  This paper exposes a control architecture enabling rehabilitation of walking
impaired patients with the lower-limb exoskeleton Atalante. Atalante's control
system is modified to allow the patient to contribute to the walking motion
through their efforts. Only the swing leg degree of freedom along the nominal
path is relaxed. An online trajectory optimization checks that the muscle
forces do not jeopardize stability. The optimization generates reference
trajectories that satisfy several key constraints from the current point to the
end of the step. One of the constraints requires that the center or pressure
remains inside the support polygon, which ensures that the support leg
subsystem successfully tracks the reference trajectory. As a result of the
presented works, the robot provides a non-zero force in the direction of motion
only when required, helping the patient go fast enough to maintain balance (or
preventing him from going too fast). Experimental results are reported. They
illustrate that variations of $\pm$50% of the duration of the step can be
achieved in response to the patient's efforts and that many steps are achieved
without falling. A video of the experiments can be viewed at
https://youtu.be/_1A-2nLy5ZE
