Extracellular water may increase with age and be independently and negatively associated with muscle strength and physical function in older adults: A cross-sectional study

Whether extracellular water (ECW) increases with aging in humans and whether it is negatively associated with muscle function remains unclear. We aimed to clarify the associations of segmental intracellular water (ICW) and ECW with age and their independent associations with muscle strength and physical performance. This cross-sectional analysis included 1102 older adults; segmental ECW and ICW in the thigh and arm were assessed by bioelectrical impedance spectroscopy (BIS). Knee extension and handgrip strength assessed muscle strength, and the timed-up-and-go test, 6-m maximum walk time, chair stand, vertical jump, and single-leg balance tests assessed physical performance. Among older age groups, covariate-adjusted thigh ECW was higher, whereas ICW and total body water were lower. Arm measures demonstrated similar age-related patterns, although less consistently, with sex-specific variation. Higher thigh and arm ECW levels were independently and negatively associated with muscle strength and physical performance in the corresponding segment (thigh: |β| = 0.12-0.31; arm: |β| = 0.12-0.20) after covariate adjustment. Conversely, higher arm and thigh ICW levels were positively associated with muscle strength and physical performance in the corresponding segment after adjustment. ICW may reflect functional muscle, whereas ECW may not be merely a passive non-contractile compartment; rather, it may be a negative correlate of muscle strength and physical performance. Assessing segmental ECW alongside ICW may be essential to detect age-related differences in muscle quality.