Sample size. Sample size calculation is based on previous investigations indicating large effect sizes between the effect of robot-assisted gait compared to walking without robot-assistance on energy consumption and cardiorespiratory load (based on a systematic review submitted for peer-review). To detect a large effect size (f = 0.40) of robot-assisted gait compared to overground and treadmill gait on energy consumption, cardiorespiratory load and perceived fatigue, in a repeated measures within subjects design (3 walking conditions and 4 measurements), with a significance level of 5% and a power level of 80%, a sample size of 21 subjects is needed (G*Power 3.1 for Mac). Sample size is inflated up to 24 subjects, so each walking order will be performed the same number of times.