Following an initial incremental shuttle walk test (ISWT) in order to determine the individual maximum capacity, patients will perform three endurance shuttle walk tests (ESWT) at 85% of the maximal pace. In randomized order, patients will complete one ESWT on CF, one on DDL (liquid oxygen) and one on DDC (portable oxygen concentrator). The time between the three ESWTs will be 24 hours in order to give enough time for regeneration. All patients will use the same oxygen devices (CF: Companion 1000 (CE 0050), Chart Industries, Inc. Garfield Heights, Ohio, USA; DD [liquid oxygen]: Caire Spirit 300 (CE 0029), Chart Industries, Inc. Garfield Heights, Ohio, USA), DD [portable concentrator]: Inogen G2, California, USA) and will carry the oxygen device in a backpack. Continuous oxygen flow in liter per minute will be compared to the level of demand delivery (e.g.: 2 l/min CF will be compared to DD level 2).