Objective: This study compared the muscle activities of the erector spinae (ES), the external oblique (EO), and the rectus abdominis (RA) on three different surfaces. The purpose of this study was to determine which surface induces the highest muscle activity during the plank exercises. The information from this study can be used to recommend plank exercises to athletes and patients with weak core muscles. Design: Cross-sectional study. Methods: The subjects include 20 adult males attending S University in Seoul. Participants completed each plank exercise on three different surfaces. To measure muscle activities, researchers used the values from electromyography. The measurement excluded the initial two and final two seconds and collected information on the RA, EO, and ES in each posture of each subject. Results: The left external oblique showed significant differences between the plank position on stable ground (ST) and the plank position using a suspension device (SL) (p<0.05) and between the plank position on the unstable ground (US) and SL (p<0.05). The right rectus abdominis and left rectus abdominis displayed statistically significant differences between the ST and the US (p<0.05) and between the ST and the SL (p<0.05). The right erector spinae had a statistically significant difference between ST and US (p<0.05). Conclusions: The plank exercise strengthens the core muscles effectively, and muscle activity is related to the posture of the exercise and the location of the muscle. These results suggest that plank exercises improve muscle activities. Additionally, plank exercises can be applied to general medical care.