A 55-year-old woman complains to a physician because she has been having chronic pelvic discomfo. On fuher questioning, she repos chronic lower back pain, constipation, difficulty with walking, and impaired coitus. Pelvic examination demonstrates that the uterine cervix lies low within the vaginal canal, but does not protrude through the introitus. For minor degrees of this patient's disorder, Kegel exercises are sometimes prescribed. These can be performed by asking the patient to do which of the following?