Intervention. Patients will be tested in 3 single walking sessions each on a separate day: walking in the Lokomat with 60% guidance force, walking on a treadmill and walking overground. Within subjects, all walking conditions will be performed at the same comfortable walking speed (CWS), with the same amount of body-weight support (BWS) (if necessary) during a total duration of maximum 30 minutes. The CWS (with a maximum of 3.2 kmph corresponding to the maximum Lokomat speed) and the amount of BWS (if necessary) will be individually determined on a separate day before the start of the study. Walking tests will be terminated early when relative or absolute indications are presented as reported by the American Heart Association or when patients are unable to continue walking. Patients will be asked to not consume food, alcohol, caffeine or nicotine at least 3 hours prior to the intervention, and not to perform additional strenuous activities at least 12 hours prior to the interventions. Walking sessions will be controlled for time of day. Before the start of the study, demographic and clinical characteristics will be collected and the CWS and the amount of BWS (if necessary) will be determined in a 10 minute walking test. At the start of each walking condition, a chest-carrying gas analysis system with mouth mask (Metamax 3B, Cortex, Germany), a heart rate belt (Polar H7) and 2 wearable foot sensors (Physiolog, Gait Up, Switzerland) will be applied. Patients will be seated for 5 minutes during which resting values (energy consumption, cardiorespiratory parameters and perceived fatigue) will be registered. After a resting period of 5 minutes, patients will walk for 30 minutes during which energy consumption, cardiorespiratory parameters, perceived fatigue and spatiotemporal parameters will be monitored continuously. Perceived fatigue will be registered every minute. Average values at rest, the beginning, middle and end of the walking sessions will be calculated offline.