Abdominal Mechanics

Abdominal function is better understood through leverage and length-tension relationships than through facilitation or inhibition labels. In a narrow infrasternal angle the external obliques have more concentric leverage because the ribcage has greater anterior-to-posterior dimensions. The transversus abdominis contributes to internal rotation of the ribs; when ribs are externally rotated it is eccentrically oriented and lacks leverage for concentric action rather than being inactive. The rectus abdominis gains leverage when the sternum is depressed or when the pelvis is posteriorly tilted. In a narrow infrasternal angle the internal obliques are placed on stretch and are more eccentrically oriented with less leverage for concentric action. When the lateral ribcage dimension expands and anterior-to-posterior depth decreases, internal obliques and transversus abdominis gain concentric leverage while external obliques become more eccentrically oriented.