## 1 Introduction  
Gait recovery is one of the primary aims of stroke rehabilitation, since almost one third of stroke survivors are unable to achieve an independent community ambulation at hospital discharge [1]. Successful approaches for gait recovery are based on intensive and repetitive task-oriented practice involving the subject's active participation [2]. In this scenario, Robot-Assisted Gait Training (RAGT) has been widely employed as a rehabilitation treatment for stroke survivors, as it lets subjects perform the gait task even if they are not able to walk [3]. Such treatments have positive practical consequences on motor and functional recovery, since robotic devices allow early lower limb mobilization [4]. Furthermore, the intensity (i.e., number of steps) and the quality of the movement executed with a robotic device are significantly higher than those performed during Conventional Gait Training (CGT) [5,6]. The efficacy of RAGT in subjects with neurological diseases is well-established in the literature, showing significant improvements in clinical outcomes in comparison with the CGT [7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18]. The robotic devices for gait training can be categorised into treadmill-based or overground, as recently defined by [PERSON] et al. [19]. The treadmill-based Robot-Assisted Gait Training (t-RAGT) is performed with stationary robotic devices that help the subject execute the cyclic ambulatory motor task with a Body Weight Support (BWS). These devices are classified into end-effector and exoskeleton systems, when the considering the physical interface between the subject and the robot [20]. In end-effector t-RAGT (such as the G-EO System [7] and the Lokohelp [8]), the physical human-robot interaction occurs at the distal segments, i.e., the subject's feet are fixed on footplates which simulate stepping. In exoskeleton t-RAGT (such as the Lokomat [7] and the ALEX [10]), the physical human-robot interaction occurs at each of the main segments of the lower limb (calves, thighs), i.e., the device's actuators move each body segment through pre-programmed physiological gait patterns. The overground Robot Assisted Gait Training (o-RAGT) is provided by wearable powered exoskeletons (such as the Ekso [11] and the Indego [21]) that allow the subject to walk overground on hard and flat surfaces by moving each lower limb with a pre-programmed physiological pattern. The main differences between o-RAGT and t-RAGT rely on the subject's participation and multisensory inputs. Indeed, the o-RAGT allows the subjects to walk overground and experience near-normal proprioceptive input during limb loading, and to explore the environment [12]. In the literature, most of the clinical studies on robotic gait rehabilitation have been carried out with t-RAGT [5; 7; 8; 13; 14; 15]; such treatments, when combined with traditional therapy, led improvements in functional ambulation outcomes. Moreover, subjects who received t-RAGT were more likely to achieve independent walking than their peers who received CGT only [15]. Systematic reviews did not find any difference in gait speed and endurance when t-RAGT was administered with the same intensity and duration as CGT [3; 6]. On the other hand, the literature on o-RAGT is limited, because wearable powered exoskeletons are a relatively new technology and have not been frequently employed in neurorehabilitation centres [11; 16; 17; 18]. A review on o-RAGT declared that subacute stroke subjects might experience further benefits from such training compared to the chronic ones [16]. A pilot observational study confirmed this hypothesis, finding that subacute stroke subjects registered a significant change in the Trunk Control Test and Walking Handicap Scale at the end of o-RAGT [17]. Moreover, the recent observational study by [PERSON] et al. [11] reported significant improvements in all the International Classification of Functioning, disability and health (ICF) domains [22].