Early Fat Loss and Plateau in Firefighter Recruits: Longitudinal Tracking of Body Composition Using Skinfold Analysis and Multifrequency Bioelectrical Impedance Analysis.

Beam, JR, Mang, ZA, Gibson, AL, Tinsley, GM, Mota, JA, Martinez, A, and Martinez, AH. Early fat loss and plateau in firefighter recruits: Longitudinal tracking of body composition using skinfold analysis and multifrequency bioelectrical impedance. J Strength Cond Res XX(X): 000-000, 2026-The purpose of this study was to examine how multifrequency bioelectrical impedance analysis (MF-BIA) and skinfold analysis (SKF) track body composition changes during a firefighter training academy. Thirty-nine male recruits (age: 27 ± 6 years; height: 176.8 ± 6.7 cm; body mass: 83.2 ± 14.5 kg; body fat percentage [BF%]: 16.9 ± 6.0%) completed structured exercise training. Body composition was assessed at 5 time points (T1-T5) using MF-BIA and SKF. Fat mass (FM), fat-free mass (FFM), and BF% were compared between methods. Cross-sectional differences and longitudinal changes were analyzed using linear mixed-effects models. No significant method × time interactions were observed for BF%, FM, or FFM, indicating that both techniques similarly tracked longitudinal changes throughout the academy. Significant main effects of time were observed for BF% and FM (p < 0.05). Planned contrasts revealed significant reductions in BF% and FM between T1-T3, T1-T4, and T1-T5 (p < 0.05) but not between adjacent time points, indicating that body composition changes occurred primarily during the early phase of training with a plateau between 12 and 18 weeks. Linear regression analyses demonstrated strong associations between MF-BIA and SKF. Bland-Altman analyses revealed outcome-specific agreement patterns. These findings support the use of either MF-BIA or SKF for consistent within-method monitoring of group-level body composition changes over time. However, despite high associations, limited individual agreement indicates the methods should not be used interchangeably. Fire departments may select the method that best aligns with resource availability, staffing expertise, and program scale without compromising longitudinal group-level monitoring.