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Active expiration during exercise can increase intrathoracic pressure yielding concavity in the expiratory flow-volume loop in COPD. We investigated the relationship between this concavity and dynamic hyperinflation (DH). 17 COPD patients (FEV₁: 38 ± 10%pred, GOLD stage 3–4) and 12 healthy subjects performed cycle ergometer incremental exercise. Expiratory limb of the spontaneous flow-volume loop was analyzed breath-by-breath using a geometric approach (rectangular area ratio (RAR), Respir. Med., 104(3):389–96, 2010). RAR below 0.5 demonstrates expiratory limb concavity. DH was determined with serial inspiratory capacity maneuvers. 5 of 17 patients displayed little end-exercise concavity (RAR = 0.52 ± 0.04, group LCONC). 12 patients had concavity at rest and end-exercise RAR reached 0.40 ± 0.03 (group HCONC). Healthy subjects showed no concavity. End-exercise RAR correlated with resting …