# Quasi-Passive Resistive Exosuit for Space Activities: Proof of Concept
## 4 Discussion & Limitations  
The resistive exosuit presented and assessed in this work (results are summarized in Table 1) is able to increase the muscular activity of both extensor and flexor muscles of  
Figure 11: (**A**) Biceps femoris MF interpolation with and without XoSoft. (**B**) Vastus lateralis MF interpolation with and without XoSoft. (**C**) Biceps femoris MF mean (bars’ height), standard deviation (vertical whiskers) with and without the exoskeleton. (**D**) Vastus lateralis MF mean (bars’ height), standard deviation (vertical whiskers) with and without the exoskeleton. Both in panels (**C**,**D**) the horizontal line with the asterisk denotes a statistically significant difference. the knees. The main goal of this work has been the quantification of the resistance generated by the modular actuation (i.e., QPA) on a single joint in bilateral configuration (i.e., right and left knee) during a simple and repetitive task (i.e., walking task). The study aimed, thus, to show the increment of fatigue required to accomplish a standard cyclical task to about its double value of muscular fatigue when accomplish terrestrial tasks. This could be interpreted as the ability for the exosuit to recreate the same fatigue required by the type of activities performed on land. The analysis and results on muscular activation and power spectrum sustain the hypothesis. In particular, the mean muscular activation overall increases of about an 97.8% of the MVC, while considering the mean of the two muscles (BF 167.1%, and VL 28.4%). Transferring this consideration to absolute values, the resistive exosuit can almost replicate the typical muscular effort generated during walking task (the typical effort would be totally replicated with an overall muscular activation increment of 100%). The power spectrum analysis shows that, even if the walking task is not a demanding task, therefore, muscle fatigue does not arise after 3 min, if the resistive exosuit is worn, the user's muscles perceive muscular fatigue. Although the short duration of the test, the clear results show a reduced value of MFs for both muscles underlining that even for an easy task (i.e., walking task), thanks to the exosuit, it turns into a more demanding task. Indeed, an overall average of MF reduction for both muscles of about 15.4% (BF 18.9% and VL 11.9%) is higher than what was demonstrated and assessed in [101], where the average MF reduction obtained after 10 min running activity was 7.5% (5% for the soleus muscle and 10% for the gastrocnemius medialis). [PERSON] et al. [102] evaluated the maximum range of MF reduction for set of muscles if subjected to high demanding task. They reported an experimental assessment where knee-extension MVC was performed against 120 kg weight, where the maximal change of median spectral frequency observed was 32% on rectus femoris muscles. Thus, confirming that the exosuit, on earth, is able to intensify fatigue by generating a muscular fatigue comparable to long-term running or physical exercise in general. This result may imply that in a zero-gravity environment this exosuit could induce a level of fatigue included between the fatigue of walking and running. Considering that, the final target is to use the exosuit at micro-gravity as muscle degradation countermeasure by modifying the training paradigm. These preliminary results start building evidence that continuous training could substitute concentrate training as micro-gravity countermeasure with potential equal benefits. The use of QPA, constituted by a passive element (i.e., EBs), which is the unique fundamental element for the generation of resistive forces, and a clutch to modulate these forces by engaging and disengaging such generation, allows the control and maximization of these resistive forces. In particular, after the EB elongation due to the user's joint motion, the specific deactivation timing of the clutch prevents from the release of the mechanical energy stored into the elongated EB (such behavior is shown in Figure 8B,C).