The incidence of gastrointestinal (GI) distress reported among long distance runners and not in practitioners of other sports with more gliding movements, such as bicycling, leads one to speculate that there is a difference in concussions and vibration of the GI region and that this may possibly be associated with the disparity in GI disorders. The present experiment was conducted to quantify the difference in vibration (accelerations/decelerations) of the GI region during running as compared to bicycling. An actometer was used which included a piezoelectric sensor. The sensor was placed on the abdomen of six subjects during identical trials of bicycling and running. Movement in three planes was detected by the sensor and recorded. The accelerometer output (AO) is an integral of these measurements. AO was used in making comparisons of the two types of movement, expressed as counts per minute. Mean AO was more than doubled in running, 859.5 +/- 130.1 (SD), versus bicycling, 425.8 +/- 149.5 (SD) (p less than .0001). The common assumption that running results in more body vibrations than bicycling has been substantiated and quantified. Although the differences are large, it can only be speculated that these differences explain the high frequency of GI symptoms among runners.