## 4 Discussion  
This study evaluated the effect of a PAM driver system that can control the contraction parameters of the PAM on the swing motion in the elderly. The contraction parameters needed to be optimized for each user to maximize the effectiveness of the PAM assistance. While walking with the assistance of this system, the joint angles of the hip and knee in the swing phase increased significantly without altering the gait performance. Figure 8 shows this graphically. The parameters that realized the most effective assistance for the individuals exhibited several variations. Most of the participants had the same settings, similar to the younger participants in our previous study [14]. However, some of the older participants also had a different setting of 100 ms delay and 200 ms contraction. Previous studies have reported that the elderly exhibited longer double support time and shorter swing time in comparison with those of the younger adults [21; 22]. The elderly also experienced a functional decline with aging and low balance capacity [23]. These age-related alterations may contribute to the large individual variations in the effective parameter settings. Our results indicate that the elderly had a larger variation in the parameter settings than that of the young population. For the elderly, parameter tuning was more important for effective swing assistance using PAM than for young individuals. On the other hand, there was no relationship between participants' walking performance and the parameter settings. Thus, the most effective parameter settings cannot be estimated from their ability to walk. Consequently, our driver system must be used in combination with real-time motion capture for clinical and living uses in the elderly. In the future, we will develop an online  
Figure 7: Differences in the hip and knee joint angles and performances between optimal pneumatic artificial muscle (PAM) assistance and lack of assistance. contraction parameter tuning system so that individual variations in effective parameters can be automatically adjusted. The peak hip and knee angles in the swing phase and the hip angular excursion were significantly increased by the PAM assistance that was set to the optimal parameters. These results are consistent with those of our previous study that focused on young individuals [14]. The height of toe clearance in the swing phase is positively correlated with the hip and knee flexion angles [6]. The increase in these angles by the PAM assistance may contribute to increasing the toe clearance. On the other hand, the increases in the hip and knee angles and the hip angular excursion of the elderly were only 15.4%, 7.3%, and 13.5% in this study, whereas those of the young adults were 19.9%, 10.7%, and 17.8% in our previous study [14]. This is because the elderly already had a large hip flexion angle due to forward trunk lean in comparison with that of the younger population [28; 29]. In our studies, this angle in the elderly (37.0\({}^{\circ}\)) was relatively large, in comparison with that of  
Figure 8: Graphical abstract. Example of body model and left hip and knee flexion angles in gait cycle activated by right initial contact (Rt. I.C. ), which triggers the pneumatic artificial muscle (PAM) contraction. The blue and red model and lines signify without and with the PAM assistance, respectively. The peak flexion angles of hip and knee joints in the swing phase and the hip angular excursion were increased by 15.4%, 7.3%, and 13.5%, respectively, by the PAM assistance. the younger participants (34.2\({}^{\circ}\)) [14]. Owing to the low capacity of the elderly to increase the flexion angle, their amplitude of increase by the PAM assistance was smaller than that in the younger population. Nevertheless, the swing assistance with the PAM driver system enabled the increase in the hip and knee flexion angles during the swing phase in the elderly. Meanwhile, no significant change was observed in the gait performance of the elderly.