## 5 Discussion and Conclusions  
Confidence in successful control of stance stabilization is crucial for physical training with an exoskeleton. In fact, fear of falling (FOF) is often a major problem for iSCI patients, especially of those in whom fall was the reason for the spinal cord injury. In addition, FOF adversely affects the efficiency of therapy, rehabilitation and use of assistive devices [37]. Given this background, it may be advantageous if patients learn and experience that they are using a human-inspired sensorimotor control system that is performing as they expect it, and this with compliant impedance-controlled actuators that are considered safer and more comfortable for physical human-robot interaction [38].  
FOF may arise if the patients experience the movements and postural actions produced by the exoskeleton as inadequate. It may be dangerous if they then try to enforce counteractions. We conceive that this may end in a dangerous \"user-device conflict\". For example, fear of falling backward may lead the patient to command an inadequate forward COM lean of the user-exoskeleton body, which the exoskeleton in turn then may try to correct by a lean backward, which in turn, likely enhances the fear and action of the patient even more--a potentially disastrous positive feedback situation.  
Currently, little is known about such user-device problems. We conceive that such problems can best be overcome by training and cautiously building up confidence, for example in that the patient is initially using stationary handrails when maintaining balance. The training may then include self-produced changes in the balancing situation, for example when raising the arms or lifting an object with a hand (compare above, Section 3.3, Balancing Experiment).  
The control model described in the previous sections relies on the presence of a vestibular system (e.g., derived from using IMUs; [17]). This is not an obvious design choice as most of the effort in exoskeleton designs is devoted to the tracking of user's movements (e.g., by means of encoders [39]) or muscle activity [40] or of the forces that the user applies to the robot [41] without focusing on the position of the exoskeleton itself in space. The compensation of external disturbances can coexist with the previously mentioned approaches that can be applied to the control of active movements, for example the servo loop can be controlled with a signal based on force sensors and at the same time the forces produced by the user can be explicitly considered in (6) so that they do not interfere with the compensation of the external forces. Overall, the development of robotic exoskeletons appears to be still long-lasting in view of the many difficulties to make these devices suitable for patients' reengagement in everyday activities [42]. The integration of posture control seems one of the possible ways to improve the usability of the exoskeletons and also to inspire design decisions, e.g., in choosing the sensors.  
The following are available online at [[http://www.mdpi.com/2076-3417/10/15/5240/s1](http://www.mdpi.com/2076-3417/10/15/5240/s1)]([http://www.mdpi.com/2076-3417/10/15/5240/s1](http://www.mdpi.com/2076-3417/10/15/5240/s1)), Video S1: Lucy_tilt_Balancing.MP4, Video S2: Lucy_Trans_Balancing.m4v.  
[PERSON] and [PERSON] were responsible for study design, data collection, analysis and manuscript writing. All authors have read and agreed to the published version of the manuscript.  
We acknowledge support by the German Research Foundation and the Open Access Publication Fund of TU Berlin.  
The authors declare no conflict of interests.