For all IIM patients, CK (0.77 [0.53; 1.02]; p < 0.0001) and, to a lesser extent, myostatin (−0.46 [−0.77; −0.15]; p = 0.003) and age (0.02 [0.004; 0.04]; p = 0.01) were independent factors associated with disease activity. Here, MSTN is linked to acquired idiopathic inflammatory myopathy.