Finite Element Analysis is a numerical modelling tool vastly employed in research facilities to analyse and predict load transmission between the human body and a medical device, such as a prosthesis or an exoskeleton. Yet, the use of FEM in a framework compatible with clinical constraints is hindered by, amongst others, heavy and time-consuming assessments of material properties. Ultrasound imaging opens new and unique opportunities for the assessment of in vivo material properties of soft tissues. Confident of these advances, a method combining a freehand ultrasound probe and a force sensor was developed in order to compute the hyperelastic constitutive parameters of the soft tissues of the thigh in both relaxed (R) and contracted (C) muscles configurations. Seven asymptomatic subjects were included for the experiment. Two operators in each configuration performed the acquisitions. Inverse Finite Element modelling allowed for the optimisation of an Ogden’s hyperelastic constitutive model of soft tissues of the thigh in large displacement. The mean shear modulus identified for configurations R and C were respectively 3.2 ± 1.3 kPa and 13.7 ± 6.5 kPa. The mean alpha parameter identified for configurations R and C were respectively 10 ± 1 and 9 ± 4. An analysis of variance showed that subject and configuration had an effect on constitutive parameters but not operator.