A Comparative Analysis of Bionic and Neutral Shoes: Impact on Lower Limb Kinematics and Kinetics during Varied-Speed Running

running biomechanics of running shoes have been extensively investigated. However, there is limited knowledge about the use of bionic shoes compared to neutral shoes, along with the velocities involved in their use. The aim of this study was to examine the biomechanical alterations associated with various running velocities of bionic shoes. By removing different thicknesses of the forefoot section, bionic shoes created a more natural shape—close to that of a human foot. The study included 16 heel strike runners running at 10 km/h, 12 km/h and 14 km/h in bionic shoes and neutral shoes, respectively. A two-way ANOVA and SPM1d were employed for examining kinematic and kinetic differences. Regarding the results for the shoes, increased ROM was observed for the bionic shoes for the hip (p< 0.001) and ankle joints (p< 0.001). Ankle positive work (p< 0.001) and negative work (p= 0.042) also showed signi cant differences. Regarding

in bionic shoes and neutral shoes, respectively. A two-way ANOVA and SPM1d were employed for examining kinematic and kinetic differences. Regarding the results for the shoes, increased ROM was observed for the bionic shoes for the hip (p< 0.001) and ankle joints (p< 0.001). Ankle positive work (p< 0.001) and negative work (p= 0.042) also showed signi cant differences. Regarding the velocity results, hip ROM (p< 0.001) increased and peak knee angular velocity (p= 0.018) increased, while knee ROM (p= 0.023) decreased. The interaction effects only existed in hip (p= 0.031) and ankle (p= 0.008) ROM. The results of this study suggested that the impact of running propulsion in the bionic shoes was minimal. However, with increased velocities, the bionic shoes demonstrated the ability to absorb more force, created a more stable training environment, and contributed to injury prevention for the hip and ankle joints. Keywords:footwear; running velocity; bionic shoes; running biomechanics 1. Introduction Running has become a prevalent leisure sports activity among individuals pursuing a healthy lifestyle [1]. Putting aside its positive impacts on health, the negative side effects of running-related injuries should also be taken into account. The annual incidence rate for running injuries typically ranges from 37% to 56%. For long-distance runners, the literature reports injury rates ranging from 2.5 to 12.1 percent per 1000 h of running [2]. The majority of running injuries are lower extremity injuries, with a notable prevalence of knee-related injuries, followed by injuries related to the ankle and hip joints. It has been claimed that lower limb biomechanics during running are linked to both running injury etiologies [3] and running performance [4]. The different characteristics of running shoes have been suggested as potential avenues for reducing the risk of running injuries, though the injury-preventive effects of various running shoes are not clear [5]. Running shoes, serving as the intermediary between the feet and the ground during a run, have the potential to substantially modify running biomechanics [6]. Bionic shoes refer to footwear that incorporates design elements inspired by biological systems or natural movement patterns [7]. These shoes frequently strive to improve comfort,

the injury-preventive effects of various running shoes are not clear [5]. Running shoes, serving as the intermediary between the feet and the ground during a run, have the potential to substantially modify running biomechanics [6]. Bionic shoes refer to footwear that incorporates design elements inspired by biological systems or natural movement patterns [7]. These shoes frequently strive to improve comfort, performance, Appl. Sci.2023,13, 12582.

Appl. Sci.2023,13, 12582 2 of 14 and overall biomechanical attributes [8]. Research speci cally exploring the biomechanical connection between bionic shoes and conventional running footwear is limited. In one study, after a 5 km run in bionic shoes, the abduction angle of the hip was reduced signi cantly to a safe condition by the neuromuscular system, sensorimotor system, and proprioception to avoid injuries [9]. Using the same bionic shoes and under experimental conditions, the bionic shoes showed decreases in vertical instantaneous loading rate (VILR) before a 5 km run, which indicated a better cushioning effect or shock absorbance. After a 5 km run, the contact time of the bionic shoes was shorter, which showed the shoes' capacity to produce average vertical GRF. These ndings provide evidence that bionic shoes can better prevent injuries [10,11]. However, these types of bionic shoe studies focused on modifying the soles of shoes to improve running gait, and limited attention has been given to researching bionic shoes with modi ed midsoles, particularly those involving changes in midsole thickness, for the study of lower extremity biomechanics. Additionally, in a previous study [12], there was little impact on foot strike pattern or stride duration when comparing with shoes with a different midsole thickness. In addition to shoes, running velocities also in uence lower limb biomechanics [13]. Overcoming the challenges encountered, Reginaldo K discovered that parameters of gait kinematics were affected by running velocity in an overall condition. For the sagittal plane, peak values of lower limb angles increased at higher running velocities, except for the peak ankle dorsi exion angle. Parameters such as joint moments, joint work, and ground reaction forces (GRF) in gait kinetics were also affected by running velocities increasing from 2.5 m/s to 4.5 m/s (9 km/h to 16.2 km/h) [14]. SCHACHE investigated the running velocity increase from 3.50 m/s to 8.95 m/s (12.6 km/h to 32.2 km/h) and found that in the sagittal plane, the knee joint work was not in uenced by the increased running velocity during the stance phase. In contrast, the ankle joint work increased from a velocity of 3.50

m/s (9 km/h to 16.2 km/h) [14]. SCHACHE investigated the running velocity increase from 3.50 m/s to 8.95 m/s (12.6 km/h to 32.2 km/h) and found that in the sagittal plane, the knee joint work was not in uenced by the increased running velocity during the stance phase. In contrast, the ankle joint work increased from a velocity of 3.50 m/s to 5.02 m/s (12.6 km/h to 18 km/h) [15]. Jesper observed that the biggest velocity effects were found from velocities of 8 km/s to 12 km/h, and compared with the knee extension moment, the peak plantar exion moment increased greater, which indicated that lower joints, such as the ankle joint, are burdened to a lager extent [16]. Taking into account the impact of running velocity and the choice of running shoes, William observed that the plantar exion at touchdown underwent alterations based on both velocity and shoe type. Notably, traditional running shoes displayed a comparatively smaller plantar exion angle. Knee angles similarly exhibited variations with velocities across all shoe conditions [17]. In a comparative study, Dustin contrasted the Nike ZoomX Vapor y Next% 2 (VFN2) with a mass-matched control shoe, evaluating their performance at 10 km/h and 12 km/h. The study indicated that while the VFN2 improved running economy, this enhancement was less pronounced than at higher velocities of 13 km/h to 18 km/h [18]. However, there are few studies on lower limb biomechanics at different velocities and the effects of different running shoes, let alone bionic shoes. Since studies related to running injuries and the running propulsions of bionic shoes are limited, especially for kinematics and dynamics, this study aimed to investigate the differences in lower limb biomechanics between male amateur runners wearing neutral shoes and bionic shoes at different velocities of 10 km/h, 12 km/h, and 14 km/h. The results may provide a reference for future studies and shoe designs related to running injuries and running propulsions. We set out to explore the following research questions: (1) Can using bionic shoes lead to improved running economy and injury prevention across varying velocities? (2) Does the ankle joint

at different velocities of 10 km/h, 12 km/h, and 14 km/h. The results may provide a reference for future studies and shoe designs related to running injuries and running propulsions. We set out to explore the following research questions: (1) Can using bionic shoes lead to improved running economy and injury prevention across varying velocities? (2) Does the ankle joint experience higher moments and power compared to the other two lower limb joints at different velocities? (3) Do bionic shoes have the potential to establish stable conditions and therefore safeguard joints against potential damage? 2. Material and Methods 2.1. Participants A total of 16 male amateur heel strike runners (mean SD: age: 27 3.7 years, height: 1.72 0.03 m, body mass: 66.7 8.2 kg, body mass index (BMI): 22.4 2.3 kg/m 2 , foot

Appl. Sci.2023,13, 12582 3 of 14 length: 255 10 mm) were recruited for this study. The standard recruitment criteria for this experiment included running for at least 20 km per week [9,19]. Additionally, all participants were right leg-dominant, as determined according to their preferred leg for ball kicking. Individuals with abnormal body posture were excluded, and those with abnor- mally shaped or tall feet were excluded from participation. Furthermore, all participants were free from running injuries, neuromuscular disorders, and lower limb defects in the previous six months. They had all been informed of the experimental protocol and signed a consent form, and ethical approval for this study was granted by the Ethical Institutional Review Board of Ningbo University, Ningbo University [20]. 2.2. Shoes Two types of shoes were included in the test: neutral shoes and bionic shoes. The bionic shoes were designed based on the natural shape of the human foot. Taking normal shoes as a model, the only change to the normal shoes for the bionic shoes was that the midsole part was thinner, as shown by the colored part in Figure.Appl. Sci. 2023, 13, x FOR PEER REVIEW 3 of 14 2. Material and Methods 2.1. Participants A total of 16 male amateur heel strike runners (mean ± SD: age: 27 ± 3.7 years, height: 1.72 ± 0.03 m, body mass: 66.7 ± 8.2 kg, body mass index (BMI): 22.4 ± 2.3 kg/m 2 , foot length: 255 ± 10 mm) were recruited for this study. The standard recruitment criteria for this experiment included running for at least 20 km per week [9,19]. Additionally, all par- ticipants were right leg-dominant, as determined according to their preferred leg for ball kicking. Individuals with abnormal body posture were excluded, and those with abnor- mally shaped or tall feet were excluded from participation. Furthermore, all participants were free from running injuries, neuromuscular disorders, and lower limb defects in the previous six months. They had all been informed of the experimental protocol and signed a consent form, and ethical approval for this study was granted by the Ethical Institutional Review Board of

those with abnor- mally shaped or tall feet were excluded from participation. Furthermore, all participants were free from running injuries, neuromuscular disorders, and lower limb defects in the previous six months. They had all been informed of the experimental protocol and signed a consent form, and ethical approval for this study was granted by the Ethical Institutional Review Board of Ningbo University, Ningbo University [20]. 2.2. Shoes Two types of shoes were included in the test: neutral shoes and bionic shoes. The bionic shoes were designed based on the natural shape of the human foot. Taking normal shoes as a model, the only change to the normal shoes for the bionic shoes was that the midsole part was thinner, as shown by the colored part in Figure 1. Figure 1. Shoe pictures and illustration of bionic midsole construction: bionic shoes. By studying the anatomical structure of feet, a portion of the bionic shoes’ midfoot was removed to better simulate human movement characteristics during running. Some other bionic shoes were customized based on individual’s foot characteristics, which pro- vided inspiration for our bionic shoes [21]. The thicknesses of the removed parts were 3 mm, 1 mm, and 8 mm for blue, yellow, and red separately. From the perspective of the vertical direction, looking from the heel to the toe, the middle part was the deepest for both the yellow and blue sections, with the depth decreasing towards the sides. During running, the metatarsophalangeal joints make deeper contact with the ground compared to other parts of the foot. 2.3. Experimental Procedure The biomechanics laboratory of Ningbo University was used to conduct all of the testing. All participants were requested to wear tights and skinny pants, making sure that all 34 reflective markers (gait 2392 model in opensim [22]), as shown in Figure 2b, stayed in the same position throughout the testing procedure. Before the formal test, static coordinates were acquired by standing on the Y-axis of a force platform with arms lifted to the side and eyes looking forward until all static coor- dinates were captured. Participants had 10 min to warm

(gait 2392 model in opensim [22]), as shown in Figure 2b, stayed in the same position throughout the testing procedure. Before the formal test, static coordinates were acquired by standing on the Y-axis of a force platform with arms lifted to the side and eyes looking forward until all static coor- dinates were captured. Participants had 10 min to warm up and familiarize themselves Figure 1.Shoe pictures and illustration of bionic midsole construction: bionic shoes. By studying the anatomical structure of feet, a portion of the bionic shoes' midfoot was removed to better simulate human movement characteristics during running. Some other bionic shoes were customized based on individual's foot characteristics, which provided inspiration for our bionic shoes [21]. The thicknesses of the removed parts were 3 mm, 1 mm, and 8 mm for blue, yellow, and red separately. From the perspective of the vertical direction, looking from the heel to the toe, the middle part was the deepest for both the yellow and blue sections, with the depth decreasing towards the sides. During running, the metatarsophalangeal joints make deeper contact with the ground compared to other parts of the foot. 2.3. Experimental Procedure The biomechanics laboratory of Ningbo University was used to conduct all of the testing. All participants were requested to wear tights and skinny pants, making sure that all 34 re ective markers (gait 2392 model in opensim [22]), as shown in Figureb, stayed in the same position throughout the testing procedure. Before the formal test, static coordinates were acquired by standing on theY-axis of a force platform with arms lifted to the side and eyes looking forward until all static coordinates were captured. Participants had 10 min to warm up and familiarize themselves with the shoes before the experiment begun. During the warm up session, participants were asked to accomplish the following: (a) jogging on a treadmill at a pace of 8 km/h for 10 min and (b) performing a series of lower limb stretching exercises, including hamstring stretches, calf stretches, and quadriceps stretches [21]. Running exercises were completed by participants running at different velocities controlled by

the experiment begun. During the warm up session, participants were asked to accomplish the following: (a) jogging on a treadmill at a pace of 8 km/h for 10 min and (b) performing a series of lower limb stretching exercises, including hamstring stretches, calf stretches, and quadriceps stretches [21]. Running exercises were completed by participants running at different velocities controlled by Brower timing lights (Brower

Appl. Sci.2023,13, 12582 4 of 14 Timing System, Draper, UT, USA) at 10 km/h, 12 km/h, and 14 km/h. Recreational heel strike runners choose speeds ranging from 10 km/h to 12 km/h as a “natural running pace” [9]. These velocities were chosen in a previous study to investigate the relation between running economy (RE) at endurance running speeds. The global running pattern was quanti ed to better understand this relation [23]. Based on existing studies in the literature, these velocities may help us understand the relationship between running injuries and bionic shoes more clearly. Participants were asked to step their right leg on to the 2 mY-axis force platform in the middle of the track over a 10 m designed track, as shown in Figurea,c. The full gait cycle was de ned as the moment from when the right heel touched the ground to the moment when the right toe was off the ground. Each velocity was tested on 5 times, with the error range for the running velocities being within 5% of the prede ned running velocity. Participants completed the 5 trials for each velocity successfully. Participants would have a 5-min rest period, during which they changed into another pair of shoes and drank water to prepare for the next procedure.Appl. Sci. 2023, 13, x FOR PEER REVIEW 4 of 14 with the shoes before the experiment begun. During the warm up session, participants were asked to accomplish the following: (a) jogging on a treadmill at a pace of 8 km/h for 10 min and (b) performing a series of lower limb stretching exercises, including hamstring stretches, calf stretches, and quadriceps stretches [21]. Running exercises were completed by participants running at different velocities controlled by Brower timing lights (Brower Timing System, Draper, UT, USA) at 10 km/h, 12 km/h, and 14 km/h. Recreational heel strike runners choose speeds ranging from 10 km/h to 12 km/h as a “natural running pace” [9]. These velocities were chosen in a previous study to investigate the relation between running economy (RE) at endurance running speeds. The global running pattern was quantified to better

Timing System, Draper, UT, USA) at 10 km/h, 12 km/h, and 14 km/h. Recreational heel strike runners choose speeds ranging from 10 km/h to 12 km/h as a “natural running pace” [9]. These velocities were chosen in a previous study to investigate the relation between running economy (RE) at endurance running speeds. The global running pattern was quantified to better understand this relation [23]. Based on existing studies in the litera- ture, these velocities may help us understand the relationship between running injuries and bionic shoes more clearly. Participants were asked to step their right leg on to the 2 m Y-axis force platform in the middle of the track over a 10 m designed track, as shown in Figure 2a,c. The full gait cycle was defined as the moment from when the right heel touched the ground to the moment when the right toe was off the ground. Each velocity was tested on 5 times, with the error range for the running velocities being within 5% of the predefined running velocity. Participants completed the 5 trials for each velocity suc- cessfully. Participants would have a 5-min rest period, during which they changed into another pair of shoes and drank water to prepare for the next procedure. Figure 2. (a) The front, back, and side positions of markers. Pink dots: markers. (b) Experimental flow. (c) Illustration of experimental design for collecting the kinematic data during the running stance phase. The kinematic and kinetic data were collected at a frequency of 200 Hz using a Vicon motion capture system (Oxford Metrics, Ltd., Oxford, UK) and 1000 Hz using a force plat form (Kistler, Winterthur, Switzerland) [24]. Brower timing lights (Brower Timing System, Draper, UT, USA) were used to control the running velocities in the experiment. 2.4. Data Collection and Processing C3D files were produced by using Vicon Nexus software (version 1.8.5A, Vicon Met- rics Ltd., Oxford, UK), which detected the kinematics and kinetics. As for the gaps of markers’ trajectories, the raw motion data were visually checked and manually filled (us- ing ‘pattern fill’ according to the shape of another trajectory

This study investigates the impact of bionic shoes on lower limb biomechanics during running at different speeds.