Background: The training of abdominal muscles has a positive impact on the functional capacity of healthy adults, being applied practically in fields of athletics and fitness through rehabilitation for lower back pain. Objective: The study compares abdominal muscle activity while performing graded isometric exercises on stable and unstable surfaces. The authors also examined perceived stability and comfort for the different surfaces. Methods: A total of 30 young, healthy adults performed 3 graded isometric exercises on a Pilates table, foam roller, and Oov (a newly developed tool). Ultrasound investigation measured transversus abdominis, internal oblique abdominis, and external oblique abdominis thickness during each task, comparing muscle thickness between conditions using general linear modeling. Results: Core abdominal activation was greater on the foam roller than the Oov and Pilates table during crook lying (bilateral leg support). Both Oov and foam roller elicited greater contralateral transversus abdominis and internal oblique abdominis thickness than the Pilates table during tabletop and straight leg raises (unilateral leg exercises). For transversus abdominis only, the foam roller elicited more muscle thickness than the Oov during straight leg raises. The Oov was rated more comfortable than the foam roller. Discussion: Exercises performed on the Oov and foam roller elicit core greater abdominal muscle thickness than those performed on a Pilates table. Unilateral leg exercises in a supine position elicit more contralateral muscle thickness than those with bilateral leg support. Conclusions: These results provide information to support choices in exercise progression from flat stable to more unstable surfaces and from those with bilateral foot support to unilateral foot support. The Oov was more comfortable that the foam roller, and this may help with exercise adherence.