A 64-year-old man is having difficulty getting out of a chair. He has lost 15 lb and feels tired all the time. On 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 creatinine kinase (CK) level is elevated and he is started on prednisone. Which of the following is the most important in monitoring response to therapy?