# Quasi-Passive Resistive Exosuit for Space Activities: Proof of Concept
## 3 Experimental Evaluation
### Overall Assessment
#### 3.2.4 EMG Assessment Results  
The \(t\)-test revealed a not significant difference in the Co-activation Index (CI) with and without wearing the exosuit \((t=-0.04,p=0.295)\), as shown in Figure 9, (both values show a CI of 6.3%). Figure 9 shows the invariance of CI, which is a global index of  
Figure 8: (**A**) Knee angle range wearing XoSoft and not wearing XoSoft. (**B**) Knee flexion resistive torque and EB elongation generated by the XoSoft, and the gait portion when the \(QPA_{kneeTlexion}\) is engaged. (**C**) Knee extension resistive torque and EB elongation generated by the XoSoft, and the gait portion when the \(QPA_{kneeExtension}\) is engaged.  
muscle activation, providing a confirmation that walking with this resistive exosuit does not provide significant modification of the walking pattern.  
The RMS of both VL and BF, averaged over five tests with and without the exosuit showed a significant increase at the paired \(t\)-test \((VL\): \(t=-22.923,p<0.001\); \(BF\): \(t=-15.052,p<0.001\)), while the MF showed a significant decrease at the paired \(t\)-test \((VL:t=25.888,p<0.001;BF:t=12.723,p<0.001)\). Figure 10 shows the mean, over five tests of the RMS, SD and significant differences. Figure 11 illustrates the mean of the MF signal (averaged over five repetitions), SD and significant differences.  
Figure 10: **(A)** Biceps femoris activation with and without XoSoft. **(B)** Vastus lateralis activation with and without XoSoft. **(C)** Biceps femoris RMS mean (bars’ height), standard deviation (vertical whiskers) with and without the exoskeleton. **(D)** Vastus lateralis RMS 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.  
Figure 9: Mean (bars’ height) and standard deviation (vertical whiskers) of the co-activation index without and with the exoskeleton. The horizontal line with the asterisk denotes a statistically significant difference.  
The EMG assessment results show that the use of the exosuit causes muscle fatigue both in the BF and VL muscles. The increase in RMS reflects the rise in muscle activity during the whole gait cycle, for both the BF and VL, when wearing the exosuit. This is showed in Figure 10A,B. The RMS, computed over the entire stride for each muscle while wearing the exosuit increases by \(167.1\%\pm 72.1\%\) for the BF (from \(20.8\%\pm 12.2\%\) to \(55.2\%\pm 13.9\%\) of MVC), and by \(28.4\%\pm 16.8\%\) for the VL (from \(25.8\%\pm 13.2\%\) to \(32.1\%\pm 12\) KF.9% of MVC). The difference in the increase of the RMS between the BF and VL is attributed to the resistive effect of the exosuit, that offers contrary forces to the user motion during both knee flexion and extension, thus, resulting in the need for greater effort from the knee flexors and extensors muscle groups.  
A further confirmation that the exosuit produces muscle fatigue comes from the power spectrum analysis. Figure 11A shows the trend for the MF with the number of strides. The MF of the BF, with the exosuit, is lower than the testing configuration without the exosuit. In all configurations the trend of the MF is negative, thus, as the number of strides increase, the MF further reduces. A similar behavior is for the the MF of the VL (shown in Figure 11B), where as the subject continues walking, the MF decreases. The MF reduces by 18.9% (from 98.4 \(\pm\) 33.4 Hz to \(79.8\pm 14.8\) Hz) for the BF and by 11.9% for the VL (from 90.9 \(\pm\) 8.6 Hz to 80.1 \(\pm\) 10.9 Hz). This confirms that the main change in the EMG signal in the frequency domain is the translation of the spectrum towards lower frequencies [109]. The difference in the rate of reduction of the MF for the BF and VL reinforces the previously advanced hypothesis that this resistive exosuit, although fatiguing both BF and VL, requires a greater effort on the extensor and flexor muscles.