Hoover sign Explanation in COPD case

At first you should know that there are 2 signs named for Charles Franklin Hoover :
1-Hoover's sign of leg paresis.
2-Hoover’s sign in pulmonology which we will take about ;
It refers to inward movement of the lower rib cage during inspiration, implying a flat, but functioning, diaphragm, often associated with COPD.
COPD, and more specifically emphysema, often lead to hyperexpansion of the lungs due to air trapping. The resulting flattened diaphragm contracts inwards instead of downwards, thereby paradoxically pulling the inferior ribs inwards with its movement.

This case for A 70-year-old man with a 70 pack-year history of smoking was referred to the emergency department for an exacerbation of chronic obstructive pulmonary disease (COPD).

On examination, he breathed through pursed lips and had paradoxical indrawing of the lower rib cage margin with inspiration (Hoover sign)(Figure 1A),A chest radiograph showed distension of the lungs and flattening of the diaphragm(Figure 1B).

See this video: