## 2 Materials and Methods
### Apparatus: Brief Description of the Ekso Exoskeleton  
The Ekso (Ekso Bionics, Richmond, CA, USA [24], Figure 1a) is a battery-powered, wearable exoskeleton that enables individuals with severe walking impairments to stand up and walk overground.  
\begin{table}
\begin{tabular}{c c c c c} \hline  & **Age [Y]** & **Gender [M/F]** & **Height [cm]** & **Weight [Kg]** \\ \hline S1 & 45 & M & 179 & 68 \\ S2 & 24 & F & 175 & 62 \\ S3 & 25 & F & 155 & 54 \\ S4 & 22 & F & 183 & 70 \\ S5 & 22 & M & 178 & 75 \\ S6 & 23 & M & 182 & 86 \\ S7 & 24 & F & 158 & 60 \\ S8 & 32 & M & 184 & 74 \\ \hline \end{tabular}
\end{table}
Table 1: Healthy subject’s data; Y: years, M: male, F: female.  
Figure 1: **(a)** Exoskeleton EIso; **(b,c)** Participants wearing the Ekso. Red dots indicate the positions of the reflective markers for the motion analysis and the blue rectangles indicate the position of the surface electromyographic (EMG) electrodes. **(d)** Markers recorded by the motion analysis system (lateral view in the center of the motion analysis lab pathway).  
In this study, we used the model 'Ekso 1', which was equipped with four battery-powered motors at the hips and knees, and 15 position sensors. The device includes two powered joints at the hip and knee, and a semi-rigid unpowered ankle joint.  
The Ekso is used always with external balancing aids: mainly walkers or crutches. Both aids are characterized by a user interface that enables the subject to interact with the device and select the desired assistance. The device provides assistance to the users by driving their lower limb joints through a predefined trajectory. The exoskeleton has different assistance settings: fixed or adaptive assistance, no assistance, as well as bilateral or unilateral assistance.  
_Bilateral assistance_ provides full stance stability and swing assistance (fixed or adaptive) to both legs. _Unilateral assistance_ provides them only for a selected limb, while the contralateral leg is automatically set free i.e., the users voluntary control the swing of that leg with their own strength and coordination, without any help by the device.  
With the _Fixed assistance_ setting, a fixed amount of assistance is provided throughout the swing phase of gait; it can vary from 0%, where the device does not provide any assistance, and the subjects should actively move, to a maximum value of 100% corresponding to the maximal assistance that allows the subjects to walk without any voluntary contribution. Thus, depending on the set fixed value, the user is required to use different levels of voluntary control to complete the swing phase. When the Ekso is programmed with a fixed assistance value, it provides a consistent amount of motor output, regardless of the patient's ability.  
The _Adaptive assistance_ setting acts based on the subjects' performance. In this control strategy, the assistive forces increase as the participant deviates from the desired trajectory [25].  
With the _No Assistance_ setting, the Ekso does not provide any assistance to the user, but its inertia is compensated i.e., the users do not feel the weight of the Ekso and voluntary control their walking patterns.  
The Ekso is also characterized by three different ways to control the initiation of each step according to the subject skills:  
* First Step: a physical therapist controls the beginning of each step by pushing a button. Usually, this modality is used in the first training session for the familiarization with the device;
* Active Step: the user controls the beginning of each step by means of an interface, placed on the crutches or on the walker, that communicate with the device.
* Pro Step: The user controls the beginning of the next step by moving his hips forward and laterally. In this case, when the device recognizes that the user is in the correct position allows the step of the contralateral leg.