The purpose of this study is to evaluate the lung function response to UMEC/VI, UMEC, and VI in individual patients using a cross-over design. This is a multicenter, randomized, double-blind, 3-way crossover study. Eligible subjects will be randomized to a sequence of UMEC 62.5mcg, VI 25mcg, and UMEC/VI 62.5/25mcg. All subjects will receive each treatment once-daily for 14 days, and each treatment will be separated by a 10-14 day washout period. There will be a 5-7 day run-in period prior to randomization.