Neck Strength: A Protective Factor Reducing Risk for Concussion in High School Sports

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1 DOI /s ORIGINAL PAPER Neck Strength: A Protective Factor Reducing Risk for Concussion in High School Sports Christy L. Collins Erica N. Fletcher Sarah K. Fields Lisa Kluchurosky Mary Kay Rohrkemper R. Dawn Comstock Robert C. Cantu Ó Springer Science+Business Media New York 2014 Abstract As the number of high school students participating in athletics continues to increase, so will the number of sports-related concussions unless effective concussion prevention programs are developed. We sought to develop and validate a cost-effective tool to measure neck strength in a high school setting, conduct a feasibility study to determine if the developed tool could be reliably applied by certified athletic trainers (ATs) in a high school setting, and conduct a pilot study to determine if anthropometric measurements captured by ATs can predict concussion risk. In the study s first phase, 16 adult subjects underwent C. L. Collins E. N. Fletcher Center for Injury Research and Policy, The Research Institute at Nationwide Children s Hospital, Columbus, OH, USA S. K. Fields Department of Communication, University of Colorado- Denver, Denver, CO, USA L. Kluchurosky Sports Medicine, Nationwide Children s Hospital, Columbus, OH, USA M. K. Rohrkemper Central Ohio Primary Care, Columbus, OH, USA R. D. Comstock (&) Pediatric Injury Prevention, Education, and Research (PIPER) Program, Department of Epidemiology, Colorado School of Public Health, Aurora, CO, USA dawn.comstock@ucdenver.edu repeated neck strength testing by a group of five ATs to validate the developed hand-held tension scale, a cost effective alternative to a hand-held dynamometer. In the second phase, during the 2010 and 2011 academic years, ATs from 51 high schools in 25 states captured pre-season anthropometric measurements for 6,704 high school athletes in boys and girls soccer, basketball, and lacrosse, as well as reported concussion incidence and athletic exposure data. We found high correlations between neck strength measurements taken with the developed tool and a hand-held dynamometer and the measurements taken by five ATs. Smaller mean neck circumference, smaller mean neck R. D. Comstock University of Colorado School of Medicine, Pediatric Emergency Medicine, Aurora, CO, USA Department of Neurology, Center for the Study of Traumatic Encephalopathy, Boston University School of Medicine, Boston, MA, USA Sports Legacy Institute, Waltham, MA, USA Department of Neurosurgery, Boston University School of Medicine, Boston, MA, USA Department of Neurosurgery, Emerson Hospital, Concord, MA, USA

2 to head circumference ratio, and weaker mean overall neck strength were significantly associated with concussion. Overall neck strength (p \ 0.001), gender (p \ 0.001), and sport (p = 0.007) were significant predictors of concussions in unadjusted models. After adjusting for gender and sport, overall neck strength remained a significant predictor of concussion (p = 0.004). For every one pound increase in neck strength, odds of concussion decreased by 5 % (OR = 0.95, 95 % CI ). We conclude that identifying differences in overall neck strength may be useful in developing a screening tool to determine which high school athletes are at higher risk of concussion. Once identified, these athletes could be targeted for concussion prevention programs. Keywords Head injury Prevention Athlete Soccer Basketball Lacrosse Introduction In the US, concussions are a common sports injury, with an estimated 300,000 recognized and 7 9 times that number of unrecognized sports-related concussions annually (Buzzini & Guskiewicz, 2006; Centers for Disease Control and Prevention, 2007; Faul, Xu, Wald, & Coronado, 2010; Gessel, Fields, Collins, Dick, & Comstock, 2007; Patel & Greydanus, 2002). Young athletes are more susceptible to concussions than are older athletes, and because of the ongoing neurocognitive development occurring throughout adolescence, concussions can have severe acute and long term complications in young athletes (Buzzini & Guskiewicz, 2006; Patel & Greydanus, 2002, Patel, Shivdasani, & Baker, 2005). Participation in high school athletics increases annually. Nearly eight million students participated in athletics during the 2011/2012 school year (National Federation of State Department of Surgery, Emerson Hospital, Concord, MA, USA Department of Neurosurgery, Neurologic Sports Injury Center, Brigham and Women s Hospital, Boston, MA, USA High School Associations, 2012). While increased physical activity has multiple benefits including managing weight, improving self-esteem, and increasing strength, endurance, and flexibility (Dietz, 1998; Pate et al., 1995; Prasad & Das, 2009), athletes are also at risk of sports-related injuries, including concussion. As the number of high school students participating in athletics continues to increase, so will the number of sports-related concussions unless effective concussion prevention programs are developed. To date, primary prevention has focused on three areas: (1) improving helmet designs, (2) considering introducing helmets to sports not currently utilizing them, and (3) adopting sport rule changes. Helmets were originally designed to prevent skull fractures, not concussions. While sport helmet design has improved considerably over time, helmets have become more effective at reducing direct focal external transfers of force but are ineffective at preventing rotational accelerations, the primary underlying mechanism of concussions (Scorza, Raleigh, & O Connor, 2012). Some sports, like girls lacrosse, have considered introducing helmets, but helmets will never prevent all concussions. Although rule changes can be effective, they do not fully protect athletes as rules are broken. Currently no research identifies a primary concussion prevention mechanism that is inexpensive, easy to adopt, widely available, and fully within the athlete s control. Neck strengthening fits these criteria, as any athlete can participate in a self-directed neck strengthening program with minimal training and no required equipment. Previous studies have found that neck strengthening programs increase neck strength in athletes and are effective as therapy for neck pain in pilots (Conley, Stone, Nimmons, & Dudley, 1997; Cross &Serenelli,2003; Mansell, Tierney, Sitler, Swanik, & Stearne, 2005; Salmon, Harrison, & Neary, 2011). In addition, one study found that, among young adult soccer players, females had greater head acceleration in response to an applied force than males, perhaps because females exhibited significantly less isometric neck strength and neck girth (Tierney et al., 2008). As a result, poor neck strength has been proposed as a risk factor for concussion. This suggests that differences in anthropometric measurements including head circumference and neck strength may be useful in identifying athletes at elevated risk of concussion. Once identified, athletes with modifiable anthropometric factors such as low neck strength could reduce their risk of concussion through a neck strengthening program.

3 Prior research evaluating anthropometric measurements as potential risk factors for concussion have largely relied upon relatively inexpensive equipment such as cloth tape measures to measure head and neck circumference and neck length. However, measurements of neck strength have required more expensive, laboratory quality equipment, which high schools generally lack. Even the least expensive option, handheld dynamometers costing approximately $750 $1,000 each, is too high for most high school athletic training budgets. A successful concussion prevention program targeting high school athletes will have the greatest public health impact given the much larger number of sports participants at the high school level coupled with concerns regarding potential acute and long-term concussion complications in young athletes. Thus, a novel method is needed to measure neck strength that is both methodologically and economically feasible in a high school setting. In this study, our objectives were to (1) develop and validate a cost effective tool to measure neck strength in a high school setting, (2) conduct a feasibility study to determine if the developed tool could be reliably applied in a high school setting by certified athletic trainers (ATs), and (3) conduct a pilot study to determine if anthropometric measurements captured by ATs in a high school setting can predict concussion risk. Methods The study was conducted in two phases. The first, administered in a controlled research environment, addressed study objectives one and two while the second, conducted in a national sample of US students in the high school athletic training setting, addressed study objective three. In the study s first phase, a sample of sixteen adult subjects (of both genders and diverse age and body size) volunteered for repeated anthropometric testing by a group of five ATs. Each AT remained at one station consisting of a straight backed chair and table with a hand-held dynamometer (the current standard for measuring neck strength among high school athletes), a hand-held tension scale (described in detail later), Velcro closure head band with D-rings, and materials to clean the headband and dynamometer between subjects. Conducting the tests were a very experienced AT with more than 20 years experience in high school, clinic, and academic settings) (tester one), a relatively inexperienced AT employed as a high school AT (tester two), a very experienced AT employed as a high school AT (tester three), and two very inexperienced ATs employed as high school ATs (testers four and five). ATs were briefed on the study s purpose and provided a brief training session on the methodology for measuring neck strength using both hand-held dynamometers and tension scales. We then received study subjects and briefed them about the study and, if they consented to participate, we instructed them to visit the five measuring stations in a first open station, first attended manner. Only two measurements for each of the four individual neck strength measures of interest (using the hand-held dynamometer and tension scale) were taken at each station, and subjects rested at least 3 min between each station to minimize risk of injury and inter-tester variation due to muscle fatigue. Study subjects each received $25 for participating. In the second phase, schools participating during the 2010 and 2011 academic years in the National High School Sports-Related Injury Surveillance Study (High School RIO TM ), a successful and long-standing sports injury surveillance study (Darrow, Collins, Yard, & Comstock, 2009; Gessel et al., 2007; Rechel, Yard, & Comstock, 2008; Shankar, Fields, Collins, Dick, & Comstock, 2007), were invited to participate in this pilot study. Immediately prior to each sport season, a convenience sample of ATs at participating schools in 25 states across the US captured head and neck circumference, neck length, and four measurements of neck strength (i.e., extension, flexion, and right and left lateral) for all athletes participating in school-sponsored boys and girls soccer, basketball, and lacrosse using a cloth measurement tape and the tension scale apparatus developed for this study. Participating ATs then used High School RIO TM, the internet-based data collection tool from the ongoing National High School Sports-Related Injury Surveillance Study, to report athletic exposure and injury data weekly throughout the academic year. This study evaluated only the concussions reported to High School RIO TM. Thus, this national sample of student athletes for whom anthropometric measures were gathered was subsequently prospectively monitored for concussion over the course of each sports season during the 2010 and 2011 academic years.

4 Fig. 1 a Developed hand-held tension scale and a Velcro adjustable head strap with a D ring. b Demonstration of proper measurement of neck extension with the developed neck strength measurement tool. c Demonstration of proper measurement of right lateral flexion Objective 1 Develop and validate a cost effective tool to measure neck strength among athletes in a high school setting. When using a hand-held dynamometer to measure neck strength in a field setting, the subject is usually seated and the hand-held dynamometer is placed in the center of the forehead to measure flexor strength, above the external occipital protuberance to measure extensor strength, above the left ear to measure left lateral flexion, and above the right ear to measure right lateral flexion. The subject is instructed to apply maximum force against the hand-held dynamometer by pushing against it for 3 s. The new apparatus developed for this study, a hand-held tension scale attached to a Velcro adjustable head strap with a D ring, measured neck strength (in pounds) in a similar manner but with the device located on the opposite side of the head and the athlete instructed to apply maximum force by pulling against the tension scale for 3 s (Fig. 1). The hand-held tension scale apparatus we developed for this pilot study costs approximately $10, offering an affordable measuring tool compared to hand-held dynamometers that can cost up to $750 $1,000 each. Measurements taken on the sixteen adult study subjects via the handheld dynamometers and tension scales determined the strength of the association between the measurements taken by the two devices. Data were also collected on head and neck circumference and neck length (in inches). Head circumference was measured at eyebrow level, and neck circumference was measured just below the base of the Adam s apple. Neck length was measured from the notch/bony protuberense (inion) at the base of the skull to the most prominent spinous process (T-1). Objective 2 Conduct a feasibility study to determine if the hand-held tension scale could be reliably utilized in a high school setting by ATs to measure neck strength. The measurements taken on the 16 adult study subjects by each of the 5 ATs were compared to assess inter-tester reliability. Objective 3 Conduct a pilot study to determine if anthropometric measurements captured by ATs can be used to predict concussion risk among high school soccer, basketball, and lacrosse players. These three sports were selected because each are played by both genders in a high school setting and have

5 relatively high concussion rates. Participating ATs were provided with a cloth measuring tape, neck strength measurement tool (i.e., tension scale and Velcro closure head band with D-rings), data recording sheet, and an anthropometric measurement instruction sheet explicitly detailing the testing protocol for all measurements. Of the 176 schools participating in the 2010 High School RIO TM study, 32 participated in this pilot study, and of the 174 schools participating in the 2011 study, 40 participated. Of these, 21 schools participated both years. Thus, a total of 51 schools from across the US participated. ATs, who are medically trained professionals, reported pre-season anthropometric measurements for 6,704 high school athletes in boys and girls soccer, basketball, and lacrosse and then reported concussion incidence and athletic exposure data during the course of each sports season via the High School RIO TM system. A unique identifier controlled by the AT linked preseason data to concussion data for each injured athlete while maintaining athlete confidentiality. Each anthropometric measurement was evaluated as a possible predictor of concussion risk in this large sample of high school athletes. Derived Anthropometric Measurements The four individual neck strength measures were based on the average of two measurements taken by ATs at each school; a total of 42 (\0.01 %) athletes were excluded due to abnormal differences between repeated measures, leaving a sample size of 6,662. Overall neck strength was calculated by averaging extension, flexion, right lateral, and left lateral measurements for each athlete. In the event of missing data for one or two of the four neck strength measures (n = 41,\0.01 %), overall neck strength was derived from the available measurements. The neck circumference to head circumference ratio and neck length to head circumference ratio were directly computed. Statistical Analysis Data were analyzed using SAS version 9.3 (SAS Institute, Cary, NC). Statistical significance was assessed at p \ Associations among anthropometric measurements were analyzed using the Pearson correlation coefficient. Intraclass correlations were also calculated. While intraclass and Pearson correlations were in agreement, Pearson correlations are reported in this pilot study as they were slightly more conservative. Differences in concussion incidence between subgroups were assessed using Wald v 2 tests with magnitude of association assessed by odds ratio (OR) and related 95 % confidence interval (CI). Means were reported with standard deviations (SD). Differences in mean anthropometric measurements between concussed and uninjured athletes were assessed by a two-sample t test with statistical significance assessed using Cochran approximations for unequal variance. Univariate logistic regression was used to assess unadjusted odds of concussion for derived anthropometric measurements, age, gender, body mass index (BMI), and sport. Multivariate logistic regression was used to assess the odds of concussion adjusted for significant covariates in the unadjusted models. This study was approved by the Institutional Review Board at Nationwide Children s Hospital. Results Validating a Cost Effective Tool to Measure Neck Strength Among Athletes in a High School Setting Our pilot study found a high correlation between the hand-held dynamometer and tension scale measurements. Correlations ranged from 0.83 to 0.94 for the four neck strength measurements (all p values\0.05). Feasibility Study to Determine if the Tool Could be Applied in a High School Setting by ATs We found a high inter-tester reliability with strong correlations among measurements taken by five different ATs. Of the 40 individual inter-tester comparisons, 30 (75.0 %) were correlated above the 0.80 level. Additionally, with only one exception, all inter-tester comparisons were significantly associated with one another (p \ 0.05). Pilot Study to Determine if Anthropometric Measurements Captured by ATs Can Predict Concussion Risk Among High School Soccer, Basketball, and Lacrosse Players Correlation of Baseline Anthropometric Measurements All four neck strength measures were highly correlated among males (range = ) and females

6 Table 1 Correlation of anthropometric measurements for girls (italicized values) and boys (bold values) Girls Neck circumference Neck length Head circumference Extension Flexion Right lateral Left lateral Boys Neck circumference Neck length Head circumference a a a a Extension Flexion a Right lateral a Left lateral a a p [ 0.05, all others \0.05 ( ) (Table 1). Other anthropometric measurements were not strongly correlated with each other or any measures of neck strength. The ratio between neck and head circumference, the ratio between neck length and head circumference, and overall neck strength, were not strongly correlated (Table 2). Gender, Sport, and Concussion Incidence Of the 6,662 athletes, 179 (107 girls and 72 boys) had diagnosed concussions (Table 3). The rate of concussion was 2.5 per 10,000 athlete exposures among boys and 4.9 per 10,000 athlete exposures among girls. Soccer had the highest rate of concussion (5.2) followed by lacrosse (3.7) and basketball (2.3). Girls had higher odds of concussion than boys overall (OR = 1.8, 95 % CI ) and in basketball Table 2 Correlation between derived anthropometric measurements for girls (italicized values) and boys (bold values) Boys Neck circumference: head circumference a Neck length: head circumference a Girls Neck circumference: head circumference Neck length: head circumference Neck strength Neck strength a a All p \ (OR = 2.7, 95 % CI ) and soccer (OR = 1.8, 95 % CI ). There was no difference between girls and boys lacrosse players (OR = 1.0, 95 % CI ). Comparison of Mean Anthropometric Measurements Between Concussed and Uninjured Athletes Concussed athletes had a smaller mean neck circumferences (p = 0.001), smaller mean neck circumference to head circumference ratio (i.e., a small neck paired with a large head; p = 0.001), and smaller Table 3 Overall and sport-specific gender differences in concussion incidence Concussion No n (%) a Yes n (%) a Total n OR (95 % CI) b p value Basketball 2,563 (97.8) 58 (2.2) 2,621 Girls 1,160 (96.7) 40 (3.3) 1, ( ) Boys 1,403 (98.7) 18 (1.3) 1, Soccer 2,674 (96.6) 95 (3.4) 2,769 Girls 1,195 (95.5) 56 (4.5) 1, ( ) Boys 1,479 (97.4) 39 (2.6) 1, Lacrosse 1,246 (98.0) 26 (2.0) 1,272 Girls 540 (98.0) 11 (2.0) ( ) Boys 706 (97.9) 15 (2.1) Total 6,483 (97.3) 179 (2.7) 6,662 Girls 2,895 (96.4) 107 (3.6) 3, ( ) Boys 3,588 (98.0) 72 (2.0) 3,660 \ a Percentages sum to % across rows b Odds ratio compares the odds of concussion among girls to the odds of concussion among boys

7 Table 4 Overall and gender-specific comparison of mean anthropometric measurements between athletes who sustained a concussion and those who did not No concussion Concussion p value b n a Mean (SD) n a Mean (SD) Overall BMI 6, (3.23) (3.12) Neck circumference (in.) 6, (3.02) (2.58) Neck length (in.) 6, (2.91) (2.75) Head circumference (in.) 6, (1.96) (2.14) Neck circumference: head circumference 6, (0.046) (0.04) Neck length: head circumference 6, (0.051) (0.05) Overall neck strength (lbs) 6, (5.16) (4.82) \0.001 Extension (lbs) 6, (5.35) (4.90) \0.001 Flexion (lbs) 6, (5.78) (5.30) \0.001 Right lateral (lbs) 6, (5.14) (4.78) \0.001 Left lateral (lbs) 6, (5.07) (4.87) Total 6, Boys BMI 3, (3.22) (2.99) Neck circumference (in.) 3, (2.53) (2.25) Neck length (in.) 3, (2.86) (2.65) Head circumference (in.) 3, (1.78) (2.02) Neck: head circumference 3, (0.04) (0.04) Neck length: head circumference 3, (0.05) (0.05) Overall neck strength (lbs) 3, (5.41) (5.30) Extension (lbs) 3, (5.66) (5.52) Flexion (lbs) 3, (6.07) (5.89) Right lateral (lbs) 3, (5.39) (5.16) Left lateral (lbs) 3, (5.32) (5.22) Total 3, Girls BMI 2, (3.23) (3.19) Neck circumference (in.) 2, (2.35) (2.05) Neck length (in.) 2, (2.85) (2.78) Head circumference (in.) 2, (1.92) (1.89) Neck: head circumference 2, (0.04) (0.04) Neck length: head circumference 2, (0.05) (0.05) Overall neck strength (lbs) 2, (4.53) (4.39) Extension (lbs) 2, (4.64) (4.35) Flexion (lbs) 2, (5.03) (4.77) Right lateral (lbs) 2, (4.57) (4.44) Left lateral (lbs) 2, (4.48) (4.54) Total 2, a Number of subjects may not sum to total due to missing data b p values from two-sample t tests comparing means

8 Table 5 Sport-specific comparison of mean neck strength measurements between athletes who sustained a concussion and those who did not No concussion Concussion p value b n a mean (SD) n a mean (SD) Basketball Overall neck strength (lbs) 2, (5.12) (4.80) Extension (lbs) 2, (5.30) (4.87) Flexion (lbs) 2, (5.77) (5.45) Right lateral (lbs) 2, (5.13) (4.66) Left lateral (lbs) 2, (5.04) (4.87) Total 2, Soccer Overall neck strength (lbs) 2, (5.06) (4.83) Extension (lbs) 2, (5.26) (4.88) Flexion (lbs) 2, (5.64) (5.20) Right lateral (lbs) 2, (5.04) (4.86) Left lateral (lbs) 2, (4.99) (4.92) Total 2, Lacrosse Overall neck strength (lbs) 1, (5.28) (4.99) Extension (lbs) 1, (5.51) (5.14) Flexion (lbs) 1, (5.86) (5.49) Right lateral (lbs) 1, (5.23) (4.94) Left lateral (lbs) 1, (5.15) (4.86) Total 1, a Number of subjects may not sum to total due to missing data b p values from two-sample t tests comparing means mean overall neck strength (p \ 0.001) than uninjured athletes (Table 4). For boys, concussed athletes had a smaller mean neck to head circumference ratio (p = 0.003) and smaller mean overall neck strength (p = 0.014) than uninjured athletes. For girls, concussed athletes had marginally smaller mean overall neck strength than uninjured athletes (p = 0.052). In basketball (p = 0.011) and lacrosse (p = 0.027), concussed athletes had smaller mean overall neck strength than uninjured athletes (Table 5). In soccer (p = 0.051) concussed athletes had marginally smaller mean overall neck strength than uninjured athletes. Evaluation of Athlete Specific Factors That May Be Useful in Identifying Athletes at Increased Risk of Concussion Overall, neck strength (p \ 0.001), gender (p \ 0.001), and sport (p = 0.007) were all significant predictors of concussion in unadjusted, univariate models (Table 6). The ratio between neck and head circumference, the ratio between neck length and head circumference, age, and BMI were not significant predictors and thus were not included in the final adjusted model. After adjusting for gender and sport, overall neck strength remained a significant predictor of concussion (p = 0.004). For every one pound increase in neck strength, odds of concussion decreased by 5 % (OR = 0.95, 95 % CI ). Discussion Using a newly developed cost effective neck strength measurement tool, this pilot study found that neck strength was a significant predictor of concussion among high school basketball, soccer, and lacrosse

9 Table 6 Regression models predicting odds of concussion injury Predictor Unadjusted a Adjusted b OR (95 % CI) p value OR (95 % CI) p value Continuous Neck strength c 0.9 ( ) ( ) Neck: head circumference 1.0 ( ) Neck length: head circumference 1.0 ( ) Age 0.9 ( ) BMI 1.0 ( ) Categorical Gender Girls 1.8 ( ) \ ( ) Boys 1 (referent) 1 (referent) Sport Basketball 1.1 ( ) ( ) Soccer 1.7 ( ) 1.6 ( ) Lacrosse 1 (referent) 1 (referent) a Unadjusted models from univariate logistic regression b Adjusted for all significant covariates from unadjusted logistic regression c Overall neck strength was calculated by averaging extension, flexion, right lateral, and left lateral measurements for each athlete athletes. Given the recent growing concern regarding sports-related concussions and limited availability of primary prevention techniques, this finding is important. The development of primary concussion prevention mechanisms that are inexpensive, easy to adopt, widely available, and fully within the athlete s control is critical. Identifying differences in anthropometric measurements, such as neck strength, may drive development of screening tools that could identify athletes at elevated risk of concussion. Once identified, these athletes could be targeted for concussion prevention interventions, such as neck strengthening programs, tailored to their specific risk factors. Although considerable research has explored concussion management and outcomes, relatively little has examined the primary prevention of concussions in a human population (Smith et al., 2012). Previous studies have found that stronger necks decrease head acceleration, rapid change in velocity, and displacement after a collision, which in turn may reduce the risk of sports-related concussion (Tierney et al., 2008; Viano, Casson, & Pellman, 2007). Even a small decrease in velocity may result in a significant reduction in the risk of concussion (Viano et al., 2007). Other studies have found that neck strengthening programs are effective in increasing neck strength among athletes (Conley et al., 1997; Cross & Serenelli, 2003; Mansell et al., 2005; Salmon et al., 2011). Therefore, neck strengthening appears to have strong potential as a primary concussion prevention mechanism. Given the increasing number of high school athletes in the United States (National Federation of State High School Associations, 2012) and the burden of sports-related concussions (Buzzini & Guskiewicz, 2006; Centers for Disease Control and Prevention, 2007; Faul et al., 2010; Gessel et al., 2007; Patel & Greydanus, 2002), developing and implementing effective primary prevention programs for concussions is imperative. Because hand-held dynamometers, currently the gold standard for measuring neck strength, are too costly for many high school sports programs, we developed a cost effective hand-held tension scale device to measure neck strength. Measurements taken with this device were highly correlated with measurements taken with a hand-held dynamometer. In addition, ATs were able to measure neck strength in a high school setting using this novel neck strength measurement tool. These results suggest that neck strength measurements of high school athletes, obtained by ATs using this affordable tool, could predict which athletes may be at increased risk of concussion.

10 Differences were identified in the association between anthropometric measurements and concussion by gender and sport. Among boys, smaller mean neck to head circumference ratio and smaller mean overall neck strength were both significantly associated with concussion. Among girls, smaller mean overall neck strength was only marginally associated with concussion, and mean neck to head circumference ratio was not associated. However, it is unclear what relative roles biophysiology, anthropomology and sociocultural constructs may play in these differences. Previous studies have found that rates and patterns of concussion in gender-comparable sports differ, with females sustaining significantly more concussions than males, reporting different symptoms, and having worse overall outcomes (Broshek et al., 2005; Dick, 2009; Field, Collins, Lovell, & Maroon, 2003; Frommer et al., 2011; Gessel et al., 2007; McClincy, Lovell, Pardini, Collins, & Spore, 2006). Tierney et al. (2005) found gender differences in head-neck stabilization, with females having a greater head-neck acceleration and displacement compared to males. While smaller mean overall neck strength was significantly associated with concussion in basketball and lacrosse, there was only a marginal association in soccer. It has long been believed that a long neck paired with a large head placed football players at an increased risk of concussion; however, we found that the ratio between neck length and head circumference was not a significant predictor of concussion in our sample of basketball, soccer, and lacrosse players. As with all studies, ours had several limitations. The sample of this pilot study was limited to high school athletes participating in three sports. Thus, these findings may not be generalizable across broader types of sports, age categories, or levels of competition. However, this was a true prospective cohort study in a large geographically dispersed sample (6,662 athletes from high schools in 25 states). Additionally, these three sports were selected because they have relatively high concussion rates and provided the ability to make gender comparisons. Because it was not possible to supervise the use of the novel neck strength measurement tool by ATs conducting measurements in these geographically dispersed schools, there was some inter-tester variability. Giving detailed instructions of the testing protocol to participating ATs both in written form and via a brief video minimized this limitation. Finally, because ATs were not blinded to the neck strength results, there was a potential for reporting bias if ATs believed that athletes with weaker necks were more likely to sustain a concussion. Despite these limitations, this pilot study makes an important contribution to the sports medicine literature by identifying neck strength as a protective factor reducing risk of concussion among high school athletes. Conclusions Using a newly developed cost effective neck strength measurement tool, this pilot study found that neck strength was a significant predictor of concussion among high school basketball, soccer, and lacrosse athletes. While we found differences in the association between anthropometric measurements and concussion by gender and sport, further research is needed to gain a better understanding as to why these differences exist and how they may affect risk of concussion. Although the findings should be confirmed in larger and broader groups of athletes, these promising results indicate targeted neck strengthening programs should be developed and evaluated as the first primary prevention mechanism for concussion. Acknowledgments We would like to thank Breg for generously donating the Velcro bands used in the developed neck strength tool. This study was funded by the National Operating Committee on Standards for Athletic Equipment (NOCSAE). Conflict of interest to disclose. References The authors have no conflicts of interest Broshek, D. K., Kaushik, T., Freeman, J. R., Erlanger, D., Webbe, F., & Barth, J. T. (2005). Sex differences in outcome following sports-related concussion. Journal of Neurosurgery, 102(5), Buzzini, S. R., & Guskiewicz, K. M. (2006). Sport-related concussion in the young athlete. Current Opinion in Pediatrics, 18(4), Centers for Disease Control and Prevention. (2007). Nonfatal traumatic brain injuries from sports and recreation activities: United States, Morbidity and Mortality Weekly Report, 56(29), Conley, M. S., Stone, M. H., Nimmons, M., & Dudley, G. A. (1997). Resistance training and human cervical muscle

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