Recently, in the field of sports medicine, there has been a large amount of research conducted on the biomechanical adaptations to running in modified footwear. Specifically, there has been an increased focus on barefoot running (Becker et al., 2014; Cheung and Rainbow, 2014; Powell et al., 2014; Rao et al., 2015; Strauts et al., 2015; Thompson et al., 2015), running strike patterns (Ahn et al., 2014; Lieberman et al., 2010; Shih et al., 2013), and various running shoe styles (Barnes et al., 2010; Bonacci et al., 2013; Hollander et al., 2014; Squadrone and Gallozzi, 2009). Researchers have found that different shoes and different strike patterns produce different biomechanical characteristics that can affect injury risk. Previous research comparing barefoot and shod running has demonstrated that barefoot running can change the landing strategy through adaptation to a forefoot strike (FFS) as opposed to a rearfoot strike (RFS), thereby reducing landing impact transients (Lieberman et al., 2010). Thus, the collision force is one of the main factors causing lower extremity running injuries during the running braking phase (Lieberman et al., 2010; Shih et al., 2013). Another comparison between barefoot running and shod running found that the barefoot running stride was shorter, with decreased contact time and higher stride frequency (De Wit et al., 2000; Divert et al., 2005). Shod running attenuates the foot-ground impact by adding damping material to avoid direct contact with the ground. Therefore, shod running may lead to a decrease in the storage and restitution of elastic energy and lower net efficiency (Divert et al., 2008). Furthermore, a comparison of the effects of running barefoot, shod and in Vibram Fivefingers (Vibram SpA, Albizzate, Italy), a lightweight minimalist shoe, revealed that runners in Vibram Fivefingers demonstrated longer strides and lower stride frequency relative to barefoot conditions and decreased contact time relative to shod conditions (Squadrone and Gallozzi, 2009).
Forefoot or midfoot strikes (MFS) are a major component of barefoot running, while most shod runners use a heel strike pattern (De Wit et al., 2000; Hasegawa et al., 2007; Lieberman et al., 2010). The cushioning capabilities of some minimalist shoes allow the runner to adopt a heel strike pattern and avoid painful heel contact with the ground (Bonacci et al., 2013; Willy and Davis, 2014). As the heel strike pattern is associated with greater dorsiflexion of the ankle joint, this pattern can reduce the ability of the ankle to attenuate impact forces (Hollander et al., 2014; Willy and Davis, 2014). However, shod running prevents a direct heel to running-surface impact during landing, leading to longer strides through modification of the contact geometry (De Wit et al., 2000). It is essential to understand how foot strike strategies are modulated by different footwear, as it has been established that different strike patterns can affect the risk of lower extremity injuries (Hamill et al., 1999; Wang et al., 2018). Therefore, if we understand how these factors change foot strike patterns, we can prescribe footwear more appropriately to individuals.
Researchers have also previously found that different types of running shoes (e.g., Vibram Fivefingers, lightweight shoes, and minimalist shoes) can affect running biomechanics and, therefore, influence the risk of injury (Bonacci et al., 2013; Sinclair, 2014; Squadrone and Gallozzi, 2009; Willy and Davis, 2014). Along with changes in design, there are also differences in the mass of running shoes: approximately 150-200 g for marathon running and Vibram Fivefingers shoes, 200-300 g for a neutral running shoe and 360 g for a traditional running shoe (Mizuno, 2018). In the past, few have explored the independent effect of mass, and its influence on running biomechanics is not well understood. Related studies have observed that minimalist shoes have moderate beneficial effects on running economy (Perl et al., 2012). It has been demonstrated that there is no detrimental effect of shoe mass on metabolic cost if the combined mass of both shoes is less than 440 g. However, if the combined mass of the shoes exceeds 440 g, there is a positive correlation with metabolic cost (Fuller et al., 2015).
It is well established that the human body will change its foot strike pattern during running in response to different designs of running shoes. The majority of shod runners use a RFS, and minimal shoes or barefoot runners use FFS and MFS, respectively (Hasegawa et al., 2007; Larson, 2014). Currently, running shoes are mainly designed as lightweight shoes, minimal shoes, and traditional cushioned running shoes. Therefore, it is unclear whether the biomechanical changes during running in different shoe types of differing mass are the result of the structural design or mass of the shoe. To address this, the current study was conducted using four identical types of shoes that differed only in mass to study the effect of shoe mass on running biomechanics in isolation. A standard, traditional running shoe was selected for this study as it provided an average amount of cushioning that would not influence running mechanics due to excessive or inadequate cushioning. It was hypothesized that greater shoe mass would result in increased vertical ground reaction force (VGRF) and a change to MFS running patterns.
Twenty collegiate male physical education students volunteered for this study and provided written informed consent. The participants' mean age, height and mass were 21.8 years (SD=1.2), 1.72 m (SD=0.03) and 68.00 kg (SD=4.32), respectively. None of the participants had a history of lower extremity injuries during the six months prior to the experiment. The runners were previously familiarized with shoes of different mass a minimum of four times at the test site, where they ran one time along the 10 meter runway in each shoe on four different days in one week. The study was approved by the Antai Medical Care Corporation Memorial Hospital (No. 15-066-B1).
Four shoe mass conditions were used in this study (Figure 1); for each condition, a single shoe was 175 g, 255 g (4x20 g lead mass), 335 g (4x40 g lead mass), or 415 g (4x60 g lead mass). Lead mass was attached to the shoes to achieve the required total mass and were evenly distributed on the four sides of the shoe. As a warm-up, participants were instructed to run back and forth for 20 minutes at a comfortable self-selected pace around the corridor, as well as perform static stretching. Following warm-up and prior to data collection, participants were asked to habituate to the runway at a comfortable self-selected pace and were given 5 practice trials in each footwear condition to become familiarized with the shoe mass. Allowing the participants to self-select the running pace facilitated a more natural response to running with additional mass on the shoes. Following familiarization, participants were instructed to run down the runway, in which two force plates were positioned to capture a right and left foot impact. Participants completed five trials for each mass condition, and three trials with stable foot contact were selected and averaged to increase the stability of measurements. Data utilized in the analysis were recorded from the participant's right foot contact with the first force plate to left foot toe-off on the