## 5 Prototype Test  
To verify the performance of the KARE-1, two sets of experimental tests were carried out on ten human subjects whose anthropometric metrics are listed in Table 5.  
In the first set of tests a human subject equipped with the body support device was instructed to walk at the speed of 4 km/h along a straight lane of 20 m length. A human subject adjusted walking speed using the markers on the lane that were placed every 1 m along the lane and a ticking sound from a metronome. A human subject made two round trips of walking for each case, with and without the KARE-1. In the second test, a human subject was instructed to do ten repetitions of the stand-sit-stand motion. One repetition is made up of four segments, i.e., stand-to-sit (3 s), keep sitting (2 s), sit-to-stand (3 s), and standing (2 s). During the stand-sit-stand test, \(\mathbf{\Re}\) of the KARE-1 is set to a constant value out of 0%, 40%, 70%, and 100% to observe its effect on the muscle loadings. The snapshots of a human user during the second and the third tests are presented in Figure 7.  
Figure 7: Human subject test conditions. Experimental setup (a,b) and snapshots (c,d). \begin{table}
\begin{tabular}{c|c|c|c|c|c|c|c|c|c} \hline  & **1** & **2** & **3** & **4** & **5** & **6** & **7** & **8** & **9** & **10** \\ \hline Weight(kg) & 75 & 58 & 71 & 61 & 74 & 69 & 85 & 84 & 68 & 62 \\ Age & 25 & 23 & 25 & 25 & 25 & 27 & 30 & 29 & 27 & 27 \\ Height(cm) & 168 & 173 & 174 & 170 & 179 & 171 & 170 & 180 & 175 & 168 \\ \hline \end{tabular}
\end{table} 
Table 5: Information of Subjects. The output kinematics of the KARE-1 show that the actuating linkage (\(\overline{\text{HG}}\) of Figure 2) was successfully controlled in the WSC mode during the walking test and in the AQSC mode during the stand-sit-stand test as in Figure 7c,d. Figure 8 shows that the KARE-1 successfully produced the output actuator torque with the consumed power loss as desired. Because the KARE-1 has been worn on the left leg only, a human subject may lean on the left side or the right and unexpected changes in muscle activities may occur. To avoid this situation, the human subjects were asked to keep their balance during the tests. During the tests, the center of pressure (CoP) of the ground reaction forces were recorded during stand-sit-stand test. In Figure 9a-c, the trajectories of the CoP during stand-sit-stand (STS) movement are compared against the case without the KARE-1. As shown, the CoP of the foot moved slightly towards the back and left, but the range of movement was small enough to say that the additional mass of the KARE-1 barely affected the balance. To evaluate the reduction in muscle loading by the KARE-1, the activation levels of the three major knee extensors were measured using an electromyography (EMG) sensor. Surface EMG signals of the vastus lateralis, vastus medialis, and rectus femoris were collected using a TrignoTM from DELSYS, Ltd. Natick, MA, USA. Additional tests were conducted to collect EMG data during the maximum voluntary contraction (MVC) according to the procedures suggested in previous work [28]. The locations of EMG electrodes are shown in Figure 5. The EMG signals were collected at 4000 Hz, offset-cancelled, rectified, and band filtered to retain signals between 50 and 200 Hz using the fourth-order Butterworth filter. The processed EMGs were again rectified and low-pass filtered with a cutoff frequency of 20 Hz using the second Butterworth filter. They were then averaged and normalized by EMGs at MVC. The muscle activations of knee extensors decreased as the body assistance ratio increased during stand-sit-stand (Figure 10b), as expected. However, there were still non-zero activations even at 100% of body assistance ratio. This accounts for the torque tracking error so that muscle forces need to compensate for the discrepancy between the gravity torque and the elastic torque, or for co-contraction of both knee flexors and extensors to stabilize the knee joint. Figure 8: Power consumption of the KARE-1.