This paper describes a case study carried out on the behaviour of an eccentric pulley transmission system housed in a manually operated abdominal-intestinal assistant device for human use. The aim is to establish a systemic framework that serves as a validation tool for mechanical systems in the initial stages of the product design processes. The proposed study system describes the device transmission process as a function of the input angle variation (dα), the height of the user’s feet (HF) and the pulley curvature function (C), resulting from the variation of its radius (dR) over time (dt). In order to explore and compare the different behaviours and identify possible solutions four different configurations of curvatures were proposed. Causal diagrams and differential equations describe the simulation scenario. The resulting application model supports the use of a systemic frame and methods as a pre-response to the validation of design proposals.