Author(s): MURAKAMI, K., AKIHARA, Y., Institution: MOMOYAMA GAKUIN UNIV, Country: JAPAN, Abstract-ID: 448

INTRODUCTION:
Body composition plays an important role in physical fitness development during adolescence. However, grade-related differences in the associations between body composition and physical fitness, assessed using fat-free mass index (FFMI) and fat mass index (FMI), have not been fully examined. This study investigated the relationships between FFMI, FMI, and multiple components of physical fitness in Japanese junior high school girls using quartile-based comparisons and correlation analyses [1].
METHODS:
This cross-sectional study included 557 Japanese junior high school girls (Grade 1: n = 228; Grade 2: n = 242; Grade 3: n = 87). Body composition was assessed by bioelectrical impedance analysis (In Body 270), and fat-free mass index (FFMI) and fat mass index (FMI) were calculated as fat-free and fat mass divided by height squared. Physical fitness was evaluated using eight items of the Japanese Physical Fitness Test. Participants were categorized into FFMI and FMI quartiles. Group differences were examined using one-way ANOVA with Bonferroni post hoc tests, and Pearson’s correlation coefficients were calculated for the total sample and by grade (p < 0.05).
RESULTS:
Higher FFMI quartiles were associated with significantly greater handgrip strength, sit-ups, side-steps, standing long jump, and handball throw performance, and shorter 50-m sprint times (all p < 0.05), with stepwise increases evident for handgrip strength and throwing performance. In contrast, higher FMI quartiles showed significantly lower performance in sit-ups, side-steps, 20-m shuttle run, and standing long jump (p < 0.05), while no significant differences were observed in handgrip strength, sit-and-reach, 50-m sprint, or handball throw; a graded decline was most evident in shuttle run performance.
In the total sample, FFMI was positively correlated with handgrip strength (r = 0.535), handball throw (r = 0.424), and sit-ups (r = 0.274), and negatively correlated with 50-m sprint time (r = −0.151). FMI was negatively correlated with shuttle run (r = −0.343), side-steps (r = −0.216), standing long jump (r = −0.230), and sit-ups (r = −0.190). Grade-specific analyses showed stronger negative associations between FMI and aerobic capacity, agility, and jumping performance in Grades 1 and 2, which were attenuated in Grade 3, whereas associations between FFMI and strength-, power-, and agility-related performance were more prominent in Grade 3.

CONCLUSION:
In Japanese adolescent girls, higher FFMI is associated with better performance in strength- and power-related fitness, whereas higher FMI is linked to poorer performance in weight-bearing and locomotor tasks. The influence of fat mass on fitness decreases with advancing grade, while the contribution of fat-free mass becomes more pronounced. Although causal relationships cannot be inferred, these findings provide large-scale, internationally comparable reference data on the role of body composition in physical fitness during female adolescence.