Muscle activity and thickness during abdominal hollowing and abdominal bracing: a scoping review

This scoping review synthesized evidence comparing Abdominal Hollowing (AH) and Abdominal Bracing (AB) with respect to trunk muscle activity and muscle thickness. The review followed the PRISMA extension for Scoping Reviews. PubMed, Scopus, and PEDro were searched, supplemented by backward and forward citation tracking. Experimental, quasi-experimental, and observational studies involving healthy adults, athletes, or individuals with low back pain were eligible when trunk muscle activity or thickness was assessed using electromyography, ultrasound imaging, or magnetic resonance imaging. Thirty studies were included. Overall, AH was more consistently associated with selective recruitment and increased thickness of the transversus abdominis and, in some studies, the internal oblique. In contrast, AB generally produced greater activation of the rectus abdominis, external oblique, quadratus lumborum, and erector spinae, reflecting broader trunk co-contraction and increased trunk stiffness. However, the relative effects of both manoeuvres varied according to body position, task demands, participant characteristics, verbal instructions, and measurement method. Most studies involved young, healthy adults, while evidence in individuals with low back pain remained limited. AH and AB should therefore be viewed as complementary rather than competing strategies. AH may be more suitable for low-load motor-control training, whereas AB may be preferable during higher-load, dynamic, or stability-demanding tasks. Further research using standardized protocols and clinical populations is warranted.