This study presents a new method for the estimation of the dynamic impedance of multi-joint leg movements. The method is based on Multi Input Multi Output (MIMO) system identification techniques and is designed for continuous torque perturbations at the hip and knee joint. Preliminary results from this study indicate that MIMO system identification can successfully be used to estimate the hip and knee impedance and the interaction dynamics between both joints. It is also concluded that, in order to create a good model representation of the leg impedance, the effect of biarticular muscles needs to be taken into account. The obtained measures for joint impedance might be used for clinical assessment and follow up of patients, as well as for the development of supportive devices.