Intracellular-to-total water ratio explains the variability of muscle strength dependence on the size of the lower leg in the elderly

Bioelectrical impedance spectroscopy (BIS) can assess intracellular water (ICW) and total water (TW) in limbs. This study aimed to examine whether BIS can explain a part of the inter-individual variation of the muscle size-strength relationship in older adults. We analyzed the data of 79 participants aged 64–86 years. The maximal voluntary isometric torques of dorsiflexion and plantar flexion on the right side were measured. The anterior and posterior muscle thickness (MT) in the right lower leg was assessed using ultrasonography. The length of the right lower leg (L) was measured, and the ICW-to-TW ratio (ICW/TW) in the right lower leg was obtained using BIS. The MT was multiplied by L to represent an index of muscle volume (MV). Correlation and stepwise regression analyses were performed. The anterior and posterior MT × L significantly and positively correlated with the muscle torque of dorsiflexion and plantar flexion (r = 0.710 and 0.649, respectively, P < 0.001). In the stepwise regression analyses, ICW/TW was selected as a significant predictor of muscle torque independent of MT × L (P < 0.05) for both dorsiflexion and plantar flexion. Electrical parameters of BIS (membrane capacitance, characteristics frequency, and phase angle) in the lower leg also significantly correlated with muscle torques. In addition, the skeletal muscle mass index (appendicular lean mass/height²) was also associated with ICW/TW (P < 0.001). The present results suggest that ICW/TW explains the interindividual variations of the muscle size-strength relationship.