Can regional phase angle reflect changes in physical fitness and body composition from late childhood to adolescence?...

It is unclear whether regional phase angle (R-PhA), measured by bioelectrical impedance (BIA), is associated with changes in physical fitness and body composition during growth. We examined longitudinal associations between R-PhA and these outcomes.

Two hundred seventy participants (122 girls) from the Physical Activity and Nutrition in Children (PANIC) study were analyzed at ages 9.7 ± 0.4 (T1) and 15.8 ± 0.4 years (T2). Multifrequency BIA provided R-PhA (arm, leg). Dual-energy X-ray absorptiometry (DXA) determined total lean mass (LM) and fat mass percentage (%FM). Handgrip strength (HS) and standing long jump (SLJ) were scaled by body mass. Peak oxygen uptake (VO₂_peak) was measured using a cycle ergometer test and scaled by LM. Changes were calculated as T2-T1 (Δ). Regression models tested associations between Δ R-PhA and Δ outcomes (adjusted for pubertal status) and whether Δ R-PhA predicted T2 outcomes (adjusted for outcome, body composition, pubertal status, and moderate-to-vigorous physical activity at T1).

ΔPhA-Arm was associated with ΔHS (β = 0.23 girls; β = 0.24 boys), ΔSLJ (β = 0.29; β = 0.38), ΔLM (β = 0.33; β = 0.45), and Δ%FM (β = -0.22 boys). ΔPhA-Leg was associated with ΔHS (β = 0.29 boys), ΔSLJ (β = 0.23; β = 0.25), ΔVO₂_peak (β = 0.25 girls), ΔLM (β = 0.26; β = 0.59), and Δ%FM (β = 0.20; β = -0.24). At T2, ΔR-PhA positively predicted HS and SLJ in both sexes, LM in boys, and ΔPhA-Arm predicted LM in girls.

Increases in R-PhA were mainly related to gains in strength, power, and LM, indicating that PhA captures aspects of muscle function and composition. Nonetheless, its use for diagnostic or decision-making purposes remains to be clarified.