Pritchard N.S., Urban J.E., Miller L.E. Lintner L., Stitzel J.D.: AN ANALYSIS OF HEAD. Vol. 12 Issue 3: 229 - 242 AN ANALYSIS OF HEAD KINEMATICS IN WOMEN'S ARTISTIC GYMNASTICS N. Stewart Pritchard1'2, Jillian E. Urban1, Logan E. Miller1,2, Laura Lintner3, Joel D. Stitzel1,2 1 Department of Biomedical Engineering, Wake Forest School of Medicine, Winston-Salem, USA, 2 School of Biomedical Engineering and Sciences, Virginia Tech - Wake Forest University, Winston-Salem, USA, 3 Department of Family and Community Medicine, Wake Forest School of Medicine, Winston-Salem, USA, _Original article Abstract Concussions in gymnastics have scarcely been researched; however, current evidence suggests that concussion rates may be higher than previously reported due to underreporting among coaches, athletes, and parents. The purpose of this study was to outline a method for collecting head impact data in gymnastics, and to provide the first measurements of head impact exposure within gymnastics. Three optional level women's artistic gymnasts (ages 11-16) were instrumented with a mouthpiece sensor that measured linear acceleration, rotational velocity, and rotational acceleration of the head during contact and aerial phases of skills performed during practice. Peak linear acceleration, peak rotational velocity, peak rotational acceleration, duration, and time to peak linear acceleration were calculated from sensor data. Kinematic data was time-synchronized to video and then sensor data was segmented into contact scenarios and skills characterized by the event rotation, apparatus, landing mat type, skill type, skill phase, landing stability, and body region contacted. The instrumented gymnasts were exposed to 1,394 contact scenarios (41 per gymnast per session), of which 114 (3.9 per gymnast per session) contained head contact. Peak kinematics varied across skill type, apparatuses, and landing mats. The median duration of impacts with head contact (177 ms) was longer than measured impacts in youth and collegiate level soccer. Results from this study help provide a foundation for future research that may seek to examine head impact exposure within gymnastics to better inform concussion prevention and post-concussion return to play protocols within the sport. Keywords: head impact exposure, gymnastics, concussion, head injury. INTRODUCTION Between 1.1 and 1.9 million sportsand recreational- related concussions occur each year among youth athletes in the United States (Bryan, Rowhani-Rahbar, Comstock, Rivara, & Bryan, 2016). While concussions are commonly associated with player-to-player collisions in contact sports such as American football (Buzas, Jacobson, & Morawa, 2014; Lincoln et al., 2011), concussions can also occur from falls or collisions with objects in sports such as gymnastics. Repeated epidemiological Science of Gymnastics Journal 229 Science of Gymnastics Journal Pritchard N.S., Urban J.E., Miller L.E. Lintner L., Stitzel J.D.: AN ANALYSIS OF HEAD. Vol. 12 Issue 3: 230 - 242 studies have shown a low incidence of concussions in both youth and collegiate level gymnastics activities (Caine et al., 2003; Marshall, Covassin, Dick, Nassar, & Agel, 2007); however, current research suggests that the incidence of concussion in gymnastics may be higher due to underreporting among athletes (Meehan, Mannix, O'Brien, & Collins, 2013), and a lack of knowledge of concussion signs and symptoms among coaches (Mannings, Kalynych, Joseph, Smotherman, & Kraemer, 2014). A recent survey by O'Kane reported that over 30% of retired gymnasts had sustained a blow to the head followed by at least one concussion symptom during their gymnastics careers (Kane, Levy, Pietila, Caine, & Schiff, 2011). Since gymnastics is not normally associated with concussions, it is possible that athletes, coaches, and parents may not be adequately educated on the symptoms, guidelines, and risks associated with the injury. A recent case report published by Knight et al. highlights this issue as the parents of a young gymnast diagnosed with a mild traumatic brain injury ignored the medical professional's recommendations and allowed their daughter to compete in a regional competition where she later sustained a second mild traumatic brain injury (Knight, Dewitt, & Moser, 2016). Gymnastics is a broad term used to describe six unique disciplines: women's and men's artistic gymnastics, rhythmic gymnastics, acrobatic gymnastics, trampoline and tumbling, and aerobic gymnastics, where athletes utilize various apparatuses to perform complex somersaulting and twisting maneuvers. Within each discipline, athletes perform a variety of distinct skills on various apparatuses (e.g. balance beam) and landing surfaces (e.g. crash mats). These combinations of skills, apparatuses, and landing surfaces result in unique movement profiles and head injury mechanisms. For instance, previous research has shown that landing forces can vary across surfaces (McNitt-Gray, Yokoi, & Millward, 2016), apparatuses (Burt, Naughton, & Landeo, 2007), and heights (Mcnitt-Gray et al., 1993). Therefore, as the environment and movement characteristics of the gymnastics skill change, the risk for head injury may also change. Understanding the specifics of head motion during play is essential to better define concussion mechanisms, risk, and return to sport safety. While the kinematics of the head in sports such as American football and soccer have been extensively studied (Cobb et al., 2013; Miller, Pinkerton, et al., 2019), only one study to date has attempted to measure the kinematics of the head during gymnastics related activities (Beck, Rabinovitch, & Brown, 1979). This study, by Beck et al., set out to understand the acceleration of the head during full body swings around the high bar (Beck et al., 1979). To do this, Beck et al. utilized a plastic helmet equipped with accelerometers that provided approximate motion of the head during the gymnastics skill. Current advancements in sensor development now allow researchers to measure head accelerations without the use of helmets, and may provide a more accurate estimate of head motion. Of these devices, a mouthpiece-based sensor has been suggested to be ideal as it provides tight coupling with the upper dentition and skull (Wu et al., 2016) and is easy to wear in a variety of sports. These devices have been utilized in previous studies with soccer athletes (Miller, Pinkerton, et al., 2019; Rich et al., 2019) and may be useful for studying head kinematics within gymnastics. Despite the growing concern over concussions in sport, there is a paucity of data examining head injury mechanisms and head impact frequency within gymnastics, a sport in which concussions can occur and head impacts may be common. Therefore, the purpose of the current study was to outline a method for measuring and analyzing head kinematics in gymnastics. A secondary goal of the current study was to provide the first Science of Gymnastics Journal 230 Science of Gymnastics Journal Pritchard N.S., Urban J.E., Miller L.E. Lintner L., Stitzel J.D.: AN ANALYSIS OF HEAD. Vol. 12 Issue 3: 231 - 242 measurements of head kinematics and head impact exposure within gymnastics. METHODS Three optional level club women's artistic gymnasts (11-16 yrs) capable of performing a wide range of gymnastics skills were recruited to participate in this study. Gymnasts were excluded from this study if they were below the optional level and/or did not participate on a competitive USA gymnastics sanctioned team. The sample size was limited to three gymnasts due to the pilot nature of this study and primary objective of developing a method to measure and analyze head kinematics in gymnastics. The study protocol was approved by the Wake Forest University School of Medicine Institutional Review Board (IRB), and parental consent and participant assent were properly acquired for participation in the study. The gymnasts were instrumented for a combined total of 34 practices over a six month period with a validated custom fit mouthpiece (Rich et al., 2019) outfit with a triaxial accelerometer and gyroscope. To prevent changes in the conformation of the mouth from resulting in sensor coupling errors, gymnasts were excluded if they had been continually wearing orthodontic braces for less than six months. The mouthpiece was custom fitted to a 3D printed dental model created from a high resolution scan (3shape, Copenhagen, DK) of the upper dentition obtained by a trained staff member and reviewed by a dental technician to ensure proper fit and tight coupling to the upper dentition. Two time-synchronized cameras, arranged such that all apparatuses were in full view of at least one camera, filmed the gymnasts during each practice. Data acquisition of the sensor is controlled by a user-defined linear acceleration trigger threshold. When this value is exceeded for a prescribed period of time, the device records linear acceleration and rotational velocity at sample rates up to 4,681 Hz and 800 Hz, respectively. Other research using the same mouthpiece-based sensor has used a sampling frequency of 4,684 Hz and an acceleration threshold of 5 g's sustained for greater than or equal to 14 samples to collect 60 ms of data per recording (Rich et al., 2019). Although there have been many previous studies that examine head impact exposure using similar instrumentation, the gymnastics environment is different from most team sports. Therefore, a frequency analysis was performed by calculating the fast Fourier transform (FFT) of all events collected during a single session at 350 Hz to identify the ideal sampling frequency for this environment. A sampling frequency of 350 Hz was chosen as it was the maximum sampling frequency that the researchers could successfully time synchronize the data with video and capture the full duration of contact events due to sensor limitations. The dominant frequencies of the head during gymnastics skill motion were at or less than 35 Hz. Therefore, a sampling frequency of 100 Hz, was deemed sufficient to capture head kinematic data. The number of pre-impact samples and post-impact samples were extended so that both contact (i.e. when a gymnast comes in contact with a surface) and aerial (i.e. when an athlete performs a skill) data could be recorded by the mouthpiece sensor. The extended time of recording not only improved video pairing, but ensured that all contact scenarios within a skill series could be recorded. The final configuration utilized a sampling frequency of 100 Hz and a trigger threshold of 4 g sustained over 3 samples. Data collected by the sensor was processed according to the methods of Miller et al. (Miller et al., 2018) and Rich et al. (Rich et al., 2019); excluding the filter since the sampling frequencies in the current study were much lower. Briefly, linear acceleration and rotational velocity data were rotated to align with an anatomic coordinate system (X points from posterior to anterior, Y points from right to left, Z points from inferior to superior), rotational acceleration was computed by numerically Science of Gymnastics Journal 231 Science of Gymnastics Journal Pritchard N.S., Urban J.E., Miller L.E. Lintner L., Stitzel J.D.: AN ANALYSIS OF HEAD. Vol. 12 Issue 3: 232 - 242 differentiating the gyroscope data using a five-point difference formula, and finally linear acceleration was transformed to the head center of gravity (CG) using rigid body dynamics. Recorded mouthpiece events were paired with observed events on film using the mouthpiece timestamp and the video time to the nearest second. A frame-by-frame analysis was conducted for each event by identifying when the initiation of the peak linear acceleration occurs. Then the mouthpiece data was synchronized to the frame of the video where the athlete initially contacted the surface. In cases where an event was triggered without surface contact (e.g., from the linear acceleration produced by the athlete's rotation during a skill) the initiation of the peak signal was synchronized to the initiation of movement by the athlete. All kinematically-significant movements by the athlete (e.g., initial contact of foot, initiation of hip circle, etc.) were then identified in the video and matched to the event recording. Contact scenarios were segmented from the time of initial surface contact to the time the athlete's body part left contact with the surface or when the athlete's motion stopped (Figure 1). Skills, defined as gymnastics-related actions performed by the gymnast (e.g., back handspring), were segmented from the time of initial contact or initiation of movement, to the time the athlete's body part left contact with the apparatus or when the athlete's motion stopped (Figure 1). Segmented contact scenarios and skills were then zeroed to the mean of the previous five samples of the recording. If the start of the scenario occurred at the beginning of the recording, the first five samples of the contact scenario or skill were used to zero the segmented data. 3.) 10 S g c o 4-f ÍD