(44) THE FAT-FREE MASS INDEX IS POSITIVELY ASSOCIATED WITH VARIOUS MEASURES OF STRENGTH IN MILITARY PERSONNEL

Purpose: Military personnel are required to be physically fit to optimally perform occupational tasks. While body fat percentage is often used to assess physical fitness, prior research has shown that fat-free mass (FFM) may better predict performance and is associated with greater strength, speed, and power in athletes and military personnel. The fat-free mass index (FFMI) is a height-adjusted metric of FFM and has commonly been used to assess relative FFM in athletes. To date, there is limited research investigating FFMI and performance among military personnel. The purpose of the investigation was to: 1) determine the relationship between FFMI and various strength measures (handgrip strength [HGS], isometric mid-thigh pull [IMTP], plank [PLK], pull-ups [PULL], and push-ups [PUSH]) and 2) determine if differences in strength performance exist based on FFMI categories (low, moderate, or high). Methods: Data from three cross-sectional studies were combined to determine the relationship between FFMI and various strength measures. Participants included 146 healthy, active-duty military personnel (n = 73 male, n = 73 female). Height was measured via stadiometer and FFM data were recorded from multi-frequency bioelectrical impedance analysis and dual-energy x-ray absorptiometry to calculate FFMI (kg·m^-2). Using newly proposed FFMI categories (FFMI ± 1.5*SD), low, moderate and high FFMI was ≤ 15.45 kg·m^-2, 15.46 – 23.64 kg·m^-2, and ≥ 23.65 kg·m^-2, respectively. A linear regression analysis examined the relationship between FFMI and each strength outcome. Between-group differences in strength measures were assessed with ANOVA. Statistical significance was set a priori as p < .05. Results: FFMI was significantly predictive of HGS (R² = 0.386, p < .001), IMTP (R² = 0.459, p < .001), LTK (R² = 0.237, p < .001), PULL (R² = 0.248, p < .001), and PUSH (R² = 0.246, p < .001), but not PLK performance (R² = 0.015, p = .146). FFMI was strongly correlated with HGS (r = .621, p < .001) and IMTP (r = .677, p < .001), and moderately correlated with LTK (r = .487, p < .001), PULL (r = .498, p < .001), and PUSH (r = .496, p < .001), but not correlated with PLK performance (r = 0.120, p =.146). When categorizing FFMI by low, moderate, and high, significant between-group differences were observed in HGS (p < .001), IMTP (p < .001), and PUSH (p < .005). There were no significant differences between groups in PLK (p = .373), LTK (p = .719), or PULL (p = .548). Those with high FFMI had greater HGS (mean difference [MD] = 47.0 kg, p< .001), IMTP (MD = 93.2 kg, p < .001), and PUSH (MD = 51.4 reps, p = .004) than those with low FFMI and greater HGS (MD = 29.5 kg, p < .001) and IMTP (MD = 46.4 kg, p = .003) than those with moderate FFMI. Additionally, higher IMTP (MD = 46.8 kg, p = .044) and PULL (MD = 33.3 reps, p = .046) were observed among those with moderate compared to low FFMI. Conclusions: FFMI was predictive of performance on combat relevant measures of strength. Military personnel with moderate to high FFMI consistently outperformed those with low FFMI in all strength measures except the PLK. Practical Applications: The current body composition standards may be modernized to include various metrics that are associated with performance. These standards could be refined to include FFMI, such that a minimum FFMI threshold can be adopted for entry into the military, while FFMI categories can be used to support more specific nutrition and training goals. Acknowledgements: None.