Sonomyography refers to the measurement of muscle activity with an ultrasonic transducer. It is a candidate modality for applications in diagnosis of muscle conditions, rehabilitation engineering and prosthesis control as an alternative to electromyography. We propose a mechanically-flexible piezoelectric sonomyography transducer. Simulating different components of the transducer, using COMSOL Multiphysics® software, we analyze various electromechanical parameters, such as von Mises stress and charge accumulation. Our findings on modelling of a single-element device, comprised of a PZT-5H layer of thickness 66µm, with a polymer substrate (E = 2.5 GPa), demonstrate optimal flexibility and charge accumulation for sonomyography. The addition of Polyimide and PMMA (Polymethyl methacrylate) as an acoustic matching layer and an acoustic lens, respectively, allowed for adequate energy transfer to the medium, whilst still maintaining good mechanical properties. In addition, preliminary ultrasound transmission simulations (200 kHz to 30 MHz) showed the importance of the aspect ratio of the device and how there is a need for further studies on it. The development of such a technology could be of great use within the healthcare sector, not only due to its ability to provide highly accurate and varied real-time muscle data, but also because of the range of applications that could benefit from its use.