We formulate bust-up and waist-down — changes in body cavity shape — not as outcomes of strength training or fat reduction, but as consequences of cross-sectional geometric change and passive volume redistribution between the thoracic and abdominal cavities. Flexor muscles, including the psoas major, constitute the outer shell of the body cavity and function as internal bracing — support muscles in the structural sense. When flexor expansion enlarges the thoracic cavity, the conservation of total body cavity volume requires that abdominal contents be passively redistributed upward. This reduces the anterior-posterior flatness ratio k of the abdominal cross-section toward unity. By the isoperimetric inequality, a circular cross-section has the minimum perimeter for a given area; therefore waist circumference decreases. Simultaneously, the thoracic cross-section circularizes under approximately constant perimeter, increasing thoracic cross-sectional area. Bust-up and waist-down are not independent phenomena; they are the superior and inferior expressions of a single coupled volume redistribution event. We present the mathematical model and supporting clinical observations.
Seiji Sato (Mon,) studied this question.