## 2 Materials and Methods
### Data Recording  
Muscle activity and kinematic data were recorded both when walking with and without the Ekso. We collected the kinematic data using a motion capture system (SMART DX, BTS Bioengineering, Milan, Italy) that consisted of eight infrared cameras, two video cameras, and reflective spherical passive markers of 15-mm diameter. Subjects walked inside the acquisition volume of the cameras [8 (walking path) \(\times\) 3 \(\times\) 2.5 (height) m] and stepped on two force platforms located halfway (Kistler, Kistler Italia, Milan, Italy or P600, BTS Bioengineering, Milan, Italy). In this study, we focused on the EMG patterns that were recorded with surface electromyography (sEMG, POCKETEMG, BTS Bioengineering, Milan, Italy). We used the kinematic data mainly for extracting the phases of the gait cycle. We used the force platforms only to verify the algorithms for estimating the gait cycle (see data analysis), but we did not consider the kinetic data; thus, we analyzed also the strides made outside the force platforms. All the data acquisition systems were hardware synchronized.  
As for the sEMG, the data were collected simultaneously from the following eight muscles of both legs (16 muscles in total, Figure 1b,c): tibialis anterior (TA), gastrocnemius medialis (GM), soleus (SOL), rectus femoris (RF), vastus medialis (VM), semitendinosus (ST), biceps femoris (BF), gluteus medius (GLM). The EMG electrodes were positioned according to the guidelines of SENIAM (Surface EMG for Non-Invasive Assessment of Muscles, seniam.org), which is a European project on surface EMG.  
As for the kinematic data, in this study, we used only markers at the foot/ankle level for computing the gait cycle events and the marker in C7 for estimating the walking speed (see data analysis). However, for completeness, we are reporting the complete set-up that we designed for future studies with people with complete and incomplete SCI. Specifically, during walking without the exoskeleton, we recorded the position of 28 markers. Of these, 21 were positioned according to the Davis protocol [26] on the spinal process of the sacrum, the spinal process of C7, bilaterally on the acromion, the anterior superior iliac spine (ASIS), the greater trochanter, the lateral epicondyle of the femur, the fibula head, the lateral malleolus, the fifth metatarsal phalangeal joint on the lateral aspect of the foot, and the heel. In both legs, four markers were placed externally, by means of bars 5-cm long in the middle between the greater trochanter and the lateral epicondyle of the femur, and in the middle between the fibula head and the lateral malleolus. The additional seven markers were placed on the sternum, on the head, bilaterally on the lateral epicondyle of the elbow joint, and on the ulnar styloid process. When subjects walked with the Ekso, the markers were positioned differently due to the exoskeleton structure. The markers were placed on the exoskeleton at torso level, in correspondence of the principal joints--hip (in proximity of the spinal process of the greater trochanter), knee (in proximity of the spinal process of the lateral epicondyle of the femur), ankle (in proximity of the spinal process of the lateral malleolus)--and on the lateral aspect of the foot in proximity of the spinal process of the fifth metatarsal phalangeal joint, as well as on the subjects at the level of the spinal process of C7, the sternum, the head, and bilaterally on acromion, elbow, and wrist (Figure 1b-d).