The aim of this study was to explore the relationship between energy cost of locomotion and stride duration variability in the region of the walk-run transition speed. Ten subjects participated in this experiment during four treadmill sessions. The first session was used to habituate subjects to treadmill walking or running. During the second session the treadmill speed was increased from 6 km.h-1 to 10 km.h-1 in steps of 0.2 km.h-1 to determine the freely chosen transition speed between walking and running (ST). The last two sessions consisted of five walks or five runs conducted at five intensities selected to represent respectively ST-1 km.h-1; ST-0.5 km.h-1; ST; ST + 0.5 km.h-1; and ST + 1 km.h-1. Exhaled gases were collected during the last two sessions, and stride duration was continuously recorded. The results indicated a significant increase in stride duration variability before ST for the freely chosen gait condition only. These findings point to an alternative hypothesis to that classically proposed to describe a transition as an energy saving mechanism.