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Background: The rectus abdominis muscle partially overlaps with the transversus abdominis muscle belly when following cephalad. This study aimed to evaluate the overlapping properties between the rectus abdominis muscle and the transversus abdominis muscle belly in relation to the performance of ultrasound-guided rectus sheath block for analgesia around the umbilicus. Methods: After obtaining institutional approval and written informed consent, ultrasound examinations of bilateral abdominal walls were performed in healthy adult Japanese volunteers. The craniocaudal distance from the level of the umbilicus to the point where the rectus abdominis muscle and the transversus abdominis muscle belly begin to overlap on the ultrasound image was measured. Results: Sixty hemi-abdominal walls were examined in 30 volunteers (15 males, 32±6 years old, and 15 females, 31±5 years old). The craniocaudal distance from the level of the umbilicus to the point where the rectus abdominis muscle and the transversus abdominis muscle belly begin to overlap on the ultrasound image (left side/right side) was 17.4±11.8/18.8±13.3 mm in male subjects and 19.5±11.5/21.2±12.4 mm in female subjects, respectively. At the level of the umbilicus, overlapping of the two muscles was observed only in 2 of 60 subjects. At 45 mm cephalad from the level of the umbilicus, the two muscles began to overlap in 58 of 60 subjects (96.7%). Conclusions: The position where the rectus abdominis muscle and transversus abdominis muscle belly begin to overlap as detected by ultrasound imaging is approximately 20 mm cephalad to the umbilicus., Acta medica Nagasakiensia, 66(1), pp.1-6; 2022