PURPOSE: We sought to quantify the impact of respiratory muscle and lower extremity strength on exercise capacity and lower extremity function (LEF) in patients with chronic obstructive pulmonary disease (COPD). METHODS: In 828 persons with COPD, we assessed the impact of reduced respiratory (maximum inspiratory pressure, MIP) and lower extremity muscle strength (quadriceps strength, QS) on exercise capacity (6-minute walk test, 6MWT), and LEF (short physical performance battery). Multiple regression analyses taking into account key covariates, including lung function and smoking, tested the associations between muscle strength and exercise and functional capacity. RESULTS: For each 0.5 SD decrement in QS, men walked 18.3 m less during 6MWT (95% confidence interval [CI], -24.1 to -12.4); women 25.1 m less (95% CI, -31.1 to -12.4). For each 0.5 SD decrement in MIP, men walked 9.4 m less during 6MWT (95% CI, -15.2 to -3.6); women 8.7 m less (95% CI, -14.1 to -3.4). For each 0.5 SD decrease in QS, men had a 1.32 higher odds (95% CI, 1.11-1.15) of poor LEF; women had a 1.87 higher odds (95% CI, 1.54-2.27). Lower MIP (per 0.5 SD) was associated with increased odds of poor LEF in women (odds ratio = 1.18; 95% CI, 1.00-1.39), but not in men (odds ratio = 1.10; 95% CI, 0.93-1.31). CONCLUSIONS: In COPD, reduced respiratory and lower extremity muscle strength are associated with decreased exercise and functional capacity. Muscle weakness is likely an important component of impairment and disability in patients with COPD.