OBJECTIVE: To quantify the relationship between strength and endurance of knee extensor and flexor and exercise capacity in patients with chronic obstructive pulmonary disease (COPD).METHODS: A total of 108 patients with stable COPD (age: 65 years, IQR25-75: 59-72) were enrolled in this cross-sectional study. Knee extensor and flexor function was evaluated using the isokinetic test, and the parameters attained were considered as independent variables. Exercise capacity was evaluated using 6-minute walking test (6MWT) and 30-second sit-to-stand test (30s SST), and the results were considered as dependent variables. The association between lower-limb muscle function and exercise capacity was assessed using multiple regression analysis.RESULTS: The patients mostly had moderate-to-severe airflow obstruction with a post-bronchodilator forced expiratory volume in 1 second (FEV1) of 57.87 ± 17.71% predicted. In multiple regression analysis, flexor total work (TW), extensor endurance ratio (ER), age and FEV1%pred were significantly associated with 6MWT (adjusted R² = 0.455, P < 0.001). Flexor TW and age were significantly associated with 30s SST (adjusted R² = 0.355, P < 0.001).CONCLUSION: Knee endurance has a significant relationship with exercise capacity, and knee flexor endurance seems to be an important factor contributing to exercise capacity in COPD patients.