Extracellular water may mask actual muscle atrophy during aging.

BACKGROUND Skeletal muscle tissue holds a large volume of water partitioned into extracellular water (ECW) and intracellular water (ICW) fractions. As the ECW may not be related to muscle strength directly, we hypothesized that excluding ECW from muscle volume would strengthen the correlation with muscle strength. METHODS A total of 119 healthy men aged 20-88 years old participated in this study. Knee isometric extension strength, vertical jump, and standing from a chair were measured as indices of muscle strength and power in the lower extremities. The regional lean volume (LV), total water (TW), ICW, and ECW in the lower leg were estimated by anthropometry (skinfold and circumference measurements) and segmental multifrequency bioelectrical impedance spectroscopy (S-BIS). Then, we calculated the ECW/TW and ICW/TW ratios. RESULTS Although ICW and the LV index decreased significantly with age (p < .001), no significant changes in ECW were observed (p = .134). Consequently, the ECW/TW ratio increased significantly (p < .001) with age (young adult, 27.0 +/- 2.9%; elderly, 34.3 +/- 4.9%; advanced elderly, 37.2 +/- 7.0%). Adjusting for this by including the ICW/TW ratio in our models significantly improved the correlation between the LV index and strength-related measurements and correlated with strength-related measurements independently of the LV index (p < .001). CONCLUSIONS The ECW/TW ratio increases in the lower leg with age. The results suggest that the expansion of ECW relative to ICW and the LV masked actual muscle cell atrophy with aging.