# Effects of a Curved Heel Shape in a Running Shoe on Biomechanical Variables and Comfort
## 4 Discussion  
This study aimed to investigate the effect of running shoe heel shape (RHS and RLHS) on impact variables, ankle joint biomechanics and shoe comfort and to analyze the contributions of biomechanical variables to shoe comfort using multiple regression analysis. This study was performed on runners who ran at the same speed and heel strike pattern in the three shoes, as foot strike pattern and running speed may affect running mechanics [38; 39]. The first hypothesis--i.e., that a running shoe with a curved heel shape would reduce the magnitude of impact variables (i.e., PVGRF and VLR) during running--was partially accepted. No differences in VLR, which may be related to injury during running [31], were shown between the shoes. However, the PVGRF of the RHS and RLHS was smaller than that of the NS. Small reductions of impact may be beneficial to reduce the risk of running injuries, especially when repetitive high loads are more pronounced, such as in long distance running [40]. The second hypothesis--that the RLHS would reduce the maximum eversion angle--was not supported. The RLHS showed a smaller inversion-eversion ROM due to a smaller inversion angle at toe-off compared with the NS and RHS. A study has suggested that a certain level of eversion or pronation of the foot is required during running to dampen the impact shock at heel contact [31]. In addition, it has been reported that excessive eversion (i.e., greater than an eversion ROM of \(15^{\circ}\)) of the foot may cause discomfort and pain followed by lower extremity joint injuries in running [18; 41; 42]. Therefore, footwear manufacturing companies have focused on the reduction of excessive foot eversion using alterations of the shoe structure and material (i.e., heel design, heel material and hardness of the midsole), but the findings are still controversial in current studies [31]. In this study, the maximum eversion angle between the shoes did not exhibit any difference. The current finding is in accordance with the result of a previous study, in which shoes with changes in the heel flare at the side of heel had no effect on the maximum eversion during the stance phase [18]. Shoe comfort is an important factor influencing running performance as well as the incidence of lower extremity joint injury in runners [43; 44]. Considering the biomechanical factors that had a positive correlation with the overall comfort of running shoes, the independent variables representing a statistically significant explanation power were dorsiflexion angle at heel contact and dorsi-plantar flexion ROM. In particular, excessive  
Figure 5: Diagram of multiple regression analysis for running shoe comfort using biomechanical factors. * indicates a significant difference between the shoes at \(\alpha=0.05\). dorsiflexion angle at heel contact has been shown to have a negative effect on the comfort of running shoes, but an increase of dorsi-plantar flexion ROM has been shown to have a positive effect on the comfort of running shoes. Based on these results, it is believed that the proper use of rounded heel shoes, considering the natural movement [45] of dorsi-plantar flexion, can help improve comfort and performance during running. Further study including more detailed information about foot kinematics and kinetics (i.e., trajectory of the center of pressure and movement coupling of the ankle joint complex) would be warranted to understand the underlying mechanism, as this would be closely related to comfort as well as running performance. Some limitations need to be mentioned for a proper interpretation of the findings and to obtain insights into the direction of future studies. As running was performed at a slow speed of 2.7 m/s, it is not clear how running shoes with rounded heels may affect running biomechanics at higher running speeds. A further study would be required to examine running motions at faster speeds to investigate the functions of a rounded heel during these higher loading conditions. Furthermore, shoe comfort was tested during a short time period.