This paper presents an EMG diagnostic Knowledge Based System, that is the first application of our methodology for reasoning with causal-functional (meta-)models. Despite past difficulties, diagnosis is still an important application of KBSs, if considered in an appropriate context of medical practice. We argue that this is the case with neurophisiology, which lends to deep modelling of the domain and associated reasoning. The results obtained with our prototype system, and the clinical context where the system may be used make it a quite promising application, not only to experiment advanced artificial intelligence techniques but also to provide an useful decision support system for medical practice.