INTRODUCTION: In older highly trained populations or master athletes, the decline in pulmonary function might play a role in limiting high-intensity endurance exercise performance. Increased airway resistance with aging can be associated during exercise with expiratory flow limitation (EFL), defined as the inability to increase expiratory flow despite increased transpulmonary pressure. EFL can lead to hyperinflation, lower inspiratory reserve volume, greater respiratory muscles work / effort, and therefore exercise intolerance. For a given lung size, the reduction of airway size increases airflow turbulence and resistance and might increase the prevalence and severity of EFL. The aim of this study is to determine how the uncoupling between lung and airway size, termed dysanapsis, determines EFL in master athletes during maximal exercise. METHODS: Ten male master athletes, (mean ± SD; age, 68.5±5.7 years; VO2MAX, 51.9±7.7 mL.min-1.kg-1) performed a maximal and graded test to exhaustion on a cycle ergometer. The maximal flow-volume (MFV) curve was determined based on flow and volume measured at the mouth from maneuvers performed prior and after exercise. To determine operational lung volumes, participants performed inspiratory capacity (IC) maneuvers during each stage of the test. The end expiratory lung volume (EELV) was calculated by subtracting the IC volume from the forced vital capacity (FVC) volume. EFL was defined as the percentage of the tidal volume that intersected the MFV curve during expiration. Subjects were flow limited when the severity of EFL was >5%. Dysanapsis was estimated using the ratio (DR) between forced expiratory flow at 50% of vital capacity (FEF50) and the product of FVC and static recoil pressure at 50% of FVC (Pst50), [i.e. DR=FEF50/(FVC* Pst50)]. RESULTS: During the exercise test (peak power output (PPO): 256±33.7 W), all participants experienced EFL at workloads (WEFL) ranging from 50% to 100% of PPO (average: 70.2 ± 20.2%). EFL severity reached 48.3 ± 16.6% at the highest workloads. DR was significantly correlated to WEFL (r=0.72, p=0.02) and participants who experienced EFL at lower workload (LOW) had a significantly lower DR than participants who experienced EFL at higher workloads (HIGH) (0.31±0.03 vs. 0.43±0.08, p=0.01, respectively). At maximum exercise, the LOW group also had a significantly higher EELV relative to FVC compared to the HIGH group (38.1±8%FVC vs. 32.4±6%FVC, p< 0.05, respectively). CONCLUSION: All master athletes experienced EFL during exercise and EFL was related to airway dysanapsis. A lower DR was associated with both an EFL occurring at lower relative exercise intensities and with a higher EELV at maximal exercise. These findings thus suggest that dysanapsis (uncoupling between lung and airway size) is an important determinant of flow limitation in male master athletes during exercise.