Comparison of Body Composition Metrics for United States Air Force Airmen

The United States Air Force currently uses AFI 36-2905 for cardiovascular fitness standards and evaluation. Regarding its fitness test, the Air Force considers waist circumference (WC) twice as important as push-ups or sit-ups. Because of this weighting, one assumes that the Air Force considers WC relatively correlated with overall fitness or at least cardiovascular fitness. To our knowledge, the Air Force has not considered on a large scale how body mass index (BMI), height-to-weight ratio (H-W), or waist-to-height ratio (WHtR) compares with WC with respect to its fitness test.

Using a 5.38 million record database from the Air Force Fitness Management System, we evaluated how WC, BMI, WHtR, and H-W correlate with fitness as assessed by the 1.5-mile run in addition to total fitness, which incorporates the 1.5-mile run time, number of push-ups and sit-ups. As this previously collected data were anonymous to us, this study fell under the definition of exempt status and approved by the institutional review board overseeing Joint Base San Antonio. For each waist metric, we performed a simple ordinary least squares regression to ascertain the correlation between that particular metric and either run time or total fitness; when incorporating more than one explanatory variable or covariate (to control for age and/or sex), we performed multiple ordinary least squares regressions. Due to the large database size and to mitigate against a type I error, we used an alpha of 0.001 for all statistical hypothesis tests.

Approximately 18% of the 5.38 million records belonged to women. With respect to sex differences, males appeared noticeably faster and performed more push-ups on average than females. The number of sit-ups completed was more comparable, with males having a slight advantage. Males also appeared to have larger WC, BMI, H-W, and WHtR measurements. We compared the ordinary least squares results between WC, H-W, WHtR, and BMI and ranked them by R2. Models varied in R2 from 1% to 46% depending on the covariates in the model, with sex having a greater effect than age. Whether individually or adjusting for age and sex, WHtR performed better than the other body composition variables with an average rank score of 1.1 and a median improvement of approximately 4% to the current Air Force metric of WC.

From our findings, we present a 20-point WHtR scoring system for the Air Force to use in lieu of its traditional usage of WC. We used this assessment chart to score all Airmen in our database and compared the results to their current scores on the abdominal circumference portion of the test with respect to predicting run time, after accounting for sex, age, and number of push-ups and sit-ups. The R2 value improved from 40.3 to 43.6, a relative improvement of approximately 8%, a fairly significant effect given the database consisted of over 5 million records. Future studies should investigate the longitudinal effect of varying waist metrics over time on run time or total fitness performance.