What Is Fitness?

And Some Ways to Improve It

Jun 10, 2025

people running on road during daytime — Photo by Mārtiņš Zemlickis on Unsplash

A while back, I noticed that many products, services, and people promote things as healthy, but no one ever bothers to define this abstract and vague umbrella term, so I took a bit of a deep dive to take a look. ICYMI: By some definitions, “health is free of disease”; by others, it’s the state of wellness—mental, physical, social, emotional; another is the ability to self-manage and adapt, and the one I like best is the ability to be in balance.)

I took a similar look at the definition of “fatigue”, which is another term thrown around aplenty in these spaces. ICYMI: fatigue is a state that takes us out of homeostasis, out of balance, and recovery works to restore that balance.

In the same way that these terms are not elucidated, it got me thinking about fitness.

What is fitness? Am I fit? Are you? Can we be fit yet unhealthy?

Surely, people can be unfit while still a healthy bodyweight, so it stands to reason that someone can be fit while also overweight, though there’s likely to be a cut-off at some point, right?

Here’s how one paper defines fitness: “Fitness is an ability to execute daily functional activities with optimal performance, endurance, and strength to manage minimalist of disease, fatigue, stress and reduced sedentary behavior.”1 Again, this definition is still abstract. We could also try to define each of these other words: performance, endurance, strength.

Another paper puts it in clearer terms: “The American College of Sports Medicine (ACSM) gives the following definition of health-related physical fitness: Physical fitness is defined as a set of attributes that people have or achieve that relates to the ability to perform physical activity.”2

The ACSM’s definition takes it a step further by isolating fitness into distinct categories: Cardiorespiratory endurance, muscular strength, muscular endurance, and flexibility.

The National Strength and Conditioning Association (NSCA), one of the most reputable organizations in the world of, well, strength and conditioning, offers a definition of fitness that parallels the ACSM’s, though it’s more directed towards athletes: “Sport performance is highly dependent on the health- and skill-related components of fitness (power, speed, agility, reaction time, balance, and Body Composition coordination) in addition to the athlete’s technique and level of competency in sport-specific motor skills. All fitness components depend on body composition to some extent.”3

The description here goes on to explain that excess body fat weighs down the athlete, increasing resistance to the body, requiring the individual to exert more effort and energy for any given task. Reducing excess body weight results in a fitter individual because it allows the same work to be done for less energy and effort.

So far, it looks like even the concept of fitness has a couple of different directions and applications for its use. With the ACSM, fitness describes the functioning of specific and different body systems, cardiovascular fitness versus muscular fitness, but then even each of these can be dichotomized: cardiovascular endurance—which is generally moderate intensity—versus maximum cardiovascular output, like sprinting. Likewise, there’s muscular endurance, like doing as many repetitions as possible versus one-repetition maximum output, which is strength. This definition of fitness overlaps with the definition of physical health in a physiological and metabolic sense.

Though the NSCA’s definition of fitness, which also overlaps with the ACSM’s, is aimed more towards athletes, the components of fitness—power, speed, agility, reaction time, and balance—are just as important for all of us. We may not need to develop these skills and abilities to the level that athletes do, but they apply to our lives. If you catch your foot on something, you need power and speed and agility and reaction time and balance to catch yourself so that you don’t fall and end up with a debilitating injury. Although working on these areas of fitness will work towards improving physical health, these are mostly neurological adaptations, essentially training and refining the central nervous to move, act, and react quickly.

It seems logical and definitely prudent to do a little bit of everything to improve our physical fitness and consequently our health.

If we are to form a holistic definition of physical fitness, it might go something like this:

The ability to perform specific physical activities and tasks quickly and efficiently with minor fatigue generated and little recovery time required; all or most systems involved with different types of movement are well trained: high-intensity (sprint work) and moderate intensity (endurance) cardiovascular movement, muscular strength (moving high loads for few reps), endurance (moving moderate loads many times), and power (moving light or moderate loads explosively). It also includes having joint mobility and muscular flexibility that allows for full or near-full natural range of motion in all movement patterns. These movements can be performed in sport as well as in activities of daily living.

What I often see in the health and fitness space are people who focus exclusively on a single area over another, one skill, ability, or body system at the expense of the others. Power and strength athletes aren’t very interested in cardio work, and endurance athletes aren’t any more interested in resistance training, but the fact is that each is a piece of the total health and fitness puzzle.

Especially as we get older, we tend to fall deeper into our silos, to pigeonhole ourselves into the area/s we like most or find ourselves good at, ultimately leading us to become less fit in some areas. Guys lift some weights, nix the cardio, and don’t even ask them to touch their toes. And as far as the weights go, it’s a lot of arm stuff. Ladies, on the other hand, like to move a little more, and when they do hit the weights, it’s 90% legs when it’s their upper bodies that need some attention, and although they’re pretty good with stretching, they don’t often pick up and move a max weight for fewer than three reps.

It’s our deficits, the hard stuff, that we need to prioritize. Whatever skill, ability, or movement is the most difficult for us, that’s where we need to spend more time. Soon enough, it’ll no longer be a deficit, and we can sort of audit other areas again, and see what needs some beefing up.

(This goes for me too. I have by no means mastered all these components. I have my own physical fitness areas that need some beefing up.)

So, how can we do this? How can we improve our overall fitness?

In exercise science, including cardiovascular work and resistance training together in a program is called concurrent training. That is, you’ll have sessions that focus on each one separately and/or you do them in the same session but separately.

Concurrent training is not cross-training, like CrossFit, where strength, power, and aerobic work are all clustered together in a few minutes of a torturous bout of activity. For example, you might do a heavy squat for three reps (strength), go sprint 20 meters (cardio), and then come back to do some power cleans (power). It’s definitely fun but not pleasant while doing it.

Concurrent training, by contrast, is something like doing some endurance exercise today, and then tomorrow hitting the weights. Or it might be doing some weightlifting today but tacking on a few sprints after your workout.

Concurrent training has been shown over multiple studies to increase physical fitness--and likely health, although health measures are not often evaluated, like blood markers--more than single-type training alone. That is, including some form of cardiovascular exercise in a training program along with some resistance training often results in more robust growth and adaptation to each compared to doing either one on its own. Systematic reviews and meta-analyses have shown this outcome in young adults4 as well as middle-aged and older adults.5

I know it’s not easy sometimes to get to the gym to do anything, so if concurrent training seems like a tall or taller order than what you’re used to, it isn’t something we need to do forever. Like everything else in health and fitness, we want to have periods and phases where we focus on different things. We rotate our pans on the stove, putting the ones from the front on to the back burners for simmering and to keep them warm. The ones in the back get moved to the front, so we can turn up the heat on them.

And concurrent training isn’t the only method to do this. It’s just one.

For example, you might take the next month and start including some specialized strength work into your weightlifting. You’d swap out some of what’s called the hypertrophy work--the muscle building--for strength work, increasing the load to the point that you can only lift it somewhere between one and five times. If you tend to focus on strength, you might do the inverse. You might swap something out for some body weight movements, like push-ups, to focus on muscular endurance. Similarly, if you’re an endurance athlete, you can likewise swap out an endurance session for a sprint session, and vice versa for the sprinters.

The great thing about focusing on one area of fitness is that it ripples into the other areas. A good sprinter’s cardiovascular fitness lends itself well to weightlifting, where muscular endurance is not the limiting factor sometimes; rather, it’s the cardiorespiratory deficit. (Try doing a long set of squats or deadlifts and see what burns more: your legs or your lungs.)

And these improvements in fitness help us in every other area of life, and you never know when, where, or how they’ll be of use. If your dog is loose or your toddler tries to run into traffic, it’s a good thing you can run faster than them. If you need to move some furniture on your own, it’s a good thing you’re strong.

The important thing about fitness is not to get complacent, that what we have is good enough, that we can hang back, and it’ll all maintain itself (which might be true to some extent).

The very cool thing is that improving our health and fitness is the easiest and quickest ways to literally improve ourselves, and when we’re better, everyone and everything around us benefits.

The Flexitorium by Joseph Anthony

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