A 64-year-old man presents to the clinic because he is experiencing generalized weakness. He notes difficulty getting out of a chair, and lifting objects above his head as examples of the muscle weakness. He has lost 15 lb and feels unwell. On physical examination, there is a blue purple rash on his eyelids and knuckles, and muscle strength in his proximal muscles is rated 4 out of 5. His laboratory investigations are abnormal for an elevated creatinine kinase (CK) level. He is started on prednisone therapy. Which of the following is the most important in monitoring response to therapy?