Sufficient physical activity during childhood is critical to support children's current and future health. Schools across the country use fitness testing to assess and track student fitness, often reporting results to parents. A new study using data from over 570 thousand fourth- through 12th-grade racially/ethically and socioeconomically diverse students in New York City public schools demonstrates a positive association between student body mass index, a measurement for childhood obesity, and the trunk lift test–one of the most commonly used measures of flexibility. FITNESSGRAM® tests are one of the most widely-used tools in the U.S. to measure public school students' physical activity, yet few studies have assessed the validity and reliability of key components of the test–the trunk extension and the sit-and-reach–to measure flexibility. Researchers found that students with overweight or obesity consistently performed better on the trunk extension test compared to students with normal weight, suggesting that this test may not accurately measure students' flexibility. The study was published in the journal PloS ONE by Hannah Thompson from the University of California, Berkeley and University of California, Division of Agriculture and Natural Resources, Nutrition Policy Institute in collaboration with Andjelka Pavlovic from The Cooper Institute®, Emily D'Agostino from Duke University School of Medicine, Melanie Napier from the University of North Carolina at Chapel Hill, and Kevin Konty and Sophia Day from the New York City Department of Health and Mental Hygiene. This study was funded, in part, by the National Heart Lung and Blood Institute (NHLBI), Grant 1K01HL151805.