Purpose <p>This study aimed to investigate the relationship between medial gastrocnemius (MG) shear modulus and viscosity, as measured using shear wave elastography (SWE), and the capacity for rapid force production, specifically the rate of torque development (RTD).</p> Methods <p>Twenty-nine healthy young men participated in this study. The passive shear modulus and viscosity (viscosity coefficient) of the MG were measured using SWE. Viscosity was quantified at 15° of plantarflexion, whereas shear modulus was measured at 0° and 15° of plantarflexion. Participants performed maximal isometric plantarflexion at 0°. RTD was calculated over 0–50 (RTD50) and 0–100 ms (RTD100) and was normalized to peak torque. The relationships between MG viscosity, MG shear modulus, RTD, and peak torque were analysed using Pearson product-moment correlation coefficients.</p> Results <p>Viscosity at 15° of plantarflexion was significantly positively correlated with RTD50 (<i>r</i> = 0.507, <i>p</i> = 0.005) and RTD100 (<i>r</i> = 0.399, <i>p</i> = 0.032). Shear modulus at 0° of plantarflexion was also significantly correlated with RTD50 (<i>r</i> = 0.479, <i>p</i> = 0.009) and RTD100 (<i>r</i> = 0.377, <i>p</i> = 0.044). No significant correlations were found between peak torque and any viscoelastic parameters. Shear modulus at 15° of plantarflexion did not correlate with RTD.</p> Conclusion <p>This study provides initial evidence that skeletal muscle viscosity may be crucial in determining rapid force production. In addition to acting as a property characterized by velocity-dependent resistance, muscle viscosity may contribute to stable and efficient force transmission during the initial phase of contraction.</p>