During the Hundred, how does precise lateral thoracic breathing directly facilitate the engagement of the transversus abdominis to stabilize the lumbar spine against limb movement?

During The Hundred, a Pilates exercise involving supine arm and leg movements, precise lateral thoracic breathing directly facilitates transversus abdominis engagement to stabilize the lumbar spine. Lateral thoracic breathing, also known as lateral costal breathing, is a controlled breathing technique where the focus is on expanding the rib cage outward and sideways during inhalation, rather than primarily forward into the abdomen or upward into the chest. As you inhale, the diaphragm, the primary muscle of respiration, descends, and the lower ribs expand laterally. During exhalation, the diaphragm ascends, and the ribs gently draw back inward and downward. This controlled movement of the rib cage encourages the precise recruitment of the transversus abdominis, which is the deepest abdominal muscle with fibers that run horizontally around the torso like a natural corset. Unlike superficial abdominal muscles, the transversus abdominis primarily functions to create circumferential tension and increase intra-abdominal pressure, which is essential for spinal stability. When performing precise lateral thoracic breathing, particularly on the exhale, the gentle inward and downward movement of the ribs naturally co-activates the transversus abdominis. This is not a forceful sucking in of the abdomen but a subtle, continuous cinching action, much like tightening a wide belt one notch. This gentle, consistent engagement of the transversus abdominis increases intra-abdominal pressure, effectively creating a rigid, pneumatic cylinder of support around the lumbar spine, which is the lower portion of the spinal column. This increased pressure acts as an internal brace, helping to resist forces generated by the limb movements in The Hundred that would otherwise cause excessive arching or flattening of the lumbar spine. The transversus abdominis also has direct connections via the thoracolumbar fascia to the lumbar vertebrae, providing segmental stability. By maintaining this low-level, continuous engagement of the transversus abdominis throughout the rhythmic breathing and limb movements of The Hundred, the lumbar spine is stabilized against potentially destabilizing forces, allowing the larger muscles responsible for limb movement to function efficiently without compromising spinal integrity.