Frequency shift ΔF at maximum inspiration and expiration. Frequency shifts ΔF were compared between healthy subjects (n = 9) and COPD subjects classified by GOLD (GOLD 1: n = 6, GOLD 2: n = 4, GOLD 3: n = 7). a) During non-forced maximum breathing, one-way ANOVA and Dunnett's post-hoc test showed significant differences between healthy subjects and each GOLD stage. Significance increased from GOLD 1 to GOLD 3 (GOLD 1: p = 0.023, GOLD 2: p = 0.008, GOLD 3: p = 0.0007). b) During forced maximum breathing, there was a significant difference between healthy subjects and GOLD stage 3 at p = 0.0008.