How to measure fitness: Operational definition practice

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It is not often that the New York Times publishes an article on operational definitions. Okay, they don’t call them operational definitions, but that’s what they are.

Introduce this assignment by describing how so much of our health is influenced by our behaviors. If behavior is involved, psychology is there. How do researchers—or ourselves, for that matter—know if changes in our behavior, e.g., exercise, more nutritious eating, is positively affecting our level of fitness? The easy answer is that we randomly assign volunteers to, say, an exercise program of some sort or to a control group—maybe even a waitlist control group—and then after a predetermined amount of time, we measure their fitness. Great! Now, how do we measure fitness?

Probably the most common way the average person on the street measures their fitness is by hopping on the scale. The more fat we carry, the greater the potential impact on our health. Since both fat and muscle have weight, the average scale does not differentiate. It is possible that the more we exercise, the more fat we lose but the more muscle mass we gain. Even though our fitness is increasing, our scales may tell us that we weigh the same or are actually gaining weight.

Then there’s the body mass index (BMI). This is another measurement that does not differentiate between fat and muscle. The BMI is not lacking for critics. As one observer pointed out, the current BMI categories are not useful. Several longitudinal studies, they report, have found being BMI overweight (BMI 25-29.9) or in the first level of BMI obese (30-34.9) had little or no impact on mortality rates (Nuttall, 2015).

What if we could just measure the amount of fat that we carry? There are scales that purport to do that. Such scales send an electrical current through your body. Water is an excellent conductor of electricity. Muscle contains more water than fat. The less resistance the electrical current encounters, the more muscle mass the scale concludes we have. The scales that have only two points of measurement—two feet—are less accurate than scales that have four points of measurement—two feet and two hands, but the two-point scales are considerably less expensive. The two-point scales tend to be reliable, but not accurate, underestimating or overestimating fat content significantly. However, if one is using such a scale to track changes, then they do fine. One more important point about these scales. If you’re dehydrated, the scale’s electrical current will meet more resistance, and the scale will say that we have more body fat than we have (McCallum, 2022).

None of these measurements—overall weight, BMI, or fat composition—identify where our fat is concentrated. Abdominal fat is associated with poorer health outcomes than, say, fat stored in the lower body. The latter may actually have protective effects (de Lemos, 2020).

If these measurements are not the best way to operationally define fitness, what are some alternatives?

If you’d like to make this an out of class assignment, ask your students to read this New York Times article (Smith, 2023). The article identifies three different approaches to measuring fitness: heart metrics, physical performance metrics, and daily living metrics. Ask students to identify at least three operational definitions of fitness provided in the article for each approach.



de Lemos, J. (2020, December 16). Why belly fat is dangerous and how to control it. University of Texas Southwestern Medical Center.

McCallum, K. (2022, April 26). How accurate are scales that measure body fat? Houston Medicine: On Health.

Nuttall, F. Q. (2015). Body mass index: Obesity, BMI, and health a critical review. Nutrition Today, 50(3), 117–128.

Smith, D. G. (2023, March 27). 3 ways to measure how fit you are, without focusing on weight. The New York Times.


About the Author
Sue Frantz has taught psychology since 1992. She has served on several APA boards and committees, and was proud to serve the members of the Society for the Teaching of Psychology as their 2018 president. In 2013, she was the inaugural recipient of the APA award for Excellence in the Scholarship of Teaching and Learning at a Two-Year College or Campus. She received in 2016 the highest award for the teaching of psychology--the Charles L. Brewer Distinguished Teaching of Psychology Award. She presents nationally and internationally on the topics of educational technology and the pedagogy of psychology. She is co-author with Doug Bernstein and Steve Chew of Teaching Psychology: A Step-by-Step Guide, 3rd ed. and is co-author with Charles Stangor on Introduction to Psychology, 4.0.