The objective of this study is to measure muscle activity during walking in order to estimate the distance that must be traveled for safe evacuation when the effect of the flow velocity changes. In the present study, electromyogram data were measured for six muscles on the right lower extremity, namely, the rectus femoris, vastus lateralis, long head of the biceps femoris, medial gastrocnemius, tibialis anterior, and gluteus medius. The experimental conditions included walking through a flooded hallway with 0.5-m-high water flowing at four different flow velocities (0, 0.47, 0.76, and 1.12 m/s in the direction opposite to the walking direction) and walking on dry ground. From the results, the tibialis anterior exhibited the highest muscle activity of up to 85% MVC with the toe off the ground when the flow velocity was the highest, suggesting that the subject tended to avoid stumbling against water flow during ankle dorsiflexion. The activity of the gluteus medius increased with the flow velocity, suggesting that the subject tried to stabilize the pelvic area to avoid falling. Empirical data for muscle activity during walking under flooded conditions in association with water height and flow velocity will contribute to better understanding safe evacuation from flooded urban underground areas.