Sporadic Inclusion Body Myositis (s-IBM) is the most common acquired myopathy above the age of 50 years. It is a disabling disease that leads to wheelchair confinement. The cause is unknown but autoimmune mechanisms have been implicated. The primary goal of the study is to search for candidate autoantigen(s) that drive the autoimmune response and identify candidate pathogenic T-cell epitopes among the endomysial T-cells extracted from the patients' muscle biopsies. It is anticipated that the search for the antigens implicated in the clonal expansion of endomysial T-cells over time will elucidate the autoimmune pathogenesis of the disease and lead to the development of antigen-specific immunomodulatory therapeutic strategies. This is an investigative study intended to better define the pathogenesis of s-IBM. No new therapy will be provided except of standard care.