Publications

• These results carry significant clinical implications, indicating that the integrated use of BIA and myotonometry allows for a more profound assessment of muscle quality in young healthy adults.
• In the future, after the development of normative ranges, the standardization of this combined, noninvasive method could lead to the creation of an integrated muscle quality index.

Background and Aims: Water plays a vital role in muscle function, and its distribution between intracellular (ICW) and extracellular (ECW) compartments serves as a key indicator of muscle quality. Bioelectrical impedance analysis (BIA) quantifies body water (BW), while myotonometry measures muscle mechanical properties; however, the relationship between these measures remains underexplored. This study aimed to assess the association between BW compartments and the resting mechanical properties of lower limb muscles.

Methods: A total of 158 healthy adults (97 men, 61 women; mean age 20.8 years) participated. BW compartments, including total body water (TBW), ICW, and ECW, were measured using a multi-frequency body composition analyser (TANITA MC-780 MA). A MyotonPRO® device assessed the mechanical properties (e.g. tone, stiffness) in the tibialis anterior, peroneus longus, and gastrocnemius muscles. Spearman’s rank correlation and multiple linear regression models were used to analyse the relationships between BIA and myotonometry.

Results: The analysis revealed a consistent bipolar pattern of associations. Absolute hydration volumes (TBW, ICW, ECW expressed in kilograms) were positively correlated with muscle tone and stiffness, with correlations ranging from weak to high (r ≈ 0.30–0.70, p < 0.001). ICW was the strongest determinant. In contrast, relative ECW (expressed as a percentage) showed moderate negative correlations with tone and stiffness. This pattern was consistent across all muscles and between limbs.

Conclusions: In healthy young adults, the resting mechanical properties of lower limb muscles are significantly associated not only with TBW but, more importantly, with its compartmental distribution. Higher ICW corresponds to greater muscle tone and stiffness, whereas a high ECW proportion is linked to lower values.

Keywords: body water compartments, mechanical properties of soft tissues, bioelectrical impedance analysis, myotonometry, muscle architecture

In this cross-sectional study of healthy young adults, parameters of BW distribution measured by BIA were consistently associated with resting mechanical properties of lower limb muscles measured by myotonometry. Absolute BW volumes (TBW, ICW, ECW in kilograms) correlated positively with muscle tone and stiffness, whereas a higher extracellular-to-total water proportion related to lower tone and stiffness (F-MYO, S-MYO), and to a reciprocal pattern for viscoelastic parameters (D-MYO, R-MYO, C-MYO). These findings suggest that muscle mechanical state reflects not only BW quantity but, critically, its compartmental distribution.