# Optimization of the Position and Stiffness of Passive Walking Assistance Devices
## 4 Discussion  
In the present study, an optimization study of walking assistance systems was conducted through simulation, unlike the previous studies that investigated the effect of walking assistance systems using experiments. The stiffness and placement of the walking assistance system were optimized by using a 2D multibody dynamics model of human walking. The optimal placement of the walking assistance system was found by proposing a design space that was aligned to the distal-proximal direction of the human body. The definition of such a design space enabled the determination of the location of the system and the corresponding rigid bodies on which it could be implemented. Then, the stiffness and free length of the spring were optimized at different walking speeds. Assisting around the hip joint was found to be the optimal placement to reduce positive joint power by using a tension-spring-like structure. [PERSON] et al. investigated a walking assistance system around the hip joint similar to the proposed optimal solution [8]. [PERSON] et al. developed a passive walking assistance system that assists the hip joint during walking and running [9]. The reason why walking assistance devices are concentrated on the ankle and hip joints to reduce the metabolic cost of walking is that the ankle and hip joints account for the largest share of the energy consumption during a gait cycle [22]. The walking assistance devices investigated by [PERSON] et al. [8] and [PERSON] et al. [9] assisted around 25-70% of the gait cycle according to experiments. The present study first identified the placement to determine the perfect supporting time and joints; then, the effect of walking speed on the optimal design was investigated by using only simulation-based design and optimization. The results obtained using multibody dynamics and optimization showed an agreement with the results of experimental studies and are promising for further applications related to walking assistance systems. After determining the optimal location of the walking assistance system, the stiffness and free length of the spring were found at different walking speeds. The effects of walking speed on the optimal parameters were investigated by considering a speed-specific stiffness and a general stiffness value for all walking speeds. Having an optimal stiffness specific to a walking speed reduced the total positive joint energy more than the optimal parameters that considered different walking speeds, as shown in Figure 6. Because the position of the system was determined at a walking speed of 1.3 m/s, the maximum reduction in total positive joint power was achieved at 1.3 m/s. The differences between the speed-specific stiffness value and the general stiffness value mostly depended on the power consumption at different walking speeds, joint angles, and optimal force-displacement characteristics. Due to the higher consumption of power at higher walking speeds, the difference between the speed-specific design and the general design was smaller at walking speeds of 1.3 and 1.5 m/s. On the other hand, the force-displacement characteristics of the general design was similar to those of the speed-specific design at walking speeds of 1.1 and 1.3 m/s. This is the reason why the difference in the performance of the general design and the speed-specific design became smaller at 1.1 and 1.3 m/s. The assistive moment characteristics should be adjusted or optimized to improve device personalization. The generated force on the springs and the moment arm are the parameters that determine the assistive moment generated by a spring system. Joint angles and the location and stiffness of the springs affect the generated force and moment arm. Previous studies on human gait showed that age, height, weight, and walking speed affect the kinematics [23; 24; 25]. Therefore, joint moments, joint angles, and body dimensions are important parameters that need to be considered during the personalization of the devices.