To determine the strenuousness and efficacy of therapy, the energy expenditure of 10 healthy and 10 hemiparetic subjects were measured while they walked on a treadmill that was combined with a robot-driven gait orthosis, the Lokomat, which physiologically exercises the legs of a patient on a moving treadmill. Subjects performed different Lokomat conditions after measurement of the baseline, i.e., standing in the Lokomat with 30% body weight support (BWS). Robotic strategies with a position control scheme used fixed gait patterns to produce the following conditions: walking with 100% BWS at a speed of 1 km/h versus 2 km/h and walking with 30% BWS at a speed of 1 km/h versus 2 km/h. Another robotic control option with a force control scheme allowed the force to be reduced on only one leg of the orthosis. In this option a reduction to 60% and to 0% assistance was tested. Oxygen consumption and heart rate were measured by a breath-by-breath respiratory gas analyzing system using standard open circuit methodology. The results for O(2) rate [ml/kg/min] indicate that: (1) walking in the Lokomat is not passive; (2) oxygen uptake is significantly increased due to an effect of loading during active stance phase; (3) speed is not a factor leading to increased oxygen consumption; (4) patients do not significantly increase their oxygen uptake due to the advanced force control scheme.