Figure 3.

Demonstrates the mechanism behind Hoover's sign. The numbers on the figure refer to the following: 1 = accessory muscles, 2 = hyper-expansion of the lungs, 3 = alteration of rib orientation to horizontal 4 = flattened diaphragm and 5 = decreased zone of apposition (adapted from Mason: Murray and Nadel's Textbook of Respiratory Medicine, 4^th Edition).