Mature locomotion in humans is characterized by an anti-phase coordination (moving in opposite directions) between the pelvic and the scapular girdles. This pattern involves a specific relationship between the arm and leg motion is deemed to be most flexible and dynamically efficient. Still, when the arms are involved in another task, like a field player running with a ball in the hands, locomotion is still possible. In order to probe the flexibility of the locomotor synergy, the present study aimed to determine the persistence and the strength of the coordination patterns between the pelvic and scapular girdles when no arm swing was allowed during walking and running. Relative phase, the time difference between the girdle rotations, measured the ongoing inter-girdle coordination of eight healthy participants asked to walk and run with or without arm swing. Results showed that an absence of arm swing led to a change from an anti-phase to in-phase pattern (girdles moving in the same direction) and that an increase in velocity strengthened the adopted pattern. Moreover, the frequency distribution of relative phase for all gait patterns with arm swing proved to be bimodal, indicating that the prevailing anti-phase pattern was always mixed with a noticeable proportion of in-phase coordination. The presence of the in-phase pattern in the easy, natural locomotion with arm swing manifests its persistence and its stability, perhaps pertaining to its prevalence in earlier times in ontogeny or evolution.