The aim of this study was to investigate the effect of supporting surface instability to trunk and lower extremity muscle activities during bridging exercise combined with core-stabilization exercise. Thirty young healthy adults (15 males and 15 females) voluntarily participated in this study. Each subject was asked to perform bridging exercise combined with core-stabilization exercise on three different supporting surfaces (stable ground surface, the wooden balancing board, and the air cushion). The muscle activities were measured using surface electromyography (EMG) during performing exercise. To test statistical significance, one-way ANOVA with repeated measures was used with the significance level of .05. The findings of this study are summarized as follows. (1) There were significant differences in muscle activities on internal oblique, external oblique, gluteus medius, semitendinosus, biceps femoris, medial gastrocnemius and lateral gastrocnemius during exercise (p<.05). (2) The biceps femoris and lateral gastrocnemius showed significantly higher muscle activity on the wooden balancing board rather than on the ground, and semitendinosus, biceps femoris, medial gastrocnemius and lateral gastrocnemius showed significantly higher muscle activity on the air cushion rather than on the ground (p<.05). Therefore, it is concluded that muscle activities in the trunk and the lower limbs during bridging exercise combined with core-stabilization exercise was affected with instability of supporting surface. Further researches are needed to investigate the long term effect of bridging exercise on muscle activity with patient group.