The response of the transverse abdominis and internal oblique muscles to different postures

The purpose of this study was to consider how the deep abdominal muscles responded to alterations in seated stability. The thickness of the right transverse abdominis (TrA) and internal oblique (IO) muscles were measured with ultrasound imaging in 30 healthy human subjects (mean age 27.7, years 22 females) in supine lying, relaxed sitting on a chair with both feet on the ground, relaxed sitting on a gym ball with both feet on the ground and sitting on a gym ball lifting the left foot off the floor. Measurements were taken at the end of both inspiration and expiration. The results showed that muscle thickness expressed as a percentage of the actual muscle thickness in supine lying did not differ between relaxed sitting on a chair and sitting on a gym ball for either muscle (P=0.012-0.054) where Bonferroni corrected P-value for significance=0.002. Raising the foot off the floor produced a significant increase in thickness for TrA and IO, when compared with the other seated postures (P<0.001). It was also found that both muscles were thicker at the end of expiration (P<0.001) which has also been established by other authors. These findings suggest that both deep abdominal muscles respond in the same way to postural changes. It also demonstrates that these muscles are automatically targeted by significantly decreasing the base of support, but in normal subjects sitting on a gym ball is not sufficient to increase their activity.