Objective: To investigate whether abdominal muscle thickness in athletes with long-standing adduction-related groin pain (LAGP) differs between Subgroups with a positive or no response to a pelvic belt. The response to a pelvic belt is defined positive in case of a decrease >= 1 on a Likert pain scale (0-10) during the squeeze test (SQT) or a decrease >= 1 on the active straight leg raise (ASLR) test score (0-10). Design: Cross-sectional study. Setting: Physical therapy practice. Patients: Fifty athletes with LAGP. Independent Variables: Squeeze test and ASLR test. Main Outcome Measures: First, the effect of a pelvic belt on pain during the SQT and the ASLR test score was evaluated. Then, thickness of in. transversus abdominis (TA) and in. obliquus internus (01) was measured using ultrasound during rest, ASLR left and right, and SQT Results: Of the 50 participants, 25 (50%) experienced a decrease in pain during the SQT when wearing a pelvic belt and 10 (20%) improved in ASLR performance with a pelvic belt. Thickness of TA and 01 at rest (both cases P > .08) and relative thickness compared with rest during tasks (in all cases P > .12) revealed no significant difference when comparing the 2 subgroups based oil the belt response during the SQT or ASLR. Conclusions: Using these methods, abdominal muscle thickness behavior in athletes with LAGP did not differ between the subgroups based on a positive or no response to a pelvic belt. However, the ultrasound method used may not have been sensitive enough to reveal differences between groups.