Science behind policy: implementing a modern circumference-based body fat equation with a physical fitness threshold is associated with lower musculoskeletal injury risk

Abstract

Body composition influences physical fitness (PF) and risk of musculoskeletal injury (MSKI). Assessing the relationship between body fat (BF), PF and MSKI risk in a large diverse military population may provide evidence basis informing health-care policies, practices, and programs for military and civilian populations. Evaluate the validity of expedient methods to estimate BF (e.g., circumference-based equation (CBE) and bioelectrical impedance analysis (BIA)) and investigate relationships between BF and PF with MSKI risk in a large diverse population. Participants were 1904 active-duty Soldiers (643 F) representing Army demographics sex, race/ethnicity (R/E), and age. PF, defined as the most recent Army Combat Fitness Test (ACFT) score and incidence of MSKI, were obtained from Army records. BF was determined by dual-energy x-ray absorptiometry (%BFDXA), bioelectrical impedance analysis (%BFBIA), and CBE using 3-site (Hodgdon, %BFHE) and 1-site (Taylor-McClung, %BFTM) equations. Results were stratified by race and sex, to evaluate differences in accuracy of estimated %BF (weighted root mean squared error from %BFDXA). Associations of BF and PF with MSKI risk were evaluated with logistic regression. CBE and BIA underestimated %BF compared to %BFDXA. %BFBIA differed from %BFDXA overall and by sex. %BFTM underestimation was uniform across both sex and R/E compared to %BFDXA. Mean differences from %BFDXA by sex (M;F) were lower when measured by %BFTM (4.38; 4.59) compared to %BFHE (5.88; 4.39). Individuals had a greater likelihood of MSKI if they failed BF standards (odds ratio 1.32). Scoring ≥ 540 total on ACFT exhibited a 31% (95% CI: 0.52, 0.92) lower MSKI risk during the following 12 months than those with a lower score. A single-site BF equation (%BFTM) maintained similar accuracy across the Soldier population by sex, age, and R/E. Implementing a PF score threshold in lieu of passing Army BF standards was associated with lower MSKI risk.