This study analyzes how different knee flexion angles affect the abdominal and pelvic muscle activity during supine bridging. Twenty healthy subjects participated in the study. We used surface electromyography (EMG) to measure how three different knee flexion angles ($100^{circ}$, $70^{circ}$, and $40^{circ}$) affected the activity of the transverse abdominis/internal oblique (TrA/IO), external oblique (EO), biceps femoris (BF), rectus femoris (RF), and gluteus maximus (GM) muscles on the dominant side during supine bridging. The one-way repeated analysis of variance (ANOVA) was used to determine the statistical significance of TrA/IO, EO, BF, RF and GM muscle activity and the GM/BF activity ratio. For the TrA/IO, EO, BF, and GM muscles, supine bridging with different knee flexion angles resulted in significant differences in abdominal and pelvic muscle activity. For the TrA/IO muscles, the post-hoc test demonstrated that muscle activity significantly increased at $40^{circ}$ compared to $70^{circ}$; however, there were no significant differences between $100^{circ}$ and $70^{circ}$ or $100^{circ}$ and $40^{circ}$. For the EO muscle, the post-hoc test demonstrated that muscle activity significantly increased at $40^{circ}$ compared to $100^{circ}$ and $70^{circ}$; no significant difference was observed between angles $100^{circ}$ and $70^{circ}$. For the BF muscle, the post-hoc test demonstrated that muscle activity significantly increased according to the knee flexion angle ($40^{circ}$ > $70^{circ}$ > $100^{circ}$). For the GM muscle, the post-hoc test demonstrated that muscle activity significantly increased according to the knee flexion angle ($100^{circ}$ > $70^{circ}$ > $40^{circ}$). However, for the RF muscle, there was no significant difference. Additionally, the GM/BF activity ratio significantly increased according to the knee flexion angle ($100^{circ}$ > $70^{circ}$ > $40^{circ}$). From these results, we can conclude that bridging with a knee flexion of $100^{circ}$ can strengthen the GM muscle, whereas bridging with a knee flexion of $40^{circ}$ is recommended to strengthen the IO, EO, and BF muscles. We can also conclude that knee flexion angles should be modified during supine bridging to increase the muscle activity of different target muscles.