## 5 Discussion

Table 3: Mean, maximum, minimum, and standard deviation of the NDPCA for groups with and without stumbling histories. angle. Therefore, we speculate that toe activity may be related to MTC. The relationship between toe activity and MTC needs to be clarified in future studies. Aging effect: The mean NDPCA evaluates the toe activity for nine walking cycles. The difference in mean NDPCAs between healthy elderly and young individuals was very small when the groups with and without stumbling histories were compared. This indicates that the mean NDPCA is a valid parameter for evaluating stumbling risk, irrespective of age. With regard to the maximum NDPCA, a high value is associated with aging (whereas a low value indicates high stumbling risk). We speculate that the high value of the maximum NDPCA resulted from a temporary action that compensated for the decline in muscle strength--in other words, a temporary increase in kicking force to raise the foot high and prevent stumbling. A low value of the minimum NDPCA was also associated with aging, although statistically significant differences were not observed. We speculate that the low value of the minimum NDPCA indicates low toe activity, which is caused by aging and leads to high stumbling risk. A large value obtained by subtracting the minimum NDPCA from the maximum NDPCA indicates considerable fluctuations in toe activity, which indicates aging. In the same context, we also evaluated the standard deviation of NDPCA. For elderly individuals with stumbling experience, the standard deviation of NDPCA was higher than that of young people. This highlights the fact that their toe activities were more volatile and may increase their risk of stumbling. The considerable variations in NDPCA could be attributed to the decline in muscle strength due to aging. Limitations and further applications: The developed sensing system has many potential applications in future research. These include the analysis of the plantar surface during walking by participants with conditions such as toe deformity and weakness [43] or plantar calluses [44], and the exploration of the plantar region to investigate plantar flexion [26] or diabetes [45]. However, the developed sensing system still has some deficiencies that need to be addressed. We provided only one size of sensor. For a few participants in certain testing environments, foot motion around the MTP joints was affected by the rigidity of the clog. Given that this study is based on image analysis, the sensor was sensitive to the illumination of the experimental environment. Illumination may affect the validity of the image data. Color-based image extraction was also affected by the camera settings. This issue can be resolved by locking the white balance of the camera and adjusting the average brightness. The effect of ambient lighting can be reduced by using the auxiliary lighting provided by the endoscope. Further improvements and application of the sensing system to other studies will be part of our future research. The experimental setup for the left and right footwear was different, but the disparity between results was minimized by conducting a practice walking session with participants. The practice session also minimized the difference in weight and toe length (due to the camera holder) between normal shoes and the developed system. However, the influence of these disparities cannot be completely eliminated. In the future, the investigation could be performed using the same experimental setup for the right and left feet, optimizing materials and topology for reducing the weight, and shortening of the length of the camera holder. Consolidating the validity of the obtained results by increasing the number of participants and its application to the assessment of stumbling or fall risk might also be part of our future work. In this study, we investigated only the case in which participants walked straight ahead.