[Purpose] The aim of this study was to compare muscle activity of the posterior oblique sling during prone hip extension (PHE) on the floor and on a round foam roll. [Subjects] Twenty-two (11 male, 11 female) healthy volunteers were recruited for this study. [Methods] The participants performed PHE on the floor and on a round foam roll. Surface electromyography (EMG) was recorded from the contralateral latissimus dorsi (LD), contralateral erector spinae (ES), ipsilateral ES, ipsilateral gluteus maximus (GM), and ipsilateral biceps femoris (IBF). A paired t-test was used to compare muscle activity under the floor and round foam roll conditions. [Results] EMG activity of the contralateral LD, ipsilateral ES, and ipsilateral GM was significantly greater when PHE was performed on the round foam roll than on the floor. [Conclusion] Performing PHE on the round foam roll induced greater posterior oblique sling EMG activity than did exercise on the floor. These results suggest that the activation pattern of the posterior oblique sling during PHE is differs according to the type of surface (stable vs. unstable) on which it is performed.