Simulation of Lower Limb Muscle Activation Using Running Shoes with Different Heel-to-Toe Drops Using Opensim

kground: Although numerous studies have been conducted to investigate the acute effects of shoe drops on running kinematics and kinetic variables, their effects on muscle forces remain unknown. Thus, the primary aim of this study was to compare the muscle force, kinematics, and kinetic variables of habitually rearfoot runners with heel-to-toe drops of negative 8 mm shoes (minimalist shoes) and positive 9 mm shoes (normal shoes) during the running stance phase by using musculoskeletal modeling and simulation techniques. Methods: Experimental data of lower limb kinematics, ground reaction force, and muscle activation from 16 healthy runners with rearfoot strike patterns were collected and analyzed in OpenSim. Using Matlab, the statistical parameter mapping paired t-test was used to compare the joint angle, moment, and muscle force waveform. Results: The results revealed differences in the sagittal

musculoskeletal modeling and simulation techniques. Methods: Experimental data of lower limb kinematics, ground reaction force, and muscle activation from 16 healthy runners with rearfoot strike patterns were collected and analyzed in OpenSim. Using Matlab, the statistical parameter mapping paired t-test was used to compare the joint angle, moment, and muscle force waveform. Results: The results revealed differences in the sagittal ankle and hip angles and sagittal knee moments between the different heel-to-toe drops of running shoes. Speci cally, it showed that the negative 8 mm running shoes led to signi cantly smaller values than the positive 9 mm running shoes in terms of the angle of ankle dorsi exion, ankle eversion, knee exion, hip exion, and hip internal and hip external rotation. The peak ankle dorsi exion moment, ankle plantar exion moment, ankle eversion moment, knee exion moment, knee abduction moment, and knee internal rotation also decreased obviously with the minimalist running shoes, while the lateral gastrocnemius, Achilleas tendon, and extensor hallucis longus muscles were obviously greater in the minimalist shoes compared to normal shoes. The vastus medialis, vastus lateralis and extensor digitorum longus muscles force were smaller in the minimalist shoes. Conclusions: Runners may shift to a midfoot strike pattern when wearing negative running shoes. High muscle forces in the gastrocnemius lateral, Achilleas tendon, and exor hallucis longus muscles may also indicate an increased risk of Achilleas tendonitis and ankle exor injuries. Keywords:minimalist running shoes; normal shoes; muscle force; heel-to-toe drop 1. Introduction Running has become one of the most popular forms of exercise globally [1,2]. Running is associated with preventing obesity and cardiovascular disease [1,3]. However, running provides us with the highest incidence of injuries. The incidence of running injuries has been reported to have increased from 19.4 to 79.3% [4]. Several studies have found that running injuries are mainly lower extremity injuries for professional and amateur runners, especially the knee joint and anterior knee (such as patellofemoral joint pain) [5,6]. Other common injuries include tibial and bular periostitis, Achilles tendonitis, plantar fasciitis, and iliotibial band syndrome [4,7]. Numerous factors might in uence running injuries, including the strike

to 79.3% [4]. Several studies have found that running injuries are mainly lower extremity injuries for professional and amateur runners, especially the knee joint and anterior knee (such as patellofemoral joint pain) [5,6]. Other common injuries include tibial and bular periostitis, Achilles tendonitis, plantar fasciitis, and iliotibial band syndrome [4,7]. Numerous factors might in uence running injuries, including the strike pattern, footwear, and high ground reaction forces during Healthcare2023,11, 1243.

Healthcare2023,11, 1243 2 of 13 running [8–10]. The relationship between biomechanical factors and the risk of running injuries has led to several methods to reduce running injury rate, such as modifying running strike patterns and limiting the running distance [11]. According to foot strike pattern, there are three main types of running patterns: rearfoot strike (RFS), midfoot strike (MFS), and forefoot strike (FFS) [12]. Research relating to running strike patterns found that 75% of runners were habitually rearfoot-strikers [13]. Strike pattern technologies were also associated with running injuries. In RFS running, there may be increases in the loading rate of the impact and knee power. However, the forefoot strike pattern would increase the ankle power and Achilles tendon force [14]. In addition, a good pair of running shoes is essential and desirable for runners. In the past 30 years, running shoes with cushioned and comfortable features have reduced running injuries [15]. Moreover, a previous study has reported that changing the running strike pattern and controlling the running distance might induce running overuse injuries [11]. Thus, many shoe manufacturers have also begun to focus on barefoot running, in which Vibram Five Fingers, Nike Free, and minimalist shod were produced using the forefoot strike pattern [16,17]. Minimalist shoes have features such as low drop, ultra-light, high bend, low cushioning, and so on [18]. Furthermore, a previous study has shown that participants running with minimalist shod may decrease the knee extension moment and patellofemoral joint contact force, enhancing the foot muscle force [19]. A meta- analysis comparing the running economy of running in barefoot, minimalist, and standard running shoes found that running in barefoot [17] minimalist shoes require less oxygen utilization [20]. Nevertheless, much research in recent years has reported that minimalist running shoes may increase the loading rate and ankle joint load [21,22]. The heel-to-toe drop is de ned as the difference between the height of the heel and the forefoot of a shoe. The heel-to-toe drop is an essential factor related to the risk of running injuries. It has been reported that lower heel-to-toe drop shoes of 0 mm and 6

shoes may increase the loading rate and ankle joint load [21,22]. The heel-to-toe drop is de ned as the difference between the height of the heel and the forefoot of a shoe. The heel-to-toe drop is an essential factor related to the risk of running injuries. It has been reported that lower heel-to-toe drop shoes of 0 mm and 6 mm instead of 10 mm were less likely to injure occasional runners than regular runners [23]. Therefore, the heel-toe drop is a vital parameter for runners to prevent running injuries. Increased shoe heel-to-toe drop might result in a rearfoot strike running gait [24,25]. Horvais et al. compared the effects of heel height and heel-to-toe drop difference on the foot-strike pattern and running kinematics, in which the foot-strike pattern was associated with the lower heel-toe drop. The results demonstrated a positive correlation between the drop of shoes and running contact time during the running stance phase. With the lower shoe drop, the foot angle at contact and contact time were decreased [24]. Chambon et al. reported that running with lower heel-toe drop shoes may in uence the foot strike pattern. For example, running in shoes with 0 mm might lead the rearfoot strike into a midfoot strike pattern [26]. Furthermore, the negative heel-to-toe drop running shoes will change the strike pattern into a midfoot strike pattern during the running stance [27]. Electromyography is an essential parameter for characterizing muscle activity during running. Lower limb muscles may provide proper joint alignment, stability, stiffness and propulsion to propel the body forward while running. Yong et al. concluded that RMS (root mean square) activity in the tibialis anterior in FFS runners was considerably reduced compared to RFS runners during the nal swing phase. In FFS runners, on the other hand, the medial and lateral gastrocnemius demonstrated much larger RMS (root mean square) activity during the terminal swing phase [28]. Fernandes Ervilha et al. showed that the iEMG (EMG intensity) of the TA (tibialis anterior) was increased when running in shoes utilizing rearfoot strike patterns than in shoes of forefoot strike pattern and barefoot running.

FFS runners, on the other hand, the medial and lateral gastrocnemius demonstrated much larger RMS (root mean square) activity during the terminal swing phase [28]. Fernandes Ervilha et al. showed that the iEMG (EMG intensity) of the TA (tibialis anterior) was increased when running in shoes utilizing rearfoot strike patterns than in shoes of forefoot strike pattern and barefoot running. Moreover, the iEMG (EMG intensity) of SO (soleus) and GM (gastrocnemius medialis) were signi cantly smaller when running in the shoes using rearfoot strike patterns [29]. The previous study found that plantar exor muscles were stimulated 11% earlier and for 10% longer in FFS runners than in RFS runners [30]. Previous research has also found modest differences in muscle activation while running barefoot versus shod. In barefoot runners, the medialis gastrocnemius, lateralis gastrocnemius, and soleus muscle activity

Healthcare2023,11, 1243 3 of 13 were signi cantly increased [31]. Numerous studies focus on muscle activities; nevertheless, there is no study on muscle force evaluation for minimalist shoes and traditional shoes. There have also been no previous studies investigating how heel-to-toe drop affects lower extremity muscle force. Thus, the main aim of this study was to use musculoskeletal modeling and simulation techniques to compare the muscle force, kinematics, and kinetic variables of habitually rearfoot runners while wearing the heel-to-toe drop of negative 8 mm shoes (minimalist shoes) or the heel-to-toe drop of positive 9 mm shoes (normal shoes) during the running stance phase. This study aimed to focus on the immediate effect of kinematic and kinetic variables during the running stance with different heel-to-toe drop shoe conditions. It was hypothesized that the plantar exors (gastrocnemius medialis /lateralis, and soleus) and Achilles tendon force might increase when running in shoes with a heel-to-toe drop of negative 8 mm (minimalist shoes). It was also hypothesized that the ankle, knee kinematics, and kinetic variables might change when running in shoes with a heel-to-toe drop of negative 8 mm (minimalist shoes) compared to running in shoes with a heel-to-toe drop of positive 9 mm (normal shoes). 2. Materials and Methods 2.1. Participants Before the test, the sample size was estimated using G*Power (Version 3.1.9.7). Taking into account the effect size of 0.4, the power value of 0.8, and the alpha level of 0.05 [27], a priori power analysis has shown that a sample size of 13 was enough to adequately power this study [27]. We therefore recruited 16 healthy recreational male runners (age:26 2.0, weight: 73.5 4.6, height: 175.8 0.5 cm) to participate in this study. The following conditions had to be met for runners to qualify as recreational: running using a rearfoot striking pattern and running 2 to 5 km a week [32]. All subjects having the target foot length of US size 9 ( 0.5) and self-reported as right leg dominant were included. None of the participants had any lower limb injuries in the past six months. All the participants had no

runners to qualify as recreational: running using a rearfoot striking pattern and running 2 to 5 km a week [32]. All subjects having the target foot length of US size 9 ( 0.5) and self-reported as right leg dominant were included. None of the participants had any lower limb injuries in the past six months. All the participants had no prior experience with minimalist running shoes and the negative value of running shoes. To avoid subjects changing their strike pattern during the running test, all the participants were not informed of the test's purpose before the study and were only informed about the experimental methodology. This study was approved by Ningbo University Health Research Ethics (protocol code: RAGH 20220116), and prior to the study, all the participants were informed about experimental conditions and provided written consent. 2.2. Experimental Shoes Condition This study used two pairs of running shoes (AT US 9). The difference between the two running shoes was the heel-toe drop (HTD). The heel-to-toe drop in running shoe design is the difference in thickness between the forefoot and heel regions of the sole [33]. Figure showed the test shoes in this study. Heel-to-toe drop differs signi cantly between regular and negative running shoes, with a 8 mm offset in in negative shoes and 9 mm in normal shoes. It is common for both conventional and negative shoes' soles to be comprised of EVA foam.Healthcare 2023, 11, 1243 4 of 14 Figure 1. Outlined in this illustration, the left running shoe has the −8 mm HTD (minimalist shoes) and the right running shoe has with 9 mm HTD (normal shoes). 2.3. Data Collection Before the test, all the participants were familiarized with test process. During data collection, running shoe conditions were assigned randomly to the participants. They were asked to make themselves adapt to each running shoes. Then runners were random- ized and wore the experience running shoes to run through a 10 m walkway. A device regulating the subjects’ speed was placed on either side of the force platform (smart speed, Fusion Sport Inc. of Burbank, CA, USA). Equipment

were assigned randomly to the participants. They were asked to make themselves adapt to each running shoes. Then runners were random- ized and wore the experience running shoes to run through a 10 m walkway. A device regulating the subjects’ speed was placed on either side of the force platform (smart speed, Fusion Sport Inc. of Burbank, CA, USA). Equipment for assessing speed was located 3.0 m away. Speed was controlled at 3.0 ± 0.5 m/s [32]. Thirty-eight reflective markers (diam- eter: 14 mm) were attached to the bilaterally lower limbs, torso, and head according to the Opensim Gait 2392 model [34]. The marker coordinates were captured using an eight- camera Vicon motion analysis system (Oxford Metrics Ltd., Oxford, UK) at a frequency of 200 Hz. A force plate (Kistler Type, 9281 B, Kistler Instrument AG, Winterthur, Switzer- land) was utilized to collect the ground reaction force (GRF) at 1000 Hz. The markers were placed on: Sternum, R. Acromion, L. Acromion, Toe. Head, R. ASIS, L. ASIS, V. Sacral, Thigh. Upper, R. Thigh. Front, R. Thigh. Rear, R. Knee. Lat, R. Knee. Med, R. Shank. Up- per, R. Shank. Front, R. Shank. Rear, R. Ankle. Lat, R. Ankle. Med, R. Heel, R. Midfoot. Sup, R. Midfoot. Lat, R. Toe. Lat, R. Toe. Med, R. Toe. Tip, L. Thigh. Upper, L. Thigh. Front, L. Thigh. Rear, L. Knee. Lat, L. Knee. Med, L. Shank. Upper, L. Shank. Front, L. Shank. Rear, L. Ankle. Lat, L. Ankle. Med, L. Heel, L. Midfoot. Sup, L. Midfoot. Lat, L. Toe. Lat, L. Toe. Med, L. Toe. Tip [35]. All participants completed a static calibration and ran at a pace of 3.0 m/s over a flat runway in a standard sports biomechanics Lab. All subjects were required to run along the walkway with the right foot stepping on the force plate, six successful trials were finally captured. One successful trial was defined as the participant’s right foot running through the whole force plate at the 3.0 ± 0.5 m/s. The surface electromyogram (EMG) wireless 32-channel system (Delsys, Boston, MA, USA) collected participants’ muscle activities during

were required to run along the walkway with the right foot stepping on the force plate, six successful trials were finally captured. One successful trial was defined as the participant’s right foot running through the whole force plate at the 3.0 ± 0.5 m/s. The surface electromyogram (EMG) wireless 32-channel system (Delsys, Boston, MA, USA) collected participants’ muscle activities during the running phase. Muscle activity in- cluded vastus lateralis (VL), vastus medialis (VM), medial gastrocnemius (MG), lateral gastrocnemius (LG), soleus muscle (SL), flexor hallucis longus (FHL), and extensor digi- torum longus (EDL) and were collected at a frequency of 1000 Hz [36]. Maximal voluntary contractions (MVC) of the muscles were performed for the normalization of muscle activ- ity (0–100%) following a previously established protocol [37]. 2.4. Data Processing Trials were processed using Vicon Nexus 1.8.5 (Vicon, Metrics Ltd., Oxford, UK), identifying anatomical and tracking markers before exporting as C3 D files. For each test, a stance phase is a right foot initial contact on the force plate to the right foot toe-off the force plate [38]. Biomechanical data were processed using Visual 3 D (V6.0, C-Motion, Germantown, MD, USA). A low pass Butterworth filter with cut-off frequencies of 20 Hz (kinetic) and 10 Hz (kinematic) was applied [39]. Then, the joint moment and joint angle Figure 1. Outlined in this illustration, the left running shoe has the 8 mm HTD (minimalist shoes) and the right running shoe has with 9 mm HTD (normal shoes).

Healthcare2023,11, 1243 4 of 13 2.3. Data Collection Before the test, all the participants were familiarized with test process. During data collection, running shoe conditions were assigned randomly to the participants. They were asked to make themselves adapt to each running shoes. Then runners were randomized and wore the experience running shoes to run through a 10 m walkway. A device regulating the subjects' speed was placed on either side of the force platform (smart speed, Fusion Sport Inc. of Burbank, CA, USA). Equipment for assessing speed was located 3.0 m away. Speed was controlled at 3.0 0.5 m/s [32]. Thirty-eight re ective markers (diameter:14 mm) were attached to the bilaterally lower limbs, torso, and head according to the Opensim Gait 2392 model [34]. The marker coordinates were captured using an eight-camera Vicon motion analysis system (Oxford Metrics Ltd., Oxford, UK) at a frequency of 200 Hz. A force plate (Kistler Type, 9281 B, Kistler Instrument AG, Winterthur, Switzerland) was utilized to collect the ground reaction force (GRF) at 1000 Hz. The markers were placed on: Sternum, R. Acromion, L. Acromion, Toe. Head, R. ASIS, L. ASIS, V. Sacral, Thigh. Upper, R. Thigh. Front, R. Thigh. Rear, R. Knee. Lat, R. Knee. Med, R. Shank. Upper, R. Shank. Front, R. Shank. Rear, R. Ankle. Lat, R. Ankle. Med, R. Heel, R. Midfoot. Sup, R. Midfoot. Lat, R. Toe. Lat, R. Toe. Med, R. Toe. Tip, L. Thigh. Upper, L. Thigh. Front, L. Thigh. Rear, L. Knee. Lat, L. Knee. Med, L. Shank. Upper, L. Shank. Front, L. Shank. Rear, L. Ankle. Lat, L. Ankle. Med, L. Heel, L. Midfoot. Sup, L. Midfoot. Lat, L. Toe. Lat, L. Toe. Med, L. Toe. Tip [35]. All participants completed a static calibration and ran at a pace of 3.0 m/s over a at runway in a standard sports biomechanics Lab. All subjects were required to run along the walkway with the right foot stepping on the force plate, six successful trials were nally captured. One successful trial was de ned as the participant's right foot running through the whole force plate at the

ran at a pace of 3.0 m/s over a at runway in a standard sports biomechanics Lab. All subjects were required to run along the walkway with the right foot stepping on the force plate, six successful trials were nally captured. One successful trial was de ned as the participant's right foot running through the whole force plate at the 3.0 0.5 m/s. The surface electromyogram (EMG) wireless32-channelsystem (Delsys, Boston, MA, USA) collected participants' muscle activities during the running phase. Muscle activity included vastus lateralis (VL), vastus medialis (VM), medial gastrocnemius (MG), lateral gastrocnemius (LG), soleus muscle (SL), exor hallucis longus (FHL), and extensor digitorum longus (EDL) and were collected at a frequency of1000 Hz [36]. Maximal voluntary contractions (MVC) of the muscles were performed for the normalization of muscle activity (0–100%) following a previously established protocol [37]. 2.4. Data Processing Trials were processed using Vicon Nexus 1.8.5 (Vicon, Metrics Ltd., Oxford, UK), identifying anatomical and tracking markers before exporting as C3 D les. For each test, a stance phase is a right foot initial contact on the force plate to the right foot toe-off the force plate [38]. Biomechanical data were processed using Visual 3 D (V6.0, C-Motion, Germantown, MD, USA). A low pass Butterworth lter with cut-off frequencies of 20 Hz (kinetic) and 10 Hz (kinematic) was applied [39]. Then, the joint moment and joint angle parameters were exported. EMG data were band-passed (20–480 Hz), full-wave-recti ed, and low-passed at 6 Hz before being amplitude-normalized by the highest signal value across all gait trials [28]. The strike index was calculated as the ratio of the location of the center of pressure at the foot strike to the length of the foot. A strike index of 0–33% indicates a rearfoot striker, a strike index of 34–67% indicates a midfoot striker, and a strike index of 68–100% indicates a forefoot striker [40]. 2.5. Muscle Force Estimation The musculoskeletal estimates were completed using the Opensim (vers. 4.3, Open- Sim) (Figure), which has been used to calculate the muscle force during the running stance [35,36]. The Opensim gait 2392 model was used

The study investigates the effects of shoe heel-to-toe drop on muscle activation in runners.