Investigating the effect of mouth guard use on aerobic performance in amateur boxers

Abstract Objectives To assess if wearing a mouth guard impacts maximal aerobic capacity in amateur boxers. Materials and Methods A prospective crossover cohort (pilot) study was conducted to assess maximal aerobic capacity in amateur boxers using the 20 m multi stage fitness test (MSFT). Two primary outcomes measures were recorded: (1) the maximum oxygen uptake (peak VO2—mL/kg/min) and (2) distance run (meters—m). Thirteen amateur boxers completed the MSFT 7 days apart under control (no mouth guard—C) and intervention conditions (mouth guard—MG). Participants also submitted data on height, weight, type of mouth guard and Rate of Perceived Exertion (RPE) during the test. Statistics Paired T‐test. Results Mouth guard use was shown to reduce peak VO2 and distance run during the 20 m MSFT from 56.31 to 54.12 mL/kg/min and 2572 to 2380 m respectively (p < 0.05). Twelve out of 13 participants wore a Boil & Bite mouthguard and recorded lower peak VO2 scores (−4.38%) when wearing a mouth guard compared to control conditions, (Mean = −2.46 mL/kg/min, Range of decrease = 4.2–0.9 mL/kg/min; p < 0.05). Ten participants submitted data on RPE—One participant with a custom made mouthguard reported no change in RPE in mouthguard conditions, while nine participants reported an average (+30.5%) increase in mean RPE in Boil & Bite mouthguard conditions compared to control conditions. Conclusions Boil & Bite mouth guard use was shown to significantly reduce aerobic performance in amateur boxers and increase the perceived rate of exertion during the 20 m MSFT.

. It remains to be seen however, if wearing mouth guards may have a secondary impact on athletic performance; with some small studies showing a negative impact (Delaney & Montgomery, 2005) and others none (Garner et al., 2011).
Whilst the debate continues regarding performance and mouth guard use, it has been left to individual sports to determine whether they wish to mandate their use. Governing bodies across all sports, have adopted varying approaches: with some mandating use at a national level by comprehensive medical guidance, and other adopting a local or regional approach to enforcement. To enforce the mandatory use of mouthguard the amateur international boxing association (AIBA) has issued comprehensive medical guidance stating that a mouth guard should be worn to "fit exactly and comfortably" during competition (AIBA Rules and Guidelines-AIBA, 2019).
During an adult amateur boxing contest, participants will be subject to three rounds of intense physical activity of between 2 and 3 min, followed by a 1-min rest period. Amateur boxers will need to rely on skill, physical conditioning, and contributions from aerobic and anaerobic respiration (Davis et al., 2014) to evade their opponent and land scoring punches. In order to win, well trained boxers will be able to pace themselves through a contest to ensure they have sufficient cardiovascular, respiratory and energy reserves throughout.
To advance the discussion on whether mouth guard use should be mandated in professional and amateur "at risk" sports. We tested the null hypothesis that mouth guard use has no effect on the maximal aerobic capacity of well-trained amateur boxers who routinely wear them.

| Orofacial injuries in sport
Whilst the exact incidence of OFT injuries in sport is not accurately known, expert opinion has advocated for their use in primary prevention of injury. The annual cost of treatment for all dental trauma is significant and thought to be ($2-5 million/1 million inhabitants) in the United States (Andersson, 2013). Data collected from over 100 pediatric Emergency Departments has shown that children are particularly susceptible, with sports and recreational activities accounting for 45.6% of all non-fatal OFT injuries seen (Kamboj et al., 2019).
A questionnaire survey of 1189 athletes across six high-risk sports reported that 28.8% of respondents had sustained at least one dental related injury during their career (Ferrari & de Medeiros, 2002).
Further single sports studies have shown that the incidence of at least one OFT during a sports career to be: 23% (Taekwondo), 11% (Soccer), 23.5% (Muay Thai), 100% (triathlon) and 73.6% (Boxing; Aljohani et al., 2017;Andrade et al., 2010;Chatrchaiwiwatana, 2016;Qudeimat et al., 2019). These studies based on self-reported injuries show that OFT injuries are a common occurrence in sports; the immediate consequences of which may require a player to be withdrawn from competition, lose time from training or be left with long term cosmetic consequences.
The American Dental association has therefore issued guidance recommending that well-fitting mouth guards are worn in 29 sports or recreational activities where participants are at risk of: "injury to the teeth, jaw and oral soft tissues (mouth, lip, tongue, or inner lining of the cheeks". Despite the recommendations from dental professionals, only a limited number of sports have mandated their use centrally; instead leaving the decision up to local sports administrators, schools, coaches and individuals to enforce. Further barrier to their universal widespread use include discomfort, restriction to talking and breathing concerns from athletes (Matalon et al., 2008).

| The role of mouth guards in performance
Several small studies have looked at the impact of wearing mouth guards, with regards to how the altered temporomandibular joint position and obstruction to the anatomical airway, effects ventilation. All three type of mouth guards are associated with a reduction in forced expiratory volume in 1 s (FEV 1 ) and functional vital capacity (FVC; Caneppele et al., 2017) at rest, but few studies have looked at the impact this has on prolonged maximal aerobic exercise. Athletes who are performing at sub maximal effort can compensate for this reduction by increasing their respiratory rate and tidal volume, if they have sufficient reserves.
Previous studies using approximations from exhaled respiratory gases have shown that amateur boxers will rely on the majority of their energy from aerobic respiration (77%; Davis et al., 2011). In addition to this VO 2 max scores in elite national (Smith, 2006) and amateur levels (Bruzas et al., 2014) boxers have consistently shown high aerobic capacities (Smith, 2006).

| Aerobic versus anaerobic exercise
Total energy production during exercise is a marker of fitness relying on elements of both "anaerobic" and "aerobic" respiration. The anaerobic pathway is responsible for producing energy without requiring oxygen (Chamari & Padulo, 2015). As this pathway does not utilize oxygen, the use of mouth guards is not thought to adversely effect it directly. Aerobic respiration however is dependent on oxygen and requires athletes to have sufficient cardiovascular and respiratory reserves to maintain energy production.
The relative contributions of these two pathways changes with a trend for an increasing reliance on aerobic respiration as the duration of exercise increases. Studies looking at runners subjected to a "maximal" steady treadmill protocol have shown that after 30 s, aerobic respiration is the predominant pathway for energy production (Spencer & Gastin, 2001; Table 1).

Recruitment-Members of the Cambridge University Amateur Boxing
club were invited to take part in this pilot study via an email advert.
Informed consent was gained prior and partition in the study was voluntary.
Ethics-Ethical approval for the study was granted by University College London (approval number 14715/001).
Inclusion Criteria-In order to be eligible for the study all participants had to have trained for a minimum of 8 weeks with an upper (maxillary) mouth guard and completed the 20 m MSFT at least once before.
Exclusion Criteria-Athletes who were injured or suffering from illness were excluded.
Data collected-Maximal aerobic capacity was assessed using the 20 m multi stage fitness test (MSFT). Two primary outcomes measure were recorded: (1) the maximum oxygen uptake (peak VO 2 -mL/kg/ min) and (2) distance run (meters-m). After each test, the participants were also asked to submit Borg scale scores for Rate of Perceived Exertion (RPE, scale 0-10). Height and weight data were collected from each participant.
Testing protocol-The order of the tests was determined by a coin toss on day 1, neither the participants nor test administrator knew which test condition was to be performed prior. Test 1 was performed in control conditions and test 2 in mouthguard conditions. Statistics-Results were analyzed on Microsoft Excel (San Diego, CA) using a paired T-test.
Sample size calculation-A minimum sample size of eight pairs (Dhand & Khatkar, 2014) was calculated on there being an expected standard deviation of the paired differences of 3.1 (mL/kg/min; Aandstad et al., 2011) and an expected mean of the paired differences of 4 (mL/kg/min).

| 20 m-Multistage fitness test
All participants completed the 20 m MSFT at the Cambridge University Sports Centre. The test is associated with a high repeat test reliability and provides an indirect measure of peak VO 2 (mL/kg/min; Cooper, 2005). The following variables were standardized and maintained in both test conditions: floor surface, footwear, temperature, time of test and a 24 h rest period prior and the test operator.
All participants completed the test prior to the weekly circuit training session to ensure they had adequate rest.

| RESULTS
Eleven males and two female participants took part in the study with The average peak VO 2 score in the control conditions was significantly higher (56.34 mL/kg/min) than the mouth guard condition (54.12 mL/kg/min; p < 0.05; Figure 1). There was an average reduction in peak VO 2 score of 4.31% (mean = −2.43 mL/kg/min, Range 4.2-0.9 mL/kg/min) across all participants. A subgroup analysis of Boil & Bite mouthguard users (n = 12) showed that Boil & Bite mouthguard use was associated with a (4.37%) reduction in peak VO 2 when wearing a mouth guard compared to control conditions (p < 0.05; Table 2).  Figure 2).

| DISCUSSION
The main aim of this pilot study was to assess if mouth guards effect aerobic performance in amateur boxers performing the 20-m MSFT.  (Garner et al., 2011;Gebauer et al., 2011;Kececi et al., 2005). where athletes would not be expected to achieve maximal aerobic capacity for prolonged periods or did not utilize true maximal exercise testing. In addition seven studies did not detail if they randomized the sequence of tests or what steps they took to minimize any potential order effect (Caneppele et al., 2017).
It is well established that mouth guards act as a physical obstruction to the airway and reduce markers of ventilation at rest. Less is known about how athletes may be able to adapt to this or alter their breathing pattern during maximal aerobic exercise (von Arx et al., 2008). We did not assess ventilation during exercise but hypothesize that this restriction to ventilation only becomes significant at maximal aerobic efforts when athletes approach their maximum minute ventilation (L/min). Previous studies with submaximal testing protocols, may not have achieved this threshold and therefore may have underestimated the effect of mouthguards on performance.
Francis et al. took direct gas measurements of athletes during exercise and proposed that wearing mouth guards may mimic purse lip breathing (PLB) patterns (Francis & Brasher, 1991 of 14 studies has shown that non-mouth guard use is associated with a 1.6-1.9 increased risk of OMF trauma across all sports (Knapik et al., 2007). Further robust epidemiological studies will be required to work out the exact incidence of OMF trauma in boxing but given the nature of the sport it is expected to be at least as common as other combat sports (Aljohani et al., 2017;Andrade et al., 2010;Chatrchaiwiwatana, 2016;Qudeimat et al., 2019).

| Limitations
The main limitations of this pilot study were that we did not standardize the mouthguard type and were only able to indirectly measure estimated Peak VO 2, via the MSFT. We plan to perform further studies with custom made (dental) mouthguards, and directly measure maximal oxygen uptake (VO 2 max-mL/kg/min), lactate thresholds and markers of ventilation during a cardio pulmonary exercise test (CPET). This will allow us to determine in more detail if mouthguards impact maximal or submaximal efforts and provide us with a mechanism of how this may occur, from athletes cardiac, respiratory and metabolic response.

| Implementing mouthguard use for safety
Individual sports must decide if the reduction in OFT, outweighs any modest impact on performance or discomfort for athletes. It has been proposed that in under 16's sport the continued safe participation of athletes, outweighs any small reduction in performance observed and other sports should consider mandating for their use at a national or organization level . Amateur and professional boxing has shown that comprehensive medical guidance can help to promote the mandatory use of mouthguards, and that this is not a barrier to athletes wanting to participants at all levels of competition.
We therefore suggested that as per the American Dental Association of America guidance (Mouth Guards, 2019): athletes at risk of injury should wear a well fitted mouth guard. Whilst the official guidance stops short of recommending one mouth guard type over another, the protective properties, relative cost and benefit must be taken into consideration when counseling athletes.
In order to reduce discomfort during training and competition, athletes may consider wearing mouthguards for a period of acclimatization, and using break periods in the sport such as between rounds, tactical stoppages or during half time to remove the mouthguard when not needed.

| CONCLUSION
The results of this study demonstrate that Boil & Bite mouth guard use is associated with a small but statistically significant reduction in peak VO 2 (mean = −2.46 mL/kg/min) when measured in amateur boxers completing the 20 m MSFT (p < 0.05). In addition to this, participants reported that wearing a Boil & Bite mouth guard increased the rate of perceived exertion by 30.5% during the 20 m MSFT (RPE-Borg scale; p < 0.05).

CONFLICT OF INTEREST
No conflicts of interest to declare.

AUTHOR CONTRIBUTIONS
I.A. and P.F. conceived and designed the study. I.A. drafted the manuscript, with supervision from C.K. and P.F. during the course of an MSc. I.A conducted the statistical analysis and interpretation of the data. All authors have read and approved the final manuscript.

FUNDING DECLARATION
No relevant declarations. The manuscript was prepared as part of the academic element of an MSc course.
F I G U R E 2 Mean distance run (m) and peak V0 2 achieved in the control group versus mouth guard group

DATA AVAILABILITY STATEMENT
In accordance with the journals policy on Data Sharing and Data Accessibility The data that support the findings of this study are available from the corresponding author upon reasonable request. Dr Irfan Ahmed On behalf of the manuscript authors.