## 3 Robotic Platform  
As specified in Section 1.3, with respect to the first feasibility study, a commercially available robot is selected because it was necessary to test the reactions of elderly individuals walking for the first time with a robot, to observe the strong points of the current robot and its limitations, to improve the future design, and to verify the effectiveness of the experimental procedures and evaluation methods. The selected robot was named Pepper, and it was the last humanoid designed by Softbank-Aldebaran [50]. A humanoid shape is an important feature that is considered for a future walking trainer. It is expected that a humanoid shape performs better in case of interpersonal communication, i.e., verbal language and non-verbal behaviours such as proxemics and body language. Thus, a humanoid can better communicate its intentions to a human partner. Pepper has 17 degrees of freedom. It can perform yaw and pitch rotations with its head. It can perform shoulder pitch and roll, elbow yaw and roll, and wrist yaw rotations with both arms, and it can close and open both hands. It can perform hip roll and pitch, and knee pitch rotations in the lower part of its body. The joints in the head and in the arms are sufficient to suggest the possibility of programming it to perform non-verbal gestures similar to the ones performed by humans (Section 1.3). With respect to locomotion, Pepper possesses a system of three wheels that is definitely more stable than that with two robotic legs. Its weight approximately corresponds to 28 kg and its height corresponds to 1.20 cm, and thus it is theoretically possible for an individual to walk together with the robot leaning on its shoulder in a comfortable manner (Figure 1). Extant studies indicated the efficacy of light touch [51] as well as of interpersonal touch [52] in increasing balance in several experiments and mostly in standing conditions. To the best of the authors' knowledge, the influence of interpersonal touch during gait was only studied by [PERSON] et al. [53] simply to observe synchronisation of gait in the case of two individuals holding hands and walking together. There is a paucity of studies to guarantee the safety of elderly individuals and to effectively enhance balance. With respect to the motivational task of our walking trainer, the design of Pepper design was potentially sufficient for the specified purpose. In addition to the humanoid shape, Pepper includes a speaker, and thus it provides positive feedback, such as encouragement and verbal praise, to increase a participant's motivation and enjoyment while walking (Section 2.1). Table 2 shows examples of the sentences spoken by [PERSON] during the experiment. Additionally, the robot has a tablet on the chest in which it shows the current score of the user, e.g., his velocity, the distance covered on a specific day, the number of steps needed to reach the goal fixed by the doctor for a specific day, and improvements gained since previous walks. With respect to tracking the human user and monitoring the gait of the user, the cameras in the eyes are definitely not sufficient, and thus it was necessary to include external cameras for the sake of the experiment. A current limitation of mobile service robots corresponds to their inability to detect humans out of their sensing range. Section 1.2 specified the advantages of using a smart environment that could provide the robot with information regarding the user's position. This should be considered for a future design as well to integrate the walking trainer with a system that successfully monitors a user's gait parameters. In order to perform the experiment and control the robot in an easier manner, a graphical user interface (GUI) was designed by using the TkInter package in Python [54]. In the control window, each button corresponded to a different behaviour: each behaviour was a combination of motivational utterance (Table 2) or and non-verbal gestures, e.g., head or arm rotation.