Aim of this study is to investigate the effects of treadmill versus cycling endurance training on Balance, gait performance and exercise capacity in patients with severe chronic obstructive pulmonary disease. Patients will be recruited during a 3-week inpatient pulmonary rehabilitation program and will be randomized into one of two intervention groups. Walking intensity in the treadmill group will be set at 80 percent of the average speed of the 6-minute walking test. The cycling group will exercise at an intensity of 60 percent according to an Initial incremental cycling test.

Patients will perform 5 to 6 training sessions per week. The total exercise duration will be progressively increased from 10 to 30 minutes. Walking or cycling intensity will also be progressively increased if perceived exertion during exercise is rated below 3 on the modified 10-point Borg scale.

Inclusion Criteria:

• COPD Stage III or IV according to the global initiative for chronic obstructive lung disease
• impaired exercise capacity (6 Minute walk distance of less than 70% of the reference values from Troosters et al.)

Exclusion Criteria:

• acute, severe exacerbation of COPD
• carbon dioxide of more than 45 mmHg at rest
• any disabilities that prevent patient form walking or cycling
• Patient is not able to walk 20 meters without any aid (for example rollator)