Assessing muscle properties is relevant in several contexts, from the study of muscle physiology/pathophysiology to the assessment of rehabilitation outcomes. Muscle activation and the resulting tissue displacement are traditionally studied with surface electromyograms (sEMGs) and ultrasound (US) images respectively. By recording sEMG signals from several locations above the skin surface (high-density sEMG - HDEMG), it is possible to study the spatial distribution of EMG activity over the muscle and to decompose the interferential signal into its constituent trains of motor unit action potentials (MUAPs). High frame rate US imaging allows to reveal fine spatial details of tissue displacement with high temporal resolution (up to 5,000 fps), enabling in vivo estimation of the mechanical muscle properties from tissue velocity fields.