Comparison of Changes in Abdominal Muscle Thickness According to Arm Position and Application of Elastic Band Resistance during Crunch Exercises

This study aimed to compare changes in the thickness of specific abdominal muscles according to arm position and the application of tubing band resistance during crunch exercises.Thirty healthy participants (15 males, 15 females) performed three types of crunch exercises: traditional crunch, shoulder extension resistance crunch, and shoulder horizontal abduction resistance crunch using a medium-resistance tubing band.The thicknesses of the upper and lower rectus abdominis, external oblique, internal oblique, and transversus abdominis muscles were measured at rest and during exercise using B-mode ultrasonography.Data were analyzed using one-way repeated-measures analysis of variance (ANOVA), followed by Bonferroni post hoc tests.The upper and lower rectus abdominis, external oblique, and internal oblique muscles showed significantly greater increases in thickness during both shoulder extension and shoulder horizontal abduction resistance crunch exercises compared to the traditional crunch exercise (p < 0.05).Compared with the traditional crunch exercise, the shoulder extension resistance crunch produced increases of 19.2%, 15.7%, 14.1%, and 15.8% in the upper rectus abdominis, lower rectus abdominis, external oblique, and internal oblique muscles, respectively (Cohen's d = 0.53-1.02).Similarly, the shoulder horizontal abduction resistance crunch produced increases of 14.7%, 11.4%, 24.9%, and 11.9%, respectively (Cohen's d = 0.50-0.94).In particular, the external oblique exhibited a 9.5% greater increase in thickness during the shoulder horizontal abduction resistance condition than during the shoulder extension resistance condition (Cohen's d = 0.32, p < 0.05).No significant differences were observed in transversus abdominis muscle thickness among the three exercise conditions.These findings suggest that incorporating arm resistance into traditional crunch exercises may influence abdominal muscle thickness responses depending on the direction of upper-extremity resistance.In particular, the shoulder horizontal abduction resistance may be useful for increasing external oblique thickness responses during crunch exercise.