaccurate to conflate the maximal attainable rate of the oxidative phosphorylation system with overall fitness.

Another problem is inconsistency across mainstream academic literature—even texts from the same publisher. For example, in the “ACSM Fitness Book,” we find the following definition: “Health-related fitness refers to the ability of your heart, vessels, lungs and muscles to carry out daily tasks and, occasionally, unexpected physical challenges with a minimal level of fatigue and discomfort.” This definition implies a fit individual will be prepared for not just the known but also occasional unknown physical challenges. This is certainly better than the two definitions provided above, which seemed to only deal with daily tasks. However, I’m not sure why “health-related fitness” only deals with four of the body’s organ systems—the heart, vessels, lungs and muscles. Are my bones not relevant to health-related fitness? My central nervous system? Again, we have a definition of fitness that seems incomplete at best.

“As a field, academic exercise science has been wandering aimlessly for 50-plus years without so much as a working definition of fitness.”

—Nathan Jenkins

After a restatement of the above-quoted definition of fitness, “ACSM Fitness Book” seems to take a turn for the better by saying that being fit means “having the reserve to do all that you want to do—and more.” But continue to the following sentence: “Becoming fit does not require high-intensity physical activity, monotonous workouts, or even an expensive health club membership (although this is sometimes motivational, as you will read in a later chapter).”

In the preceding text, we find a lie, a truth and a statement that can only be described as bizarre.

The lie is that high-intensity exercise is not required to become fitter. We know that it is. This fact is not disputed by any informed participant in a discussion about the health benefits of exercise. A better statement would be that high-intensity exercise is not required to increase longevity—and this is supported by a tremendous amount of data in the research literature. But longevity and fitness are wholly different entities.

The true statement is that monotonous workouts are not required. This seems obvious to CrossFit athletes who perform constantly varied functional movements at high intensity, but it is good for the readers of the ACSM text to have this point explicitly mentioned.

The bizarre statement is that health-club memberships are not needed but might be motivational. This claim is incomprehensible. We know it is not true, as evidenced by the copious number of globo-gym memberships that are sold but never actually used. It becomes a de facto monthly donation to the business rather than a payment for services actually rendered.

In addition to having fallacious definitions, sometimes as a field we have not even bothered to define the term when it is used. In these instances, the word “fitness” is used with an apparent assumption that the definition is already known. For example, in the NSCA article “Central Concepts Related to Periodization,” the term “fitness” is used 14 times without being defined.

Similarly, in the book “Foundations of Fitness Programming,” the NSCA instructs personal trainers to perform an initial fitness assessment but neither defines fitness nor provides any guidance on how to assess.

The bottom line: As a field, academic exercise science has been wandering aimlessly for 50-plus years without so much as a working definition of fitness. We have been using the term loosely and unscientifically. We have advised health practitioners, trainers and coaches to assess fitness without telling these consumers of our research what fitness is and how to assess it.

ALT TEXTPower graphed against duration of effort and age: Increase the area under the curve to become healthier. (Shaun Cleary/CrossFit Journal)

Teaching the Definition of Fitness in an “Active Learning” Experiment

Initially disheartened by the lack of anything useful in the academic literature, I realized something very important in 2015: I had Glassman’s definition of fitness. That’s right: In my university classrooms, I teach that the definition of fitness is work capacity across broad time and modal domains. For my introductory discussion of the topic, I actually shorten it to “work capacity over time.” We introduce the time domains and modal domains later because there is a lot to unpack regarding exercise duration and exercise modality (e.g., how low-skill versus high-skill activities can affect fitness for a given workout independent of metabolic capacity).

Indebted to Glassman for his scientific contribution to the field, I wanted to find a way to effectively teach his revolutionary concept. As an instructor, I believe active-learning approaches that require hands-on experience coupled with synthesis, evaluation and thoughtful discussion of course content are more effective and engaging than traditional teaching methods that require only passive reception of information. Applying this philosophy, I came up with a simple activity in which all students could participate.

The class is divided into three or four heats. Students are told that each heat will exercise for four minutes, which will be divided into eight intervals of 20 seconds of work and 10 seconds of rest; i.e., a Tabata interval pattern. The work will be a simple exercise that can be performed anywhere: push-ups. Students are then briefed on the movement standards for a proper push-up. They are then shown the scaling option (push-ups from knees). Finally, it is made clear that “bro reps” (failure to achieve full range of motion) will result in an automatic F for the entire course. Students are told to track their reps for each of the eight rounds. Data from the spring 2018 semester’s experiment are shown in the figures below.

ALT TEXTFigure 2—Performance data for male and female students (left panel) and male students with the author’s data extracted for an important comparison: Is the professor more or less fit than male students about 15 years his junior? Why? (Nathan Jenkins)

Students are presented with several questions:

  1. We have eight scores—one rep count for each round. Is there a way to simplify the data to a single metric that’s reflective of overall performance on this test?
  2. Why did push-up performance decline linearly for three to four rounds and stay relatively steady for the remaining rounds?
  3. Do you think this performance-versus-time curve is unique to the Tabata push-up challenge? Or would we expect a similar response to maximal efforts in longer time domains?

In discussion, the students come to conclusions, some of which will be familiar to CrossFit athletes but foreign to the academy.

First, we can quantitatively summarize the performance on the Tabata push-up challenge with a single metric: the area under the rounds-versus-repetitions curve. We can see that students who perform better on the push-up test have more area under the curve. With regard just to push-up performance for now, what is fitness? It is the area under the curve. We’re finally getting somewhere.

As to why we see the characteristic rapid decline in performance over the first three or four rounds (after which reps bottom out at about five to seven per interval), we link this back to the concept of the metabolic energy systems, the subject of a previous lecture.

Briefly, the phosphocreatine energy system, which generates ATP for muscular work rapidly but is short-lived, is the primary energy source for the first one or two rounds. As phosphocreatine is depleted, the muscles increasingly rely on anaerobic glycolysis for energy production.

Anaerobic glycolysis produces ATP relatively quickly but not as fast as the phosphocreatine system. Anaerobic glycolysis also produces hydrogen ions as a byproduct, which causes discomfort and fatigue. The relatively lower capacity for power output and the increased production of hydrogen ions can explain the decline in performance from about the second to fifth rounds.

For the last three rounds, power output is low and fatigue and pain are high. From what we know of metabolism, we can infer that the phosphocreatine system is depleted, the anaerobic glycolysis system is operating at maximal capacity and has begun to taper off, and we are increasingly relying on oxidative phosphorylation for ATP generation. In fact, oxidative phosphorylation has been working at maximal capacity the entire time; the problem we run into in the later stages of the push-up challenge is that this system is not particularly helpful for rapid force generation, but it is all that is left once the phosphocreatine and anaerobic glycolysis systems have reached their limits.

Finally, we can broaden our focus beyond this specific challenge. It turns out that for any and all physical challenges, each individual will have a maximal power-output-versus-time curve. To illustrate, I show a screenshot of a recent workout log of mine on Beyondthewhiteboard.com (see below). Although each individual data point is a different workout (combinations of monostructural, gymnastics and weightlifting elements), they can all be expressed in terms of average power output; i.e., work done per unit time and total time to complete the work.

As Glassman has pointed out, the area under that curve is fitness quantified.

ALT TEXTFigure 3—The author’s performance data from workouts logged on Beyondthewhiteboard.com illustrate that fitness defined as work capacity across broad time and modal domains is quantifiable. Each green dot is a data point showing the average power output sustained for a given workout performed over the last two or three years. The red curve (drawn by hand) represents a rough approximation of the line that best fits the data. The total area under this curve is my fitness. (Nathan Jenkins)

I have found that this definition of fitness, and the visualization of the concept through objectively measured exercise performances, provides a foundation for students to better understand every concept discussed throughout the entire exercise-physiology course curriculum: Work capacity for a given time domain is dependent upon the metabolic energy systems contributing to ATP turnover. The neuroendocrine response to exercise differs as a function of the exercise stimulus, which can be quantified in terms of the power-output-versus-time curve. We can conceptualize changes in fitness with exercise training by envisioning changes to the area under the curve. We can discuss how the curve might look for athletes in different specialized disciplines; e.g., distance running versus weightlifting.

These are but a few examples of instances in which students are reminded that fitness is defined as work capacity across broad time and modal domains.

Perhaps the most significant discussion that follows from a useful definition of fitness is on optimal exercise prescription for the general population. If fitness is measured as our work capacity, and if we believe that increased fitness is something to be sought after for health, then it seems reasonable that the training program recommended to the public should have a scientific basis for increasing work capacity across broad time and modal domains.

But this is not the case. We are, as Glassman recently put it, in a very strange and shocking state of affairs with regard to messages about exercise:

“Public health is just an amazing joke. … (There is) this problem with it: People won’t do what they’re being asked to do, and what they’re being asked to do is deadly.”

I couldn’t agree more. If an exercise regimen does not improve fitness, then by definition it is not improving health, and we are regressing toward sickness.

Credit: www.journal.crosfit.com