# Effects of a Curved Heel Shape in a Running Shoe on Biomechanical Variables and Comfort
## 1 Introduction  
In running, impact is absorbed through ankle joint version with dorsi-flexion from when the heel contacts the ground until mid-stance [1]. It has also been suggested that excessive version of the foot may induce limb injury during running [2, 3, 4]. Thus, a strategy to reduce impact and maintain lower extremity joint stability is necessary to reduce injury risk factors and run efficiently [5, 6].  
Shoes play an important role in protecting the foot and lower extremity joints [7]. Shoes not only absorb impact upon the foot's contact with the ground during various physical activities that include running but offer stability [7]. Therefore, the materials and structures of shoes are crucial factors that directly affect a human body's motion. Many previous footwater studies have examined the effect of changes in the hardness of the sole on impact and ground reaction force (GRF) variables, such as loading rate and planter pressure, as well as the ankle joint range of motion during running [8, 9, 10, 11, 12, 13, 14, 15]. Other studies investigated changes in motion and impact with changes in other material properties of a shoe related to a shoe's thickness, elasticity and bending stiffness during running [8, 16, 17].  
While previous studies have mainly examined the effect of a shoe's material characteristics, only a few studies have investigated the influence of a shoe's structure and shape. For example, an earlier study investigated the effects of the flare at the heel of a shoe on ankle joint version and impact in running [18]. Recently, the biomechanical function of a rounded heel in badminton shoes has been investigated, and results showed that the vertical loading rate (VLR) was smaller with a rounded heel shoe (RHS) than othertypes of shoes during a lunge motion. Therefore, it was reported that an RHS would be effective in reducing the impact on the human body during the lunge motion in badminton. These findings suggest that an RHS may lead to a reduced impact in running, and the changes in the structure and shape of the heel are required to reduce excessiveevers and improve the stability of the ankle joint [19]. Several previous studies have speculated that heel shape would alter lower extremity biomechanics (i.e., cushioning and motion control) and comfort perception during running [18; 20]. Therefore, a shoe manufactured with a curved heel is expected to have a positive effect on the reduction of impact as well as lower extremity stability in running.  
In addition to evaluating how running shoes affect biomechanical factors, the perception of comfort is also an important factor [21]. It has been reported that overall comfort increases as loading rate decreases and the total pressure to the sole is evenly distributed [22]. Therefore, it is necessary to investigate the relevance of a shoe's subjective comfort, and examining biomechanical factors that have a positive effect on the comfort of a shoe can provide meaningful information.  
The purpose of this study was to examine the effects of a curved heel shape of a running shoe (i.e., a rounded heel shoe (RHS) and rounded lateral heel shoe (RLHS)) on impact and lower extremity stability as well as relationships with comfort during running. The hypotheses of this study were as follows: first, a running shoe with a curved heel shape (RHS and RLHS) would reduce the peak vertical ground reaction force (PVGRF) and the vertical loading rate (VLR) during the stance phase, second, the RLHS would reduce the maximumeversion angle compared with the RHS and NS. In addition, changes in biomechanical variables would be closely related to shoe comfort.