Author(s): ADACHI, R., KODESHO, T., NAKAO, G., NARA, G., ISHIYAMA, K., SEKIGUCHI, K., KIDA, K., SAWANO, J., HIRAYAMA, S., ANDO, R., TANIGUCHI, K. , Institution: SAPPORO MEDICAL UNIVERSITY, Country: JAPAN, Abstract-ID: 405

INTRODUCTION:
Evaluating viscoelasticity, a mechanical property of skeletal muscles, aids in understanding muscle conditions. Recently, ultrasound shear wave elastography (SWE) enables quantitative assessment of elasticity and viscosity. Previous research reported a significant positive correlation between skeletal muscle elasticity and the rate of torque development (RTD), an index of rapid force production [1]. However, few studies have focused on muscle viscosity and its relationship with exercise performance remains unclear. Therefore, this study investigated the relationship between the elasticity and viscosity of the medial gastrocnemius and RTD.
METHODS:
The participants were 29 healthy young (mean age; 20.8 ± 1.6 years, height; 174.7 ± 6.3 cm, weight; 64.3 ± 6.8 kg). The elasticity and viscosity of the medial gastrocnemius muscle were measured in the prone position using SWE (SuperSonic Imagine, MACH 30) at 0° (neutral) and 15° (plantar flexion), respectively. The shear modulus (kPa), calculated from the shear wave velocity (SWV) generated by the acoustic radiation force, was used as the elasticity index. For viscosity, the viscosity coefficient (Pa.s) was calculated as the viscosity index based on the frequency dependence of SWV using frequency dispersion analysis. To assess the RTD, maximal isometric plantar flexion was performed at 0°. Peak torque and RTD at 50 ms (RTD50) and 100 ms (RTD100) from torque production onset were calculated. Correlations between the viscoelastic properties and RTDs were analyzed using Pearson’s correlation coefficients. Statistical significance was set at p < 0.05.
RESULTS:
Elasticity was significantly and positively correlated with RTDs (RTD50: r = 0.479, p = 0.009; RTD100: r = 0.377, p = 0.044). Viscosity was significantly and positively correlated with the RTD (RTD50: r = 0.507, p = 0.005; RTD100: r = 0.399, p = 0.032). Contrastingly, peak torque was not significantly correlated with elasticity (r = -0.020, p = 0.918) nor viscosity (r = -0.123, p = 0.526).
CONCLUSION:
Elasticity and viscosity were significantly and positively correlated with RTD, whereas no relationship was observed with peak torque in the present study. Viscoelastic properties of skeletal muscles contribute to the early phase of force production rather than to maximal muscle strength itself. Considering that high-viscosity materials generate substantial reaction forces in response to rapid loading [2], the passive reaction forces associated with explosive force production may mechanically contribute to the RTD. Therefore, the viscosity of skeletal muscles, as well as their elasticity, may be crucial factors in determining rapid force production.

REFERENCE
[1] Ando, R., Eur J Appl Physiol. 2024
[2] Rehorn, M. R. et al., J Biomech. 2014