Background: Obesity negatively affects balance and core muscles playing a crucial role in postural stability. However, little is known about the relationship between abdominal muscle thickness and balance in normal weight women. Objectives: This study aimed to investigate the relationship between abdominal muscle thickness and balance ability in normal-weight women in their 20s. Design: Cross-sectional observational study. Methods: Twenty-four healthy normal-weight women participated. Abdominal muscle thickness including the external oblique (EO), internal oblique (IO), and transverse abdominis (TrA) was measured by ultrasonography. Static balance was assessed during the One-leg standing test and Tetrax system, while dynamic balance was assessed using the Y-Balance and single leg triple hop tests. Pearson correlation analysis was conducted to examine the relationship between muscle thickness and balance performance. Results: IO thickness showed a negative correlation with static balance (r=- .42). EO thickness showed a positive correlation with single leg triple hop tests (r=.49-.58) and a negative correlation with posterolateral distance during YBalance test. Conclusion: Our findings suggested that abdominal muscles, specifically the IO and EO, are moderately related to balance performance in normal-weight young women. These findings demonstrated the potential benefit of targeted core strengthening interventions to improve postural stability and functional movement. Further studies involving additional muscles and diverse populations are recommended.

Background: Core muscles provide important dynamic stabilization of the lumbar spine. The twist sit-up can simultaneously stimulate the external oblique (EO), internal oblique (IO), and transverse abdominis (TrA) muscles. Objectives: The purpose of this study is to investigate the impact of knee angles during twist sit-up exercises and determine the most efficient knee angle for performing this exercise. Design: Cross-sectional study. Methods: The study recruited 30 students from S University regardless of gender. The subjects performed a total of 10 twist sit-ups at five different knee angles (0, 40, 65, 90, and 105 degrees), divided between the ipsilateral and contralateral sides. The thickness of abdominal muscles (EO, IO, and TrA), was measured using ultrasonography on rest position and full contraction position for each angle of knee flexion and compared between angles. Results: There were significant differences in the ipsilateral EO, IO, and TrA muscle thickness at different knee flexion angles (P<0.5). There was a significant difference in the thickness of EO between 0 and 105 degrees and between 40 and 105 degrees (P<0.5), a significant difference in the thickness of IO between 0 and 105 degrees (P<0.5), and a significant difference in the thickness of TrA between 0 and 105 degrees, 40 and 90 degrees, and 65 and 90 degrees (P<0.5). Conclusion: In conclusion, the muscles involved in axial rotation i.e., EO and IO, maximally contract at a knee flexion angle of 105 degrees. Therefore, this may be the most optimal angle to perform twist sit-ups.