The effect of foam padding on the head response in soccer heading

Fu Yang Tan; Mohd Hasnun Arif Hassan; Nasrul Hadi Johari; Mohd Nadzeri Omar; Iskandar Hasanuddin

doi:10.4103/mohe.mohe_37_22

ORIGINAL ARTICLE

Year : 2022 | Volume : 11 | Issue : 2 | Page : 108-114

The effect of foam padding on the head response in soccer heading

Fu Yang Tan¹, Mohd Hasnun Arif Hassan¹, Nasrul Hadi Johari¹, Mohd Nadzeri Omar¹, Iskandar Hasanuddin²
¹ Human Engineering Research Group (HUMEN), Faculty of Mechanical and Automotive Engineering Technology, Universiti Malaysia Pahang, Pahang, Malaysia
² Department of Mechanical and Industrial Engineering, Faculty of Engineering, Universitas Syiah Kuala, Banda Aceh, Indonesia

Date of Submission	27-Dec-2022
Date of Decision	29-Dec-2022
Date of Acceptance	31-Dec-2022
Date of Web Publication	22-Feb-2023

Correspondence Address:
Mohd Hasnun Arif Hassan
Human Engineering Research Group (HUMEN), Faculty of Mechanical and Automotive Engineering Technology, Universiti Malaysia Pahang, Pahang
Malaysia

Source of Support: None, Conflict of Interest: None

DOI: 10.4103/mohe.mohe_37_22

Abstract

Background: Soccer is regarded as the most popular sports in the world, with millions of people are actively involved in the game. Being a contact sports in nature, soccer players are susceptible to various kinds of injuries such as lower extremities muscle injury. In addition to those familiar injuries that soccer players sustain during the game, traumatic brain injury is also a possibility. Head impacts in soccer could be a result of head-to-head impact with an opponent player, head-to-elbow impact, an impact with the goal post, impact with the ground, as well as an impact with the soccer ball, which occurs during a heading manoeuvre. Soccer allows the players to use their head to hit the ball to pass it to a teammate, or even perform heading to score goal. Although soccer heading is perceived as less harmful as compared to head impacts with other hard objects, many studies have shown compelling evidence that this repetitive heading might harm the brain, thereby leading to traumatic brain injury. Protective headgears designed, especially for soccer players have been commercially available in the market for some years.
Objective: This article investigates the effectiveness of two padding foams by means of heading experiment.
Methodology: An anthropometric test device known as Hybrid III head and neck dummy instrumented with an inertial sensor that consists of a triaxial accelerometer and gyroscope installed at the centre of gravity of the head was used in the experiment. A soccer ball launching machine was used to propel the ball at several inbound velocities. Peak linear acceleration and peak rotational acceleration (PRA) were recorded, and the head injury criterion (HIC) and the rotational injury criterion (RIC) were calculated.
Results: Poron X-ray diffraction (XRD) foam was found to provide the best protection in both linear and rotational components. However, for protection against HIC, it is evident from the findings that both foams were very effective at lower inbound ball velocity and became drastically ineffective as the inbound velocity increases up to 22 m/s (i.e. equal to 79.2 km/h). This could be attributed to the foam being completely compressed by the high-velocity ball impacting it. The same situation is seen for RIC, however with less significant decline in protective performance.
Conclusion: Overall, it can be concluded that the Poron XRD foam could be a better foam to be used in soccer headgear as compared to the yoga mat foam.

Keywords: Head injury, head injury criterion, padding foam, rotational injury criterion, soccer heading

How to cite this article:
Tan FY, Hassan MH, Johari NH, Omar MN, Hasanuddin I. The effect of foam padding on the head response in soccer heading. Malays J Mov Health Exerc 2022;11:108-14

How to cite this URL:
Tan FY, Hassan MH, Johari NH, Omar MN, Hasanuddin I. The effect of foam padding on the head response in soccer heading. Malays J Mov Health Exerc [serial online] 2022 [cited 2023 Sep 25];11:108-14. Available from: http://www.mohejournal.org/text.asp?2022/11/2/108/370245

Introduction

Soccer is considered the most popular sports in the world with a participation of millions of people. A publication by the International Federation of Football Association (FIFA) in 2007 revealed that there were some 270 million people actively involved in soccer (International Federation of Football Association, 2007). That was 15 years ago, and the numbers could have been doubled or tripled by now. In a contact sports such as soccer, sustaining injuries is a definite possibility. Soccer players are susceptible to various kinds of injuries such as ankle injury, knee injury, as well as concussion, to name a few (Koutures et al., 2010). In a study conducted in 2021, soccer was found to result in the highest rate of injury in 1000 h of playing (Prieto-González et al., 2021). Although some regard soccer as less aggressive as compared to the American football or ice hockey, the rate of concussion in soccer is comparable to that of both aforementioned sports (Hubertus et al., 2019; Mooney et al., 2020; Wahlquist and Kaminski, 2021).

Concussion in soccer can be a result of head impact with other player's head, elbow, goal post, ground, as well as impact with the ball when performing soccer heading (Hassan et al., 2018a). It was reported that player-to-player contact is the major cause of concussion in soccer (Comstock et al., 2015). More than half of the reported concussion cases in soccer happened in soccer heading encounters (Andersen et al., 2004). It was also reported in the same study that 41% of concussion in soccer were sustained due to impacts with another player's elbow, arm or hand (Andersen et al., 2004). With regard to gender, studies have shown that female soccer players have a higher tendency to sustain concussion as compared to the male players (Caccese et al., 2018; Dave et al., 2022; Weiner et al., 2022). Nevertheless, it has been reported that almost 40% of soccer players who have sustained concussion during the match did not report the incidence, thereby could lead to negative consequences such as a higher tendency to sustain further concussion in the following matches (Nelson and Albright, 2020).

Although studies have shown that player-to-player contact is the main cause of concussion among soccer players, purposeful heading of the ball in soccer was also considered one of the main factors of concussion. The difference between head impacts resulted from player-to-player contact and soccer heading is that a single heading in soccer may not be harmful, but the repetitive impact might. It was reported that a soccer player may have performed more than 600 headings in a year (Levitch et al., 2018). Poor neuropsychological performance such as psychomotor speed, attention task and working memory has been associated with heading frequency (Bruno and Rutherford, 2021; Stewart et al., 2018). These are some of many evidence that suggests repetitive head impact due to soccer heading may be harmful to the brain.

Studies have been conducted to investigate the effect of soccer heading on the brain function of soccer players. The neuropsychological function of soccer players was evaluated using a validated structured questionnaire known as HeadCount-2w, which evaluates the exposure to heading within 2 weeks (Lipton et al., 2018). High exposure of soccer heading within a time span of 12 months was found to impair learning ability among women soccer players (Ye et al., 2022). Another study found that recent heading (within 2 weeks before testing) and long-term heading (within 12 months before testing) resulted in neuropsychological functions (Levitch et al., 2018). In addition to HeadCount, other methods of neuropsychological evaluation were conducted, with similar results that demonstrate suspected concussion due to heading and poor working memory among soccer headers that are essential for daily life (Ashton et al., 2021).

In addition to the neuropsychological test, magnetic resonance imaging technique, known as the diffusion tensor imaging, was conducted to evaluate the effect of repetitive heading on the player's brain. A study on 37 amateur soccer players has found that repetitive heading could result in abnormal white matter microstructure as well as poor neurocognitive performance (Lipton et al., 2013). A recent Canadian study has also reached a similar conclusion that repetitive head impacts led to a lower value of fractional anisotropy in those who had been subjected to repetitive subconcussive head impacts, which suggests a degraded brain function (Lefebvre et al., 2021). It was also found that female players experienced more changes in the white matter microstructure as a result of soccer heading as compared to male players (Rubin et al., 2018). This could be attributed to the weaker neck muscle of female players.

Furthermore, computer simulations such as finite element analysis (FEA) (Hassan et al., 2018b; Hassan and Taha, 2015) and mathematical model (Taha et al., 2013; 2015) have also been conducted to study the effect of soccer heading on the brain. It was demonstrated that the brain motion induced by soccer heading could be represented by a mass-spring-damper model with acceptable accuracy (Taha et al., 2015). A more advanced FEA allows the researcher to study in detail the underlying mechanism of brain injury due to soccer heading such as the influence of ball impact angle on brain deformation (Hassan et al., 2022). To perform FEA analysis on soccer heading, a validated human head finite element model (Hassan et al., 2020) is required, in addition to a validated finite element model of soccer ball (Taha and Hassan, 2017).

The objective of this study is to evaluate the effectiveness of two commercial soccer headgear, the Full90 and the ForceField headgears. Both headgears were designed, especially for soccer players. However, their performance in reducing the head injury risk when performing soccer heading has not been studied well. The headgears will be tested in a simulated soccer heading experiments on an anthropometric test device (ATD) known as Hybrid III head-and-neck dummy. The head injury predictor (i.e. the parameter used to measure the severity of a head impact) used to evaluate the effectiveness of the headgears are the peak linear acceleration, peak rotational acceleration (PRA), head injury criterion (HIC) and rotational injury criterion (RIC). We hypothesise that both foam paddings will not reduce the HIC and RIC significantly as compared to barehead heading. The following sections explain the methodology of the study.

Methodology

Soccer ball launching machine

To perform soccer heading experiment, a soccer ball launching machine that can operate with adjustable speed is needed. A bespoke machine was developed, as shown in [Figure 1]. It utilises a two counter-rotating wheels mechanism to propel the ball forward. The machine comprises of Arduino Uno as a microcontroller, HC-06 Bluetooth module and two alternating current (AC) motors having a power of 150 W, 1.15 Nm torque and a speed of 1245.56 revolution/min. The machine is able to launch the ball forward at a speed of up to 24 m/s (Hassan et al., 2018). The AC motor is an economical option; thereby it is used to spin the wheel. The machine was configured to shoot the ball at its maximum speed, which produces an average inbound ball velocity of 22.76 m/s. It is essential to use a soccer ball launching machine to ensure the repeatability of the experiment.

Figure 1: The soccer ball launching machine

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Anthropometric test device

To perform the soccer heading experiment, an ATD, known as Hybrid III headform and neckform dummy, was used, as shown in [Figure 2]. Hybrid III represents the 50^th percentile male head and neck, that weigh 4.54 kg and 1.54 kg, respectively (Clark et al., 2018). Hybrid III crash test dummy has been widely used in evaluating the safety performance of passenger cars. In addition, it has been used by many researchers to evaluate the effectiveness of protective helmets in sports (Clark et al., 2018; 2021; Oeur et al., 2021).

Figure 2: Hybrid III headform and neckform

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The dummy is mounted on a platform made of aluminium profiles, and the movement of the dummy is locked in every direction, thereby not allowing it to move back and forth, or left and right on impact. This was done to replicate an actual soccer heading, in which the player is fully aware of the incoming ball. When performing intentional soccer heading, the players usually strengthen their neck muscle and torso to head the ball. Hence, the ATD was fixed on the platform.

Inertial sensor

To measure the severity of head impact due to soccer heading, Shimmer3 200 g inertial sensor (Shimmer, Ireland), as shown in [Figure 3]a, was used. The sensor consists of a triaxial accelerometer with a capability to measure linear acceleration up to ± 200 g, and a gyroscope that can measure angular velocity up to ± 2000°/s. The sensor was placed inside the Hybrid III head at its centre of gravity, as depicted in [Figure 3]b. The capability of the inertial sensor in measuring linear acceleration up to 200 g makes it suitable to measure head impact in soccer heading. Both accelerometer and gyroscope record the data at 1000 Hz. The data were streamed to a laptop through Bluetooth.

Figure 3: (a) Shimmer3 200g inertial sensor, (b) Sensor installed at the centre of gravity of the Hybrid III head

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Experimental setup

The experiment was conducted outdoor due to the rebound of the ball after impact. The soccer ball launching machine was slightly elevated and placed near to the Hybrid III head dummy (aprrox. 1 m away) and positioned accordingly such that the ball propelled by the machine will hit the forehead of the head dummy, as shown in [Figure 4]. Five trials were performed for every condition. Only the trials, which the ball hits the centre of the forehead were considered acceptable. Otherwise, the experiment was repeated.

Figure 4: The experimental setup

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The inbound ball velocity was measured using an iOS application, SpeedClock – Video Radar developed by Sten Kaiser. The application utilises the iPhone slow-motion camera function that records the footage at 240 frames/s. The application tracks the movement of the ball in each frame, as shown in [Figure 5]. The diameter of the ball (which is 22 cm) was used as the basis to measure the distance the ball travels. The application converts this distance travelled by the ball in pixels into metre, thereby calculates the velocity of the ball by dividing this distance by the time taken. To measure the ball velocity, we chose the last five frames just before the ball hits the dummy head. The accuracy of this application was proven to be comparable to that of timing lights (Stanton et al., 2016). We found that the use of the SpeedClock application is satisfactory in a soccer heading experiment.

Figure 5: Measurement of the ball inbound velocity in SpeedClock application

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Foam padding

The objective of this study is to evaluate the effectiveness of commercial soccer headgear. Two foam paddings were used in this study, namely a Poron X-ray diffraction (XRD) foam and a yoga mat foam. [Figure 6] illustrates both foam paddings. The experiment was also conducted on the head dummy without wearing any headgears. This serves as the 'control group' or the baseline in evaluating the effectiveness of the foam paddings. This study is only limited to evaluating the performance of both foams in soccer heading situation and not in an impact with objects other than the soccer ball.

Figure 6: (a) Poron XRD foam, (b) Yoga mat foam. XRD: X-ray diffraction

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Data analysis

As mentioned earlier, the data recorded by the inertial sensor were streamed to a laptop, which was then used for further analysis. Since the output of linear acceleration generated in all three axes was in millivolts (mV), to obtain linear accelerations in m/s², the values of linear acceleration need to be converted in MATLAB. This was achieved by exporting the uncalibrated data from the 200 g accelerometer. Then, the Shimmer3 200 g sensor was placed on the surface such that the positive X-axis is pointing in the direction of the gravity vector (vertically up), hence pointing away from the direction of the gravity vector (vertically down). These steps were repeated for both the remaining Y-axis and Z-axis to obtain the offset, sensitivity and alignment matrices. After that, the calibration parameters were applied to the uncalibrated accelerometer output using the available calibration code to derive a calibrated 200 g accelerometer output into m/s².

In the meantime, the gyroscope measures the angular velocity in all three axes. Therefore, to obtain the angular accelerations in each respective axis, the values of angular velocities were numerically differentiated. To avoid the accumulation of errors, the angular velocities were initially filtered using the Butterworth filter before the differentiation. This was applied using add-ins in Microsoft Excel developed by Wassenbergh (2007), where a cut-off frequency of 167 Hz was applied which corresponds to the one-sixth of the sampling rate of the Shimmer sensor of 1024 Hz. Finally, the resultant linear and angular accelerations were calculated.

The data obtained from the experiment were further used to calculate the linear impact severity indexes HIC and rotational impact severity indexes RIC during the time of impact using Microsoft Excel using the following equations:

where a_lin(t) represents linear acceleration in g while a_rot(t) represents rotational acceleration in rad/s² as a function of time, respectively and t₂–t₁ represents the time span of 15 ms which the HIC and RIC were differentiated (Dunn et al., 2020).

Results and Discussion

The aim of this study is to evaluate the effect of putting foam padding on the forehead of the ATD on the head response in the soccer heading situation. Two foam paddings were used in the study, which are the Poron XRD foam and a yoga mat foam. The simulated soccer heading generates six head kinematics data, namely the linear accelerations in all three axes and the angular velocities in all three axis. The linear accelerations were used to calculate the HIC, and the angular velocities were differentiated and used to calculate the RIC. The HIC indicates the linear component of head injury severity, while the RIC shows the rotational component of head injury severity.

[Figure 7] shows the calculated maximum HIC of all three conditions: Barehead, with Poron XRD foam, and with yoga mat foam. It is noted that both foam paddings reduce the HIC at lower ball inbound velocity, with the Poron XRD foam provided a better protection. However, it is evident that the performance of both foams reduces as the inbound ball velocity increases. At the inbound velocity of 22 m/s and beyond, the protective performance of both foams is significantly reduced. The trend of the percentage of HIC reduction when using the foams can be better visualised in [Figure 8].

Figure 7: The maximum HIC₁₅ of barehead impact, with Poron XRD foam and with yoga mat foam. HIC: Head injury criterion, XRD: X-ray diffraction

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Figure 8: The percentage of maximum HIC reduction across the inbound ball velocity tested. HIC: Head injury criterion

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Based on the percentage of HIC reduction, it is apparent that Poron XRD foam outperforms the yoga mat foam. At 18 m/s inbound ball velocity, the Poron XRD foam reduces the maximum HIC by 120% [the negative sign in [Figure 8] indicates a reduction], while the yoga mat foam reduces the maximum HIC by almost 40%. As the inbound ball velocity increases, the percentage of reduction decreases. At 22 m/s inbound ball velocity, the maximum HIC was reduced by about 20% for Poron XRD foam and almost reaching 0% for yoga mat foam. It is a 100% difference for Poron XRD foam across 4 m/s inbound velocity (18–22 m/s) and 40% difference for yoga mat foam. We speculate that this occurs due to the fact that the foam might have been compressed fully or at least more than 80% compressed at large inbound velocity, thereby reducing its capability of mitigating the HIC.

[Figure 9] depicts the comparison of maximum RIC across the tested inbound velocity for barehead impact, as well as impacts with Poron XRD foam and the yoga mat foam. As mentioned earlier, RIC indicates the rotational components of the head impact, derived from the angular accelerations of the head. It is evident in [Figure 9] that Poron XRD is superior to the yoga mat foam as well. However, a different trend is observed between the RIC and the HIC. It is observed that both foams seem to provide a steady reduction of RIC as the inbound ball velocity increases.

Figure 9: The maximum RIC₁₅ of barehead impact, with Poron XRD foam and with yoga mat foam. RIC: Rotational injury criterion, XRD: X-ray diffraction

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The trend of RIC reduction can be clearly seen in [Figure 10], which illustrates the percentage of RIC reduction across the inbound velocity tested (between 18 and 22 m/s). The percentage of reduction again demonstrates the superiority of Poron XRD foam in protecting the head while performing soccer heading. At 18 m/s inbound velocity, there is an almost 90% reduction by Poron XRD foam, while there is only about 30% reduction by yoga mat foam. A similar, but less drastic trend is seen when it comes to the protective performance of both foams, where the protection decreases as the inbound ball velocity increases. However, from 18 m/s to 22 m/s, the decrement of protection by Poron XRD foam and yoga mat foam is only 35% and 12%, respectively. This suggests that the protection against the rotational component of head impact is not significantly influenced by the inbound ball velocity, as what have been seen in the linear component of head impact (the HIC).

Figure 10: The percentage of maximum RIC reduction across the inbound ball velocity tested. RIC: Rotational injury criterion

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The experiment was conducted to study the effect of padding foam on the head response in terms of HIC and RIC when performing soccer heading. It was found that both foams were able to reduce both HIC and RIC, with Poron XRD producing a much better reduction than the yoga mat foam. With regard to HIC that indicates the linear component of the head impact, both foams performed well at low-speed ball impact, but the performance reduced significantly as the inbound ball velocity increases. This might be attributed to significant deformation of both foams at higher velocity impact. We speculate that the foams might have been fully compressed by the ball, thereby its capability to reduce the HIC degrades with the increase of the ball inbound velocity.

With respect to the rotational component of the head impact represented by the RIC, different behaviours can be observed. Poron XRD foam still outperformed the yoga mat foam. However, the performance of both foams in reducing the RIC is much more consistent across the inbound ball velocities tested. This shows that although the foams were compressed significantly by the fast-travelling ball, both foams were able to still protect the brain against the rotational component of head injury. However, as the inbound ball velocity increases, we did notice a slight reduction in the performance of both foams. We believe that there is a need to improve soccer headgear design to provide a better protection to the soccer player's brain when performing soccer heading. Using only foam in the headgear may not be effective within the range of ball-to-head impact velocity that may occur in the soccer heading situation. A combination of foam and plastic could be explored in searching for an optimal soccer headgear design. The addition of plastic may prevent the foam from being fully compressed by the ball, thereby maintaining its protective performance at higher ball velocity.

Conclusion

In this study, soccer heading experiments were conducted to evaluate the effectiveness of two foam paddings, namely the Poron XRD foam and a yoga mat foam in providing the protection against head injury when performing soccer heading. In general, both foams provided some protection against the linear component of head impact indicated by the HIC and the rotational component of head impact indicated by the RIC. Poron XRD foam was found to provide the best protection in both linear and rotational components. However, for protection against HIC, it is evident from the findings that both foams were very effective at lower inbound ball velocity and became drastically ineffective as the inbound velocity increases up to 22 m/s (i.e. equal to 79.2 km/h). This could be attributed to the foam being completely compressed by the high-velocity ball impacting it. The same situation is seen for RIC, however with a less significant decline in protective performance. Overall, it can be concluded that the Poron XRD foam could be a better foam to be used in soccer headgear as compared to the yoga mat foam. However, some improvements need to be introduced in the headgear design to compensate for its low performance at higher inbound ball velocity. A mixture of foam and plastic may provide a better protection, thereby warrants further investigation.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

References

1.	Andersen, T. E., Árnason, Á., Engebretsen, L., & Bahr, R. (2004). Mechanisms of head injuries in elite football. British Journal of Sports Medicine, 38(6), 690-696. doi: 10.1136/bjsm.2003.009357.
2.	Ashton, J., Coyles, G., Malone, J. J., & Roberts, J. W. (2021). Immediate effects of an acute bout of repeated soccer heading on cognitive performance. Science and Medicine in Football, 5(3), 181-187. doi: 10.1080/24733938.2020.1846769.
3.	Bruno, D., & Rutherford, A. (2021). Cognitive performance in former professional football (soccer) players is affected by estimated heading frequency. Alzheimer's & Dementia, 17(S10), e057577. doi: 10.1002/alz.057577.
4.	Caccese, J. B., Buckley, T. A., Tierney, R. T., Rose, W. C., Glutting, J. J., & Kaminski, T. W. (2018). Sex and age differences in head acceleration during purposeful soccer heading. Research in Sports Medicine, 26(1), 64-74. doi: 10.1080/15438627.2017.1393756.
5.	Clark, J. M., Connor, T. A., Post, A., Hoshizaki, T. B., & Gilchrist, M. D. (2021). The influence of impact surface on head kinematics and brain tissue response during impacts with equestrian helmets. Sports Biomechanics, 20(6), 737-750. doi: 10.1080/14763141.2019.1599062.
6.	Clark, J. M., Hoshizaki, T. B., & Gilchrist, M. D. (2018). Assessing women's lacrosse head impacts using finite element modelling. Journal of the Mechanical Behavior of Biomedical Materials, 80, 20-26. doi: 10.1016/j.jmbbm.2018.01.020.
7.	Comstock, R. D., Currie, D. W., Pierpoint, L. A., Grubenhoff, J. A., & Fields, S. K. (2015). An evidence-based discussion of heading the ball and concussions in high school soccer. JAMA Pediatrics, 169(9), 830-837. doi: 10.1001/jamapediatrics.2015.1062.
8.	Dave, U., Kinderknecht, J., Cheng, J., Santiago, K., Jivanelli, B., & Ling, D. I. (2022). Systematic review and meta-analysis of sex-based differences for concussion incidence in soccer. Physician and Sportsmedicine, 50(1), 11-19. doi: 10.1080/00913847.2020.1868955.
9.	Dunn, M., Davies, D., & Hart, J. (2020). Effect of football size and mass in youth football head impacts. Proceedings, 49(1), 29. doi: 10.3390/proceedings2020049029.
10.	Hassan, M. H. A., Taha, Z., Shaharudin, M. A. H., Wee, L. K., & Anuar, Z. (2018). Development of a soccer ball launching device. In Lecture Notes in Mechanical Engineering (pp. 591-598). Singapore: Springer. Retrieved from https://doi.org/10.1007/978-981-10-8788-2_53. [Last accessed on 2022 Dec 20].
11.	Hassan, M. H. A., & Taha, Z. (2015). Finite element analysis of soccer heading. Procedia Engineering, 112, 46-51. doi: 10.1016/j.proeng.2015.07.174.
12.	Hassan, M. H. A., Mohd Anni, M. A., Tan, F. Y., Johari, N. H., & Omar, M. N. (2022). The influence of ball impact angle on the brain deformation in soccer heading: A finite element analysis. In Lecture Notes in Mechanical Engineering. 313: 320. Retrieved from https://doi.org/10.1007/978-981-16-4115-2_25. [Last accessed on 2022 Dec 20].
13.	Hassan, M. H. A., Taha, Z., Hasanuddin, I., & Mohamed Mokhtarudin, M. J. (2018a). Concussion in soccer. In SpringerBriefs in Applied Sciences and Technology (pp. 1-9). Singapore: Springer. Retrieved from https://doi.org/10.1007/978-981-13-0271-8_1. [Last accessed on 2022 Dec 20].
14.	Hassan, M. H. A., Taha, Z., Hasanuddin, I., & Mohamed Mokhtarudin, M. J. (2018b). Simulation of soccer heading manoeuvre. In SpringerBriefs in Applied Sciences and Technology (pp. 29-37). Singapore: Springer. Retrieved from https://doi.org/10.1007/978-981-13-0271-8_4. [Last accessed on 2022 Dec 20].
15.	Hassan, M. H. A., Taha, Z., Hasanuddin, I., Majeed, A. P. P. A., Mustafa, H., & Othman, N. A. (2020). A simplified human head finite element model for brain injury assessment of blunt impacts. Journal of Mechanical Engineering and Sciences, 14(2), 6538-6547. doi: 10.15282/JMES.14.2.2020.01.0513.
16.	Hubertus, V., Marklund, N., & Vajkoczy, P. (2019). Management of concussion in soccer. Acta Neurochirurgica, 161(3), 425-433. doi: 10.1007/s00701-019-03807-6.
17.	International Federation of Football Association. (2007). FIFA Big Count 2006: 270 Million People Active in Football. FIFA Communications Division, Information Services.
18.	Koutures, C. G., Gregory, A. J. M., McCambridge, T. M., Benjamin, H. J., Brenner, J. S., Cappetta, C. T., Demorest, R. A., Halstead, M. E., LaBella, C. R., Martin, S. S., & Rice, S. G. (2010). Clinical report – Injuries in youth soccer. Pediatrics, 125(2), 410-414. doi: 10.1542/peds.2009-3009.
19.	Lefebvre, G., Guay, S., Chamard, E., Theaud, G., De Guise, E., Bacon, B. A., Descoteaux, M., De Beaumont, L., & Théoret, H. (2021). Diffusion tensor imaging in contact and non-contact university-level sport athletes. Journal of Neurotrauma, 38(5), 529-537. doi: 10.1089/neu.2020.7170.
20.	Levitch, C. F., Zimmerman, M. E., Lubin, N., Kim, N., Lipton, R. B., Stewart, W. F., Kim, M., & Lipton, M. L. (2018). Recent and long-term soccer heading exposure is differentially associated with neuropsychological function in amateur players. Journal of the International Neuropsychological Society, 24(2), 147-155. doi: 10.1017/S1355617717000790.
21.	Lipton, M. L., Ifrah, C., Stewart, W. F., Fleysher, R., Sliwinski, M. J., Kim, M., & Lipton, R. B. (2018). Validation of HeadCount-2w for estimation of two-week heading: Comparison to daily reporting in adult amateur player. Journal of Science and Medicine in Sport, 21(4), 363-367. doi: 10.1016/j.jsams.2017.08.008.
22.	Lipton, M. L., Kim, N., Zimmerman, M. E., Kim, M., Stewart, W. F., Branch, C. A., & Lipton, R. B. (2013). Soccer heading is associated with white matter microstructural and cognitive abnormalities. Radiology, 268(3), 850-857. doi: 10.1148/radiol.13130545.
23.	Mooney, J., Self, M., Refaey, K., Elsayed, G., Chagoya, G., Bernstock, J. D., & Johnston, J. M. (2020). Concussion in soccer: A comprehensive review of the literature. Concussion, 5(3), CNC76. doi: 10.2217/cnc-2020-0004.
24.	Nelson, N., & Albright, C. (2020). Injury reporting in collegiate soccer players and the impact of non-reporting. Internet Journal of Allied Health Sciences and Practice, 18(4), 1-5. doi: 10.46743/1540-580x/2020.1908.
25.	Oeur, R. A., Gilchrist, M. D., & Hoshizaki, T. B. (2021). Parametric study of impact parameters on peak head acceleration and strain for collision impacts in sport. International Journal of Crashworthiness, 26(1), 16-25. doi: 10.1080/13588265.2019.1634336.
26.	Prieto-González, P., Martínez-Castillo, J. L., Fernández-Galván, L. M., Casado, A., Soporki, S., & Sánchez-Infante, J. (2021). Epidemiology of sports-related injuries and associated risk factors in adolescent athletes: An injury surveillance. International Journal of Environmental Research and Public Health, 18(9), 4857. doi: 10.3390/ijerph18094857.
27.	Rubin, T. G., Catenaccio, E., Fleysher, R., Hunter, L. E., Lubin, N., Stewart, W. F., Kim, M., Lipton, R. B., & Lipton, M. L. (2018). MRI-defined white matter microstructural alteration associated with soccer heading is more extensive in women than men. Radiology, 289(2), 478-486. doi: 10.1148/radiol.2018180217.
28.	Stanton, R., Hayman, M., Humphris, N., Borgelt, H., Fox, J., Del Vecchio, L., & Humphries, B. (2016). Validity of a smartphone-based application for determining sprinting performance. Journal of Sports Medicine, 2016, 7476820. doi: 10.1155/2016/7476820.
29.	Stewart, W. F., Kim, N., Ifrah, C., Sliwinski, M., Zimmerman, M. E., Kim, M., Lipton, R. B., & Lipton, M. L. (2018). Heading frequency is more strongly related to cognitive performance than unintentional head impacts in amateur soccer players. Frontiers in Neurology, 9, 1-10. doi: 10.3389/fneur.2018.00240.
30.	Taha, Z., & Hassan, M. H. A. (2017). A reaction-force-validated soccer ball finite element model. Proceedings of the Institution of Mechanical Engineers, Part P: Journal of Sports Engineering and Technology, 231(1), 43-49. doi: 10.1177/1754337115626636.
31.	Taha, Z., Hassan, M. H. A., & Hasanuddin, I. (2015). Analytical modelling of soccer heading. Sadhana – Academy Proceedings in Engineering Sciences, 40(5), 1567-1578. doi: 10.1007/s12046-015-0383-5.
32.	Taha, Z., Hassan, M. H. A., Aris, M. A., & Anuar, Z. (2013). Predicting brain acceleration during heading of soccer ball. IOP Conference Series: Materials Science and Engineering, 50(1), 012023. doi: 10.1088/1757-899X/50/1/012023.
33.	Wahlquist, V. E., & Kaminski, T. W. (2021). Purposeful heading in youth soccer: A review. Sports Medicine, 51(1), 51-64. doi: 10.1007/s40279-020-01376-8.
34.	Weiner, A. R., Durbin, J. R., Lunardi, S. R., Li, A. Y., Hannah, T. C., Schupper, A. J., Gal, J. S., Jumreornvong, O., Spiera, Z., Ali, M., Marayati, N. F., Gometz, A., Lovell, M. R., & Choudhri, T. F. (2022). Incidence and severity of concussions among young soccer players based on age, sex, and player position. Orthopaedic Journal of Sports Medicine, 10(1), 1-8. doi: 10.1177/23259671211059216.
35.	Ye, K., Fleysher, R., Lipton, R. B., Zimmerman, M. E., Stewart, W. F., Sliwinski, M. J., Kim, M., & Lipton, M. L. (2022). Repetitive soccer heading adversely impacts short-term learning among adult women. Journal of Science and Medicine in Sport, 25(11), 935-941. doi: 10.1016/J.JSAMS.2022.08.011.