This paper presents an idea of using a structure traversal graph (STG) to characterize whether program comprehension is progressing smoothly. Inspired by electrocardiograms that are used to measure heart rhythms, STGs are an attempt to depict the rhythm of program navigation. STGs are created by abstracting navigation between files to the level of a conceptual architecture graph. We analyzed the navigation patterns of fifteen developers performing a modification task in the laboratory and found that the STG was a good organizing principle for program comprehension behavior