## 4 Discussion  
The aim of this study was to investigate the effect on metabolic cost of a specific torque profile assisting hip flexion during walking. Results indicated that walking with Exoband significantly reduced the metabolic cost with respect to free walking. Further, the specific hypothesis that the investigated assistive profile would have provided more consistent metabolic reduction across participants than previously achieved [15] was verified. In the present work, each participant reported a reduced metabolic cost while walking with Exoband, while in a previous work involving a passive system [15] not all the participants were able to benefit from the use of the device thus reporting a reduced metabolic cost. The hypothesis was also confirmed by a slightly higher reduction in metabolic cost (4.2% vs. 3.3%) and a lower SD (1.9% vs. 3.0%). The variability of metabolic cost responses displayed by users walking with exoskeletons has been reported by several other studies, thus motivating the need to develop optimization strategies to improve users' metabolic outcomes [17]. The interesting finding of this work relies on the fact that a unique and constant assistive profile was able to provide a consistent reduction across all participants. This can have practical advantages in tuning the assistance of these devices thus favoring the adoption of simpler passive devices, which are lacking of any control architecture. Furthermore, these results are of particular interest considering that a lower peak torque was applied in this work than before (0.023 Nm/kg vs. 0.042 Nm/kg), thus highlighting the key role of a later onset timing (~20% vs. ~6%) in the reduction of metabolic cost provided using Exoband. Few studies have been conducted on the metabolic cost of walking involving passive assistive devices, the outcomes and designs adopted are summarized in Table 2. Although the results in term of metabolic cost are in line with previous work on passive exoskeletons [11; 15; 28; 29], it has to be highlighted that all the previous studies applied a higher assistive torque than the present study. Moreover, some of the work investigated the effect of springs of different stiffness assisting the hip joint [28; 29] but considering the range evaluated, peak torques applied were higher than the present study. This seems to fur  
Figure 4: Net metabolic power for each participant during the two conditions of testing. FreeWalk condition (black) and EXO condition (red). ther suggest the importance of exploring different onset timing other than the magnitude of assistance. Spatiotemporal parameters were found to be unaltered between the two conditions of testing (Table 1), indicating that walking with Exoband has no effect on these variables. This confirms the finding that the passive nature of the system did not impose a different kinematics to the elderly, and this could be a sign of a preserved walking comfort in the use of this device [15], regardless of the different assistance applied in the present study. In addition, previous work has shown that when step length is different than the preferred one, metabolic cost of walking increases [30]. Unfortunately, other works involving passive hip systems [28,29] did not report spatiotemporal parameters, thus preventing a direct comparison between devices on these parameters. This work reports interesting outcomes with respect to metabolic cost reduction associated with the use of a passive assistive device. Nevertheless, at this stage, the lack of muscle-level data prevents us to drive conclusions on the determinants of these findings. Spatiotemporal parameters were found to be unaltered between the two conditions, but this does not exclude the presence of joint-level differences reflecting altered muscle dynamics. The earlier onset of hip flexors assistance could have been responsible for the reduced metabolic cost as suggested from simulation studies on load carriers [22].