Synchronizing Gait with Cardiac Cycle Phase Alters Heart Rat... : Medicine & Science in Sports & ExerciseFinally, if diastolic stepping is truly advantageous for exercise performance, we wondered if there is any natural tendency for elite runners to step diastolically without prompting. We observed that 3 of the 10 subjects stepped accurately at the diastolic target timing (45% ± 15% RRI) during a substantial portion of their final, unprompted (silent) control period (e.g., see Fig. 2B). In one of those cases, the subject was guided to systolic step timing during the final prompted period but naturally reverted to stepping during diastole when the auditory prompt was removed. By comparison, none of the subjects stepped at a consistent systolic phase during the free stepping control stages. These anecdotal observations are also consistent with the findings of earlier authors, including Niizeki (10), which suggested that the heart automatically adjusts its timing, when HR = SR, to prevent ventricular systole from coinciding with maximal peripheral skeletal muscle contraction.