# Biomechanical Analysis of Running in Shoes with Different Heel-to-Toe Drops
## 1 Introduction  
As one of the most popular sporting activities, running is an effective way to improve cardiovascular function and physical fitness [1]. However, running-related injuries are very common. A study has shown that 79% of long-distance runners suffer running-related injuries every year [2], such as patellofemoral pain, tibial stress fractures, Achilles tendonitis, and plantar fascitis, with 46% of these injuries being recurrent. As the key equipment for running activity, well- designed running shoes are very important for optimizing the impact force and lower extremity loading that are related to running injuries. In spite of the constant innovation in running shoe design and advances in cushioning technology in the past few decades, running-related injuries and recurrence rates remain surprisingly high [3]. Various factors may lead to the etiology of running-related injuries. The amount, frequency, and timing of vertical impact force are considered to be the key factors causing lower extremity running-related injuries [4; 5; 6]. Vertical impact force can be affected by a few factors, including running shoes, foot strike patterns, and spatiotemporal parameters such as step length and frequency [7; 8; 9; 10; 11]. Therefore, mitigating the factors associatedwith vertical impact force seems to be the focus when aiming to decrease the risk of running-related injuries. In order to decrease the vertical impact force, running shoe companies have launched higher heel-cushioned shoes, hoping to provide better cushioning and reduce more shocks due to impact force. A study revealed that additional cushioning reduced the vertical loading rate in running [12]. In recent years, some shoe companies altered the amount of cushioning in their shoes, offering running shoes with different drops (i.e., the thickness difference between the forefoot and heel of running shoes). Minimalist shoes are the representatives of shoes with lower drops which aim to simulate barefoot running and may promote non-rearfoot strike patterns. Studies have shown that running in minimalist shoes increased the vertical impact and vertical loading rate [8; 13; 14; 15]. In addition, some studies have found that the cushion of running shoes has no immediate effect on the vertical impact force or vertical loading rate [5; 16]. The conflict in previous studies' results may be caused by the differences in running speed, running environment, and limited range of drops. More importantly, previous studies used different brands and models of running shoes which had different midsole construction designs and midsole materials. Thus, it is unclear whether the observed different biomechanics during running in different types of shoes results from the drop or other structural aspects of running shoes. Only a few studies have specifically focused on the drop of running shoes, which was reported to be a key parameter affecting the strike patterns. Studies have reported that an increased drop in running shoes promotes a rearfoot strike running gait [17; 18]. Running in shoes with large drops increases the foot inclination angle at initial contact and decreases the vertical loading rate in the standing phase compared with running in shoes without a drop for female runners [7]. In addition to the biomechanical changes, a prospective study indicated that running in shoes with a 10-mm drop yields a higher injury rate for the knee joint and lower injury rate for the ankle and foot compared with running in shoes without a drop [19]. Few studies have focused on the biomechanical analysis of running shoes with various drops, which limits the understanding of the relationship between the drop of running shoes and lower extremity biomechanics. The purpose of the current study was to compare the strike patterns, vertical impact forces, and lower extremity biomechanics during running in shoes that varied only in terms of heel-to-tee drop.