In this context, Gottesman and McCool4 found that Tdi alone cannot distinguish between a chronically paralyzed atrophic diaphragm and a functioning diaphragm in patients with generalized muscle wasting or in small individuals. Change in thickness during inspiration or thickening fraction (TF or ΔTdi), calculated as TF = (thickness at peak inspiration or total lung capacity − thickness at end expiration or FRC)/thickness at end expiration or FRC, might be more definitive. The function, TF vs lung volume, for a range of volumes is linear.5 Moreover, as the diaphragm shortens during contraction, it thickens, and measures of ΔTdi are inversely related to changes in diaphragm length (Ldi) (ΔTdi is approximately 1/ΔLdi). Hence, it is intriguing to compare a diaphragm with chronically reduced length, as in COPD, with that of a normal population by measuring ΔTdi with respect to per unit change in Ldi. This might shed some light on the pathophysiologic differences in diaphragm between COPD and normal population.