The passive elastic properties of a muscle-tendon complex are usually estimated from the relationship between the joint angle and the passive resistive torque, although the properties of the different structures crossing the joint cannot be easily assessed. This study aimed to determine the passive mechanical properties of the gastrocnemius medialis muscle (GM) using supersonic shear imaging (SSI) that allows the measurement of localized muscle shear modulus (μ). The SSI of the GM was taken for 7 subjects during passive ankle dorsiflexion at a range of knee positions performed on an isokinetic dynamometer. The relationship between normalized μ and the length of the gastrocnemius muscle-tendon units (GMTU) was very well fitted to an exponential model (0.944<R²<1) to calculate muscle stiffness (α) and slack length (l(0)). This relationship was compared to the normalized force-length relationship obtained using Hoang's model. In addition, the reliability of the μ-length obtained with the knee fully extended was calculated. The μ-length relationship was highly correlated with the force-length (0.964<R²<0.992) although muscle force was slightly underestimated (RMSE=31.0±14.7 N, range: 7.8-56.0 N). α and l(0) measured with the knee extended were similar to that reconstructed from all knee angles and displayed good intra-session reliability (for α, SEM: 9.7 m(-1); CV: 7.5%; ICC: 0.652; for l(0), SEM: 0.002 m; CV: 0.4%; ICC: 0.992). These findings indicate that SSI may provide an indirect estimation of passive muscle force, and highlight its clinical applicability to evaluate the passive properties of mono- and bi-articular muscles.