Associations between Fat Mass and Fat Free Mass with Physical Fitness in Adolescent Girls: A 3-Year Longitudinal Study

SIMPLE SUMMARY: During the adolescent period, the associations between body composition and health-related physical fitness have been less studied. By examining such associations, health-related and physical education professionals will be able to monitor and track both components and their inter-correlation characteristics. Therefore, the main purpose of this study was to examine the longitudinal associations between the two components of body composition (namely fat mass and fat free mass) and physical fitness performance tests. In 240 adolescent girls, the findings showed that fat mass was inversely associated with standing broad jump, sit-ups in 60 s, and squats in 60 s, while positive associations with the 800 m and the 400 m run were observed. Fat free mass was positively associated with standing broad jump, sit-ups in 60 s, and squats in 60 s, while negative associations with the 800 m and the 400 m run were found. Fat mass and fat free mass seem to be similarly, but oppositely associated with physical fitness tests in adolescent girls. Thus, interventions that target lower fat mass and higher fat free mass values in order to improve physical performance should be advocated. ABSTRACT: The main purpose of the study was to examine the longitudinal associations between fat mass and fat free mass with health-related physical fitness. Two-hundred and forty 15-year old adolescent girls were measured at the baseline and after a period of 3 years (17 years). Health-related physical fitness included the following tests: (1) explosive power of the lower extremities (standing broad jump); (2) muscle endurance of the trunk (sit-ups in 60 s); (3) flexibility (sit-and-reach test); (4) muscle endurance of the lower extremities (squats in 60 s); (5) aerobic endurance (the 800 m run test); and (6) speed endurance (the 400 m running test). Fat mass and fat free mass were assessed using the bioelectrical impedance method. Longitudinal associations were analyzed with linear mixed model estimates. After adjusting for body mass index, fat mass was negatively associated with standing broad jump (β = −1.13, p < 0.001), sit-ups in 60 s (β = −0.27, p < 0.001), and squats in 60 s (β = −0.27, p < 0001), while positive associations with the 800 m running test (β = 0.02, p < 0.001) and the 400 m running test (β = 0.02, p < 0.001) were observed. On the other hand, fat free mass was positively associated with standing broad jump (β = 1.14, p < 0.001), sit-ups in 60 s (β = 0.28, p < 0.001), and squats in 60 s (β = 0.28, p < 0001), while the 800 m running test (β = −0.02, p < 0.001) and the 400 m running test (β = −0.02, p < 0.001) exhibited negative associations. This study shows that fat mass and fat free mass components are longitudinally, but oppositely associated with health-related physical fitness in adolescent girls.