## 4 Discussion  
As a technique for presenting the target torque to the knee joint, it is possible to use the torque motor, and the effective result may be obtained. However, exoskeletal suits using torque motors are not easy to adjust for differences in wearer body size. PGM is one of the most suitable actuators for the presentation of the sense of motion, which is the purpose of this research, because it is lightweight, flexible, and wearability is high because movement is not interrupted. The experiments verified whether presenting a muscle sense by force feedback can induce the perception of stair-climbing. As shown in Figure 11a, the activity level of the vavats lateralis muscle in the illusionary system is equivalent to that of physical stair-climbing. This result confirms that force feedback by PGM effectively promotes the muscle activity of the vavats lateralis muscle. However, the muscle activities of the gastrocnemius muscle were significantly different between physical and virtual stair-climbing. One reason for this difference is the limited motion analysis considered in the kinematic model. When walking on stairs, the body's center of gravity is lifted by a kicking force, which increases the activity of the gastrocnemius muscle. In this study, the illusion intensity in the exertion force design was limited to the knee joint torque, which does not sufficiently account for activity of the gastrocnemius muscle. By placing additional PGMs that measure the ankle joint torque, we might generate similar muscle activity to that of actual stair-climbing over a wider range of lower-limb muscles. The questionnaire responses to the immersion perception were positive under all three conditions, indicating that the visual display in the virtual environment greatly con \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & \multicolumn{4}{c}{**(a)**} \\ \hline
**Condition** & **Difference** & **Lower** & **Upper** & **\(p\) Value** \\ \hline B-A & 0.195 & \(-\)0.829 & 1.22 & 0.879 \\ C-A & 0.514 & \(-\)0.510 & 1.54 & 0.423 \\ C-B & 0.319 & \(-\)0.705 & 1.34 & 0.711 \\ \hline \hline  & \multicolumn{4}{c}{**(b)**} \\ \hline
**Condition** & **Difference** & **Lower** & **Upper** & **\(p\) Value** \\ \hline B-A & 0.964 & \(-\)0.411 & 2.34 & 0.201 \\ C-A & 1.93 & 0.552 & 3.30 & 0.00579 * \\ C-B & 0.964 & \(-\)0.411 & 2.34 & 0.201 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Results of a three-group test using one-way analysis of variance at the 0.05 significance level: (**a**) immersive experience; (**b**) illusion intensity. *: \(p\) value <= 0.05  
Figure 14: Statistical results of the subjective evaluation experiment: (**a**) immersive results; (**b**) illusion intensity results. tributed to the sense of immersion. The immersion perception scores were not significantly different among the three conditions, but were largest under the visual+force condition. The intensity of the illusion differed between the visual condition and the visual+force condition, implying that the force display enhanced the illusionary effect. Kinestheia is a conscious awareness of body position and movement, maintained by afferent inputs from peripheral receptors (peripheral sensation) and centrifugal input from the center (motor command) [21]. In this experiment, the subject's toes were displayed on the steps of the virtual stairs, so their height position differed from their actual position. Therefore, the kinesthesia generated by the actual joint motion did not always match the visual information presented by the HMD. In contrast to the conventional method, which uses the haptic stimuli of the protrusions, the proposed method attempts to match the muscle activities of the actual and virtual stair-climbing activities, thus compensating for the discomfort caused by the disparity between what the user is watching and perceiving.