Phase angle identifies a muscle quality phenotype associated with physical performance in U.S. marines.

OBJECTIVE: To evaluate whether phase angle (PhA) provides physiological information beyond DXA-derived body composition for explaining physical performance in active-duty U.S. Marines. METHODS: We analyzed data from 2,177 active-duty U.S. Marines with bioelectrical impedance analysis, dual-energy X-ray absorptiometry (DXA), and standardized physical performance testing. Phase angle was expressed in degrees and standardized within sex to define Phase Angle Muscle Quality (PhA-MQ) quartiles. Outcomes included Physical Fitness Test (PFT), Combat Fitness Test (CFT), estimated VO2max, and performance normalized to lean mass. Multivariable linear regression models adjusted for DXA-derived fat-free mass index (FFMI), percent body fat (%BF), age, and sex were used to assess independent associations. Incremental explanatory value was quantified using change in R2 (ΔR2) and partial R2. Nonlinear associations were examined using spline models. RESULTS: Physical performance increased progressively across PhA-MQ quartiles, with the greatest separation observed for lean-mass-normalized outcomes. Across PhA quartiles, mean PFT scores increased from 236 to 271 points; while CFT scores increased from 240 to 272 points. In adjusted models, PhA remained independently associated with PFT (ΔR2 = 0.013, p < 0.001) and CFT (ΔR2 = 0.006, p<0.001). Stronger effects were observed for performance efficiency measures, including PFT per kg lean mass (ΔR2 = 0.020; partial R2 = 0.055) and CFT per kg lean mass (ΔR2 = 0.012; partial R2 = 0.051). In contrast, associations with VO2max were minimal and not statistically significant (p = 0.088). Joint models demonstrated that PhA retained independent explanatory value alongside FFMI and %BF, particularly for lean-mass-normalized outcomes. Continuous spline analyses supported monotonic relationships between PhA and performance. CONCLUSIONS: Phase angle identifies a physiological phenotype associated with enhanced physical performance independent of DXA-derived body composition. These findings support phase angle as a marker of muscle quality-related physiological status rather than a direct measure of muscle quality.