The SMOB project intends to contribute to fill the gap with reliable and operational outcome measures for type III and IV SMA. In analysing the reliability in imaging (spinal and muscular), electrophysiology analysis (MUNIX), and evaluate the evolution of respiratory function for 50 patients' cohort. The investigators would also take the opportunity to collect biologic samples in order to investigate genetic markers and to assess quality of life of patients by QoL-gNMD questionnaire. The investigators aim to build a database that will allow us to evaluate the effectiveness of a new therapy for adult SMA patients by studying the natural history of the disease. The investigators have distributed the various expertise in Work Package where several centers are involved. This study is original in that it evaluates the parameters of qMRI and MUNIX in correlation with blood biomarkers. To our knowledge, there are no quantitative MRI (spinal and muscular) biomarkers and/or electrophysiological (MUNIX technique) highlighted for tracking the progression of the adult form of SMA type III and IV. This pilot study would allow identification of predictive markers of the disease progression, and to have validated, sensitive to change and relevant measurement tools that could be used as endpoints in future therapeutic trials.