Documentation:FIB book/Boxing Injury Biomechanics

Introduction

The world of boxing has seen many anti-boxing campaigns due to the high rate of head injuries and fatal events in this sport. Baird et al. (2010) investigated the deaths in professional boxing and the variables related to mortality from 1950 to 2007. The death of Duk Koo Kim in 1983 was a turning point in the history of boxing, after which regulations changed, and the number of rounds was reduced from 15 to 12.

588 deaths were found between 1950 and 2007, from which 339 deaths were in professional boxing. The mean age of the dead boxers was 24 ± 3.8. 64% and 15% of all deaths were a result of KO (KO is the abbreviation for a knockout. A full knockout is considered any legal strike that renders an opponent unable to continue fighting.) and TKO (technical KO), respectively. Mortality was most common in Featherweight class. The United States saw the highest rate of mortality, with 23 percent of all deaths. Sixty-one percent of preterminal events (loss of consciousness) occurred in the ring.

Figure 1. Number of boxing mortalities by decades. DOI: 10.1227/01.NEU.0000373207.04297.13

After 1983, there was a decrease in mortality (219 vs. 120). There was no evidence that the reduced mortality rate after 1983 was because of the reduction in the number of rounds. In the past, boxers had a longer career time, there was not enough medical supervision during the match, the gloves were less safe, and games could last up to 40 rounds.

There is a significant relationship between KO and mortality. Therefore, medical groups should intervene as immediately as possible after a KO occurs. Another method to improve the mortality rate can be mandating all the boxers who have undergone a KO to go through neuroimaging and hospital evaluation.

Due to the tendency to cause severe injuries, boxing has increasingly drawn the attention of the public. Researchers have also done a tremendous amount of work attempting to understand the fundamental biomechanisms behind injuries endured in boxing and develop suitable protective devices such as headgear to attenuate the head impact. In this article, we will go through three major areas summarized from our literature search result:

1. biomechanical measurements in boxing

2. headgear used in boxing

3. comparison between boxing and other combat sports. The first part will discuss the biomechanical properties of boxing and how they can be measured, including peak linear and rotational acceleration, impact speed, peak force, as well as others.

The second part will focus on investigating how headgear is tested in boxing and how well it can protect boxers, and this part will also compare headgear used for boxing vs. other combat sports such as taekwondo. The last part will compare the biomechanical characteristics of boxing with other combat sports which are also very dangerous to the athletes, and compare their various fighting skills. At the end of this article, we will summarize our findings from reading all these research papers by exploring their strengths, limitations, controversies, and future directions.

Biomechanical Measurement in Boxing

One of the most critical parts in studying an injury mechanism is being capable of measuring biomechanical parameters such as acceleration, velocity, force, duration of impact, etc. This is an essential step because otherwise, we cannot compare different injurious scenarios or different protection devices in terms of how dangerous they are. Researchers have been trying various methods to measure biomechanical parameters in boxing. These efforts can be divided into two main categories: Ex-vivo, methods in which external devices such as dummy heads are used, and in-vivo, where researchers struggled to measure different parameters inside the ring from the head of real boxers. This section is aimed to summarize some of these studies.

Ex-Vivo

J Atha et al. (1985) attempted to determine the mechanical properties of a boxing punch by measuring the acceleration, velocity, and peak force of boxing punches. Data were gathered from a world-ranked professional heavyweight boxer, as he punched an instrumented, padded target mass suspended as a ballistic pendulum (Figure 2.). Within 0.1 seconds of the start, the punch had traveled 0.49 m and attained a velocity on the impact of 8.9 m/s. The peak force on the impact was 4096 N, attained within 14 ms of contact. This force can reach a value of 6320 N at the end of the impact duration. Thus, the peak force to the human head can be estimated as 6320 N. The transmitted impulse generated an acceleration of 520 m/$s^{2}$ in the target head. The blow is equivalent to swing a padded wooden mallet of 6 kg at 20 mph (Table 1).

Figure 2. The target mass used by Atha et al. instrumented with accelerometer, force transducer and markers. Punches were delivered to this platform. DOI: 10.1136/bmj.291.6511.1756

Walilko et al. (2005) focused on the biomechanics of the head for punches to the jaw and the risk of head injury from translational and rotational acceleration. The experiment was set up by seven Olympic boxers from five weight classes delivering 18 straight punches to the frangible face of the Hybrid III dummy (Figure 3.). Translational and rotational head acceleration, neck responses, and jaw pressure distribution were measured. The average punch force was 3427 N, and the effective punch mass was 2.9 kg, which resulted in 6343 rad/$s^{2}$ rotational acceleration and 994 N neck shear force. Hand reached a mean velocity of 9.14 m/s at the impact point. Generally, the severity of the punch increased with heavier weight classes (Table 1).

Table 1. A summary of the measured biomechanical parameters in two fundamental studies

Study	Number of tested boxers	Mean peak impact force (N)	Delta t (ms)	Mean impact speed (m/s)	Mean peak linear acceleration (g)	Mean peak rotational acceleration (rad/s^2)	HIC	Nij
Atha et al. 1985	1 (Heavyweight)	4096	14	8 to 9	53	NA	N/A	N/A
Walilko et al. 2005	7 (Five weight classes)	2625 (Middleweight) 4345 (Heavyweight)	11.4	9.14	58	6343	71	0.27

Figure 3. The experiment performed by Walilko et al. in which Olympic boxer punched an instrumented Hybrid III head. DOI: 10.1136/bjsm.2004.014126

In-Vivo

One problem when investigating the biomechanics of head for boxing is not being able to measure head accelerations during real matches. The HIT (Head Impact Telemetry) system was designed to measure real-time dynamics of the head with six accelerometers inside sporting helmets and was used in sports like football but not in boxing. Beckwith et al.(2007) validated an Instrumented Boxing Headgear (IBH) to measure the severity and location of the impact (Figure 4.). In the validation of the IBH system, an impactor hit a dummy head on the left aspect of the lower jaw.

The IBH consisted of 12 single-axis linear accelerometers instrumented inside a boxing headgear. Linear regression was used to evaluate the accuracy and repeatability of the IBH system through comparing the results (five variables: peak linear and rotational accelerations, Head Injury Criterion (HIC), Gadd Severity Index (GSI), and predicted locations) with the data collected from a Hybrid III head form. There was an acceptable correlation between the results from Hybrid III and IBH (Figure 5.). Peak linear and rotational acceleration: $r^{2}$ = 0.91; HIC: $r^{2}$= 0.88; GSI: $r^{2}$ = 0.89; Location error: 9.7±5.2°.

Figure 4. Linear relationship between IBH results and Hybrid III data. DOI: 10.1123/jab.23.3.238

Most of the studies on boxing punch biomechanics use Hybird III head form. The results from the dummy head may not be valid because of the lack of biofidelity of the jaw and head. Stojsih et al. (2008) used the IBH system to collect real-time biomechanical data of the head during sparring sessions from both male and female boxers. Assessments of cognitive function were done before and after the sessions using IMPACT software.

Peak translational and rotational accelerations, HIC and GSI, were significantly higher in male boxers. There was no significant difference in neurocognitive conditions between genders. Impacts on either the side or back of the head exhibited, on average, significantly higher accelerations and HIC scores than the frontal impacts. The neurocognitive assessment revealed a significant decrease in the delayed memory scores for both males and females. There was no significant change in other cognitive scores.

Discussion

Strengths

The work of Atha et al. was one of the first papers in boxing biomechanics, which became a basis for many future studies. Although the experimental equipment was rudimentary, the results are, to a reasonable extent, consistent with the future studies on dummy heads. The applied force to the head is reported 4069 N which is close to the 3427 N reported in future studies with Hybird III head form. The same is true for the reported punch velocity (8-9 versus 9.14 m/s).

The IBH system can be a beneficial device to improve the risk of head injuries. It can pave the way for correlating biomechanical parameters and clinical diagnosis for in-ring brain injuries, creating an extensive database for ringside physicians to decide on the severity of impacts and stop the game, if needed.

Controversies

One strange point was that the number of deaths was the highest in Featherweight class, which is a light class. On the other hand, Walilko et al. showed that generally mechanical parameters were higher for heavyweight class boxers than the lightweight class boxers. So, why mortal events were higher in Featherweight class? One explanation can be the fact that lightweight boxers are faster and deliver much more punches during one single match than heavyweight boxers. Also, neck muscles might be weaker in lightweight class and this can affect to what extent they can decelerate their heads.

As shown in table 2, the biomechanical parameters (accelerations and HIC) collected from sparring sessions using the IBH system were lower in magnitude than the previously reported values in the literature, possibly because in previous studies, the boxers were asked to apply their best punches. However, in sparring sessions, we see a wide variety of punches and usually no heavy punches, because boxers are training. The interesting point was that the mean acceleration of frontal punches was less than back and side punches. This can be due to the higher frequency of frontal punches, preparing neck muscles to abate the impacts.

Table 2. A comparison of the measured parameters in the discussed three studies

Study	Number of tested boxers	Mean peak impact force (N)	Delta t (ms)	Mean impact speed (m/s)	Mean peak linear acceleration (g)	Mean peak rotational acceleration (rad/s^2)	HIC	Nij
Atha et al. 1985	1 (Heavyweight)	4096	14	8 to 9	53	N/A	N/A	N/A
Walkiko et al. 2005	7 (Five weight classes)	2625 (Middleweight) 4345 (Heavyweight)	11.4	9.14	58	6343	71	0.27
IBH (Beckwith JG et al. 2007)	30 male 30 female	N/A	N/A	N/A	30	2571	43	N/A

Limitations

Using a Hybird III head form was a common technique in most of the papers which were trying to simulate the punching scenario. Although using this method for comparison purposes seems reasonable, the biofidelity of the head structure of the Hybird III dummy is doubtful. Also, the jaw bone structure is an integral part of the punches, and it is not incorporated appropriately in Hybird III head forms. As experimental units used in different laboratories are not similar, it is difficult to compare the results in different studies. How the dummy head is mounted to the platform, the device used to land the punch, and different boxers with different levels of skill can add to the inconsistency between the studies. Furthermore, the skull bone, the jaw bone and the face’s soft tissue all have energy absorption capabilities which are not considered in these studies

Regarding the IBH system, possible slippage of the headguard and non-optimized instrumentation of the accelerometers to keep them away from direct impacts can be problematic, but the authors provide reasonable solutions.

Future directions

Future research should be more focused on finding more accurate methods of measurement. Also, the IBH system, if gets the permission to be used in real matches, can be used for creating a reliable database. This data can be used to identify the best injury criterion or injury predictor of severe brain injuries during boxing matches.

Protective Headgear

Headgear plays a critical role in attenuating the impact and increasing the protection of the boxers. This section will show some previous research on comparing the biomechanical response between wearing and not wearing headgear, comparing headgear used in boxing and other combat sports such as taekwondo, and then concludes with a discussion on the strengths, controversies, limitations, and future directions for this sub-area.

With vs. Without Headgear

Andrew S McIntosh and Declan A Patton (2015) provided an approach for measuring how well a boxing headgear can attenuate the head impact. In this approach, a spring driven linear impactor (shown in Figure 6.) was used to create a range of impacts at different speeds (between 4.1 and 8.3 m/s) and delivered the impacts to an instrumented Hybrid III Crash Test Dummy head and neck complex system with and without an AIBA (Association Internationale de Boxe Amateur)-approved headgear. The mass of the linear impactor was about 4 kg and a semi-rigid fist-glove interface was used to imitate more realistic boxing punches. Acceleration data show that there were large and/or significant differences in head-impact responses in the lateral and frontal tests between the AIBA-approved headgear and bare headform. More specifically, the following things were observed:

1. In the 8.3 m/s fist-glove impacts, mean peak resultant linear accelerations for bare headform were approximately 130 g compared with roughly only 85 g wearing the headgear.

2. In the 6.85 m/s fist-glove impacts, mean peak resultant angular head accelerations for bare headform were in the range of 5200–5600 rad/$s^{2}$ and nearly halved by wearing the headgear.

3. Linear and angular accelerations in 45° forehead and 60° jaw impacts were both reduced by the headgear to a large extent.

Overall, the article shows that existing AIBA-approved headgear can play a significant role in reducing head impact and the risk of head injury (measured by HIC15) in competitive boxing events where punch-speeds range from 5 to 9 m/s.

Figure 6. A: Disc-pad vs. B: Fist-glove interface used in a spring-driven linear impactor. DOI: 10.1136/bjsports-2015-095094

Boxing vs. Taekwondo Headgear

Aside from comparing the attenuation performance with vs. without headgear, DM O'Sullivan et al. (2016) compared the impact attenuation performance of boxing vs. taekwondo headgear by measuring the head's peak linear and rotational acceleration in a more recent study. A standardized martial arts headgear rotating striker (shown in Figure 7.) was used to impart impacts to a Hybrid III Crash Test Dummy head and neck complex system. Two boxing (Adidas and Greenhill) and two taekwondo (Adidas and Nike) headgear were selected and were fitted to the dummy head. For each headgear, the head was subsequently subjected to five impacts at the front and side locations by the striker at a speed of 8 ± 0.3 m/s. Linear and rotational acceleration were recorded throughout the experiment. The article' s results show that:

1. None of the headgear was able to pass the American Society for Testing and Materials (ASTM) criterion to reduce head acceleration below the cut-off threshold at 150 g.

2. There were significant interactions of the impact location and headgear brand on the rotational acceleration.

3. There were significant effects of both impact location and headgear brand on the linear acceleration.

4. Pairwise comparisons adjusted by Bon-ferroni correction show significant differences between the front and side locations for both linear and rotational acceleration.

The authors also indicated in the paper that increases in the thickness of headgear padding did not always correspond with a reduction in head acceleration. This points out that future studies should focus more on examining material properties that may increase headgear’s impact reduction to help better protect athletes in boxing, taekwondo, and other combat sports.

Figure 7. The rotating striker used for comparing boxing and taekwondo headgear. DOI: 10.1080/17461391.2016.1161073

Discussion

Strengths

These two papers aimed at investigating 1). how well headgear can protect the boxer's head and 2). the effect of wearing different headgear. Altogether they looked into several different punch regions: forehead (both), side (both), and jaw (one). Also, they both found that impact location (frontal vs. side) has a significant influence on the outcome.

Controversies

The two papers both used Hybrid III dummy head for the test and they used punch speeds around 8 m/s (both) and around 7 m/s (one). However, under similar speed conditions (8 m/s), the first paper has a much lower linear acceleration result (< 100 g) compared to the second one (> 200 g). The linear acceleration variance could be caused by inconsistent ways used to create the impact load, inconsistent mass of the impactor (3.88 kg vs. 4.5 kg), inconsistent mechanical properties of the headgear, whether the ‘fist’ interface wears a glove, whether the dummy head and neck complex were mounted to the body structure, and a lot of other considerations. In a nutshell, the inconsistent results in linear acceleration between the two papers show that the specific way a boxing-punch test is set up has a significant influence on the outcome and data measured.

Table 3. Compare the tests conducted by the two headgear papers

	Andrew S Mclntosh, Declan A Patton (2015)	DM O'Sullivan et al. (2016)
Mean impact speed (m/s)	8.34	8.00
Impactor mass (kg)	3.88	4.50
Headgear brand	TopTen	Adidas, Greenhills, Nike
Wearing glove	Yes	No
Dummy	Hybrd III head and neck complex	Hybrid III head and neck complex
Mean peak resultant linear acceleration (g)	< 100 (Able to pass ASTM criterion at 150 g)	> 200 (Unable to pass ASTM criterion at 150 g)

Limitations

All these punching tests on headgear involve using Hybrid dummy head and neck complex. However, the biofidelity of Hybrid III has never been verified for a boxing-punch scenario, which makes measurements in these two papers questionable in terms of how biofidelic the head's responses were. As we could imagine, repetitive punches on the head are not the same as those head impacts experienced in frontal car crashes.

Future directions

In the future, we hope that a more consistent test platform for boxing-injury-biomechanics research can be developed such that people can share and compare their results more reasonably and objectively. Furthermore, Hybrid III dummy's biofiedlity for boxing injury scenarios needs to be verified rigorously. Or new dummies for boxing studies need to be developed to resolve the concern over dummy's biofidelity.

Comparison Between Boxing and Other Combat Sports

Taekwondo is a traditional Korean martial art dating back more than 2000 years. It first appeared in Olympic games at 1988 and show up as a demonstration sport in the Olympic games until now. Karate is a traditional Japanese martial art developed over 1000 years ago from China. Taekwondo and Karate combine in total have over 130 million participants worldwide. Both Taekwondo and Boxing are the Olympic games, which scored by head hits and body shots. All three types of fighting styles are wildly using in UFC, mixed martial arts fighting game.

Therefore, we believe that Taekwondo and Karate are two good sports comparing with Boxing. Since the analysis on Taekwondo and Karate are relatively new, using results and methods from boxing analysis can accelerate our understanding of the mechanism of Taekwondo and Karate.

Boxing vs. Taekwondo

First investigation made by GP Fife et al. (2013) is to examine the difference between taekwondo kicks and boxing punches in head linear acceleration (RLA), head injury criterion (HIC15) and impact velocities. Hybrid II Crash Dummy head, which instrumented with a tri-axial accelerometer mounted inside (Figure 8.), was used for taekwondo head impacts. Hybrid III Crash Dummy head instrumenting with an array of tri-axial accelerometers mounted at the head center of gravity was used for boxing impact tests. From the data results, expect Clench axe，all accelerations produced by taekwondo techniques are greater than the maximum acceleration boxing skill, hook punch. Significant difference in RLA and HIC15 can be observed between turning kick (130.11 ± 51.67 g, 672.74 ± 540.89) and hook punch (71.23 ± 32.19 g, 78.96 ± 69.84). The authors made the conclusion that taekwondo kicks demonstrated significantly larger magnitudes than boxing punches for both RLA and HIC15.

Figure 8. Hybrid II head test setup for taekwondo studies. DOI: 10.5604/20831862.1077551

The second investigation was made by GP Fife et al. (2018) to compare rotational and linear head accelerations as a result of taekwondo kicks and punches. Turning kick, spinning hook kick, hook punch, straight punch, and jab punch executed by Taekwondo athletes on a Hybrid III Crash Test Dummy head, which instrumented with tri-axial accelerometer and an angular rate sensor mounted inside. linear (RLA) and rotational accelerations produced by different techniques were recorded and put into comparison. According to the results, taekwondo kicking techniques presented a higher magnitude in both mean linear accelerations, and mean rotational accelerations than taekwondo punching techniques. Turning kick has the largest linear and rotational accelerations among five techniques.

Boxing vs. Karate

ML Schwartz et al. (1986) compared the mechanical properties between karate kicks and boxing punches. To investigate the relative force of kicks and punches, a dummy head was mounted 175 cm above the floor (to simulate a 50th-percentile man standing erect) and 125 cm above the floor (to simulate the man in a crouched position) on a universal joint permitting motion about three axes. The mechanism was contrived to provide constant rotational stiffness, and springs provided constant restorative moments about the three axes. The texture of soft tissue was simulated by a mask of visco-elastic foamed materials. Fourteen karate experts punched and kicked the dummy. Accelerometer measurements in the 90-G to the 120-G range indicated that safety chops (hand protectors) and safety-kicks (foot padding) did not reduce the acceleration of the dummy. Ten-ounce boxing gloves mitigated peak acceleration to some extent. Kicks and punches produced accelerations in the same range.

Discussion

Strengths

Dr. GP Fife investigated the mechanism of Taekwondo and did the comparison with the data acquired from Olympic boxers during his first investigation. Since one of the major parameter is linear acceleration, they did the second investigation on comparing linear acceleration and rotational acceleration in Taekwondo. In the third paper, the experiment was set up to investigate the mechanism of Karate by comparing Karate with Boxing.

Table 4. A comparison of the measured parameters in different techniques

Sports	Techniques	Mean peak linear resultant acceleration (g)	HIC
Boxing (Fife GP et al. 2013)	Hook punch	71.23	78.96
	Forehead punch	52.26	47.81
	Uppercut punch	17.11	24.10
Taekwondo (Fife GP et al. 2013)	Turning kick	130.11	672.74
	Jump back kick	83.82	462.95
Taekwondo (Fife GP et al. 2018)	Turning kick	70.80	N/A
Karate (Schwartz ML et al. 1986)	Karate punch	44 (typical peak linear accelerations)	N/A
	Karate kick	30 (typical peak linear accelerations)	N/A

Controversies

With the first investigation, GP Fife et al. (2013) argued that different techniques produced varying peak mean linear acceleration and taekwondo turning kick produced the highest among all. According to the article's results, taekwondo kicks have significantly larger magnitudes on both linear acceleration and injury criterion than boxing punches. Obvious difference in linear acceleration can also be found between different taekwondo techniques such as clench axe (54.95 ± 20.08 g) and turning kick (130.11 ± 51.67 g). During a follow-up investigation, GP Fife et al. (2018) confirmed previous findings from GP Fife et al. (2013), which is that taekwondo turning kick produced the highest RLA compared to other tested techniques. However, the RLA value of taekwondo turning kick decreased to approximately 70 g, which is much more similar to the RLA of hook punch in boxing compared to taekwondo turning kick. After comparing two experiments, one explanation can be found that a Hybrid dummy III head was used in investigation two instead of Hybrid dummy II head. Even though the other instrumentations are the same, the inconsistency of turning kick could be due to the different biofidelity between Hybrid III and Hybrid II dummy head. Another theory is that the volunteers in first investigation were a group of 2004 taekwondo Olympic athletes while volunteers in the second investigation were formed by taekwondo practitioners. Therefore, since the experiment focus on the peak mean value, it can be argued that the variation in skill level of the volunteers in the experiments causes the differences in the final results. An interesting side observation is that linear acceleration produced by hook punch in taekwondo (47.3 ± 23.8 g) is less than the hook punch in boxing (71 ± 32 g).

Limitations

All the impact tests are using Hybrid dummy head and neck complex. As we know Hybrid dummy is designed for frontal crash. Therefore, whether Hybrid dummy head is reliable in all the punching tests or not is questionable. The variation skill level of volunteers can not be unified, which could result inconsistency as discussed above. In addition, most of the experiments can only use two comparison parameters, HIC and peak mean linear acceleration. The effectiveness of Head Injury Criterion for head impact in combative sports is questionable.

Future directions

Further research needs to be conducted to look into each biomechanism. Most of the investigation about combat sports are missing rotational acceleration, which plays a big role in mechanism. By using the analysis, creating a new injury criterion for sports also could be a direction for future studies.

Conclusion

One major part of research in boxing injury biomechanics is measuring acceleration, force and velocity during the injury mechanism. Two papers were summarized in which researchers used a device and a HIII dummy head to measure biomechanical parameters when boxers were punching them. As dummies are not validated to be used in such applications, researchers developed a new device called IBH to measure desired parameters during real matches. IBH is a boxing headgear instrumented with accelerometers. Later, this device was used during sparring matches to measure head's centre of gravity acceleration, HIC and location of impact.

The two papers regarding protective headgear investigated: 1. how well headgear can protect the boxer's head, and 2. the effect of wearing different headgear. Altogether they looked into several different punch regions: forehead (both), side (both), and jaw (one), and commonly found that impact location has a significant influence on the outcome. However, there exist some controversies in terms of the inconsistent measurements of the linear acceleration measurements: < 100 g vs. 200 g, and, thus, the consequent inconsistent final arguments: protect well vs. unable to protect well. Speaking of major limitation, Hybrid III head and neck complex was used in both headgear papers, without showing convincing evidence that Hybrid III's biofidelity is suitable for boxing headgear tests. Based on these findings, future research in protective headgear involves two important aspects: 1. developing a more unified and consistent research platform for testing headgear, and 2. verifying Hybrid III's biofidelity for headgear tests or developing more suitable dummies.

Based on the previous boxing researches, it is easier to investigate the mechanism of other combative sports. According to the papers on comparing boxing with other sports, the conclusion can be made that different fighting techniques can result in different head impact biomechanisms and, thus, head responses. The first two papers investigate the mechanism of different taekwondo techniques and put in comparison with different boxing techniques. Even though both investigations agree on turning kick is the most powerful technique in taekwondo, an inconsistent value on the linear acceleration of turning kick was found between two papers. All the investigations are facing the same limitations that the questionable biofidelity of Hybrid dummy head and head injury criterion in combative sports. Future research could lead to entirely new injury criterion based on the revised restrictions. In the meantime, more precise experiments should be developed to investigate more effective prevention mechanisms in different combative sports.