Athletic Training and Sports Health Care

Original Research 

Using In-ear Accelerometers to Measure Head Acceleration in Rough Stock Riders: A Pilot Study

Charles H. Mathers, MD, MPH; Jason P. Mihalik, PhD, CAT(C), ATC; Donald M. Andrews; Sharmila D. Watkins, MD, MPH; Patrick Puzzuto; D. Dale Kiecke, PT, SCS, ATC, LAT

Abstract

Previous studies on athletes in helmeted sports have evaluated the biomechanics of sports-related concussion, yet there is limited data related to nonhelmeted sports such as rodeo. We explored the feasibility of ear-mounted accelerometers for measuring head acceleration in rough stock riders. We also compared the head acceleration experienced by bareback and bull riders. Nine bull riders and 8 bareback (horse) riders were recruited to wear ear-mounted triaxial accelerometers and angular rate sensors. Data on peak resultant linear head acceleration, angular rate, and angular acceleration were computed. Bareback riders experienced significantly greater linear head acceleration and angular rate compared with bull riders. A non-significant trend was observed, suggesting bareback riders also experienced greater angular head acceleration than bull riders. In-ear accelerometers may offer a feasible means of measuring head acceleration in nonhelmeted sports, although more study is needed to overcome limitations of this design.

Abstract

Previous studies on athletes in helmeted sports have evaluated the biomechanics of sports-related concussion, yet there is limited data related to nonhelmeted sports such as rodeo. We explored the feasibility of ear-mounted accelerometers for measuring head acceleration in rough stock riders. We also compared the head acceleration experienced by bareback and bull riders. Nine bull riders and 8 bareback (horse) riders were recruited to wear ear-mounted triaxial accelerometers and angular rate sensors. Data on peak resultant linear head acceleration, angular rate, and angular acceleration were computed. Bareback riders experienced significantly greater linear head acceleration and angular rate compared with bull riders. A non-significant trend was observed, suggesting bareback riders also experienced greater angular head acceleration than bull riders. In-ear accelerometers may offer a feasible means of measuring head acceleration in nonhelmeted sports, although more study is needed to overcome limitations of this design.

Dr Mathers is from the NASA/UTMB Aerospace Medicine Residency Program, Preventive Medicine & Community Health, and Dr Watkins is from Preventive Medicine & Community Health, The University of Texas Medical Branch, Galveston; Mr Andrews is from the J. Pat Evans Research Foundation, Dallas; Mr Kiecke is from Justin Sportsmedicine Team, Flower Mound, Texas; Dr Mihalik is from the Matthew A. Gfeller Sport-Related Traumatic Brain Injury Research Center, Department of Exercise and Sport Science, The University of North Carolina, Chapel Hill, North Carolina; and Mr Puzzuto is from Diversified Technical Systems Inc, Seal Beach, California. Dr Mathers is also from the Division of Preventive, Occupational, and Aerospace Medicine, Mayo Clinic, Scottsdale, Arizona.

The authors thank Steve Moss from Diversified Technical Systems Inc, for his contributions to this project. This study was funded by the J. Pat Evans Research Foundation and the National Space Biomedical Research Institute. Diversified Technical Systems Inc and the Indy Racing League provided equipment and technical expertise for this project. The Justin Sportsmedicine Team provided on-site support and supplies for the Houston Livestock Show and Rodeo. Mr Puzzuto is a paid employee of Diversified Technical Systems Inc but has no other financial interest related to the study hardware. The authors have no other financial or proprietary interest in the materials presented herein.

Address correspondence to Charles H. Mathers, MD, MPH, Division of Preventive, Occupational, and Aerospace Medicine, 13400 E. Shea Boulevard, Scottsdale, AZ 85259; e-mail: mathers.charles@mayo.edu.

Received: May 04, 2011
Accepted: February 01, 2012
Posted Online: March 09, 2012

Sport-related concussion remains one of the more clinically difficult conditions for sports medicine professionals to manage. They are prevalent, with reports of as many as 3.8 million sport-related traumatic brain injuries sustained in the United States each year.1 Much of what clinicians understand regarding concussion presentation on the field, sideline clinical evaluation, and management and return to play of the concussed athlete is from the study of athletes following injury. Sports medicine professionals are challenged to study methods of preventing injury so that athletic participation by athletes of all ages remains safe.

Several studies have evaluated the biomechanics associated with sport-related concussion. These have ranged from observational studies2–5 to studies of the clinical manifestations following potentially injurious head impacts.6 Much of what is known can be related to the in vivo studies in this field. Not all severe head impacts cause injury, and impact severity does not appear to relate to clinical symptom status, postural control, or even cognitive function following sport-related concussion.2,7 Unfortunately, in vivo biomechanical studies of athletic head acceleration have been limited almost entirely to helmeted sports, including football and ice hockey. Although those sports tend to produce a high incidence of sport-related concussion, several other nonhelmeted traditional sports are also discussed in sport-related concussion circles, including, but not limited to, soccer (men’s and women’s), women’s lacrosse (men’s lacrosse is a helmeted sport), field hockey, gymnastics, cheerleading, and pole vaulting. To date, instrumentation allowing for proper study of head acceleration is lacking in these sports.

Current instrumentation needs to be extended to nonhelmeted sports. Some devices that have been proposed for this purpose include mouthpiece accelerometers8 and in-ear accelerometers.9,10 In-ear accelerometers have been developed and tested with the Indy Racing League and the Air Force Research Laboratory.9 Knox11 compared ear-mounted accelerometers with rigidly mounted sensors in manikins and found good correlation regarding measured head acceleration. Follow-up work by Begeman et al12 found that ear-mounted sensors did not remain firmly anchored to the head when exposed to higher amounts of acceleration. However, the recent work by Salazar et al13 improved on the sensor mounting to the earplugs by placing them into the external auditory canal and improving the mounting medium. The authors reported good correlation between ear-mounted sensor data and accelerometers implanted in cadaver skulls.13 Ear-mounted accelerometers have been tested in vivo among IndyCar race drivers, primarily focusing on head linear acceleration during crash impacts.9 A need exists to extend these devices into the study of nonhelmeted sports so clinicians may begin to better understand the nature of head acceleration.

Rodeo is a fast-paced sport with more than 700 major rodeos sanctioned by the Professional Rodeo Cowboys Association (PRCA) occurring annually in the United States and Canada.14 Head injuries have been recognized as a major cause of morbidity during rough stock events. The Justin Sportsmedicine Team published a study15 on injuries sustained during PRCA rodeo events from 1981 to 2005. Injuries during bull riding were most commonly reported, accounting for 50% of all injuries. Bareback riding caused the second most number of injuries, at 20%. Of all injuries reported, the head and face were injured the most, accounting for 16% of all the injuries reported. Concussion was the most common major injury reported in rodeo events, accounting for 50% of major injuries.15

In an injury analysis of Canadian Professional Rodeo from 1995 to 1999, Butterwick et al16 found that head injuries were the second most common injury experienced, with concussions accounting for 8.6% of all injuries. It was common for the cause of these concussions to remain unknown, and the authors point out that “violent whiplash” may have been responsible for some of the injuries.16 A recent review by Meyers and Laurent17 found that cranial trauma comprised 11% to 14% of all rodeo injuries reported in the literature, with concussion incidence rates of 3.4 per 1000 competitive events. Neurologic outcomes varied widely from no reported sequelae to severe injury, including seizures, quadriplegia, and death. The authors pointed to the “underestimation of trauma because of the limited body of knowledge available in this sport” and called for better injury surveillance in rodeo.17

The primary objective of the current study was to determine the feasibility of using in-ear accelerometer instrumentation to record head acceleration among rough stock riders during the 2009 Houston Livestock Show and Rodeo. Our goal was to examine whether an in-ear accelerometer system can be used to elucidate the injury mechanisms that cause concussions and other head injuries in rough stock riders. An earlier case report10 found that a bull rider’s head might experience peak linear accelerations of 26 g along the longitudinal axis during dismount alone, and bareback riders experienced 46-g head accelerations along the same axis. Thus, the secondary purpose of the current study was to use novel technology to address whether a difference in head acceleration exists between bull riders and bareback riders.

Method

Design and Participants

This study used an observational cohort design aimed at identifying whether differences in head linear acceleration and angular rate existed between bull riders and bareback riders. A convenience sample of 9 male bull riders (mean age = 25.56±4.19 years; mean height = 172.72±4.75 cm; mean mass = 70.20±6.95 kg) and 8 male bareback riders (mean age = 26.00±3.96 years; mean height = 171.45±4.29 cm; mean mass = 71.31±5.82 kg) were recruited to participate. This study recruited only male participants because there were no female riders participating in the bareback or bull riding events. The institutional review board of the lead author’s institution approved the study procedures. On the day of each rider’s event, the rider was introduced to the study team and hardware. Each participant provided written informed consent before participating.

Instrumentation

For the current study, the Slice Nano (Diversified Technical Systems Inc [DTS], Seal Beach, California) data recorder was selected. Its total volume measures approximately 20 mm3. The Slice Nano data recorder, the 9-volt battery, and required wires were encased in a plastic cover. The assembled data recorder unit was approximately the size of a smartphone. Small wires connected the data recorder to earplugs with embedded accelerometers and angular rate sensors. Another wire from the data recorder connected to a small red button and a green LED light. Pressing the red button began the data collection, whereas the LED light provided a visual confirmation of activation. The data recorder unit, earplugs with wires, and USB cable are depicted in the Figure.

Study hardware with data recorder unit exposed (wire with red button and green LED light not shown).

Figure. Study hardware with data recorder unit exposed (wire with red button and green LED light not shown).

Diversified Technical Systems Inc, assembled 2 sets of earplugs with embedded triaxial sensors. The left earplug was embedded with 3 uniaxial linear accelerometers (Single-Axis, High-g, iMems Accelerometers, serial number ADXL 193; Analog Devices, Inc, Norwood, Massachusetts). These sensors measure 5×5×2 mm and record up to 250 g. The right earplug was embedded with 3 angular rate sensors (ARS-8K High Performance Angular Rate Sensor; DTS), which measure 7.6×10.2×14.6 mm and record up to 139.6 rad/sec. These sensors were validated through testing with the National Highway Traffic Safety Administration.18 This configuration offers significant advantages allowing for the collection of 6 channels of data and allows for both linear acceleration along the x-, y-, and z-axes, and angular rate along the x-, y-, and z-axes.

Angular rate sensors, which measure angular displacement over time, were used because sensors to detect angular acceleration were too large to incorporate into earplugs at the time of this study. Data collection frequency was set at 2 kHz, and synchronous (linear acceleration and angular rate) data were recorded for the duration of the participants’ events. Size constraints limited each earplug to 3 imbedded sensors. The earplugs each had a small hole drilled in them, permitting the rider to hear study investigators, crowd noise, and medical personnel should an injury be sustained during the event. At the onset of data collection, the earpiece accelerometers filtered the raw signal data using a 400-Hz 9-pole Butterworth filter.

Procedures

Approximately 45 minutes prior to the rider’s scheduled event, the rider was outfitted with the study hardware. The data recorder unit itself was placed in a commercially available camera case for added protection and then strapped to the rider’s upper abdomen using an elastic strap. Next, the earplugs were placed in the rider’s ears. The earplugs were secured in place using a standard Cover-Roll (BSN Medical BmbH & Co KG, Hamburg, Germany) stretch adhesive bandage. Benzoin Compound Tincture USP (Paddock Laboratories, Inc, Minneapolis, Minnesota), a substance that helps secure and sweat-proof the tape, was placed on the rider’s ears with a swab stick capsule prior to tape application. The wires connecting the earplugs to the data recorder unit were taped above the rider’s clavicle on each side with adequate slack to prevent impeding the riders’ head motion. The rest of the wires and data recorder unit were secured underneath the riders vest or shirt. The red button and LED light were taped to the rider’s gripping arm (ie, the arm the rider uses to hold on to the horse or bull during the event).

After the hardware was secured, the rider proceeded to the arena. Approximately 15 minutes prior to their scheduled event, the study team met with the rider again to arm the system. This arming process consisted of placing the 9-volt battery into the data recorder unit, plugging the unit into a laptop computer using the USB cable, performing a final system check of the hardware, and arming the unit for data collection. Because the 9-volt battery provides approximately 45 minutes of charge to the data recorder unit, the arming process had to occur in the arena just prior to the event. Approximately 1 minute prior to the rider’s event, a representative from the event sports medicine team depressed the red button taped to the rider’s shoulder, thus beginning the data collection. The rider then completed his rough stock event (ie, bull ride or bareback ride). The hardware recorded data for 10 minutes continuously after triggering of the button. Data on linear acceleration and angular rate were recorded to the Slice Nano data recorder at 2000 samples per second.

Upon completion of the event, the rider returned to the study team. The hardware was removed and the data recorder unit was plugged into a laptop computer to begin downloading the data. The data were analyzed using commercially available software from DTS and a laptop computer (Inspiron 1525; Dell Inc, Round Rock, Texas). In total, data were collected on 9 bareback and 9 bull rides. One bareback rider was able to complete 2 rides within the 10-minute recording period.

Data Reduction

Raw accelerometer data were imported into TDAS Control Software (DTS). Raw data were then cropped to include only those data collected during the actual rodeo event. As stated earlier, the system is capable of recording 10 minutes of continuous data, but the actual rough stock rodeo events are meant to last no more than 8 seconds for a successful ride attempt. After cropping, the data were then filtered using a Society of Automotive Engineers class 180 (300 Hz) 4-pole phaseless Butterworth filter. They were then imported into MATLAB (MathWorks, Natick, Massachusetts), where the peak linear acceleration raw data collected along the x-, y-, and z-axes were then used to compute a resultant linear acceleration using a sum of squares method, in essence applying Pythagorean’s theorem to 3 dimensions [resultant = (x2 + y2 + z2)1/2]. This method is used by other head impact biomechanics technologies including the Head Impact Telemetry (HIT) System (Simbex, Lebanon, New Hampshire). Linear acceleration was measured in terms of gravity force (ie, 1 g = 9.81 m/s2). The same sum of squares procedure was used to compute the resultant angular rate from the individual channels (x, y, and z). Angular rate data, originally recorded in degrees per second, was converted to radians per second. Instantaneous angular acceleration profiles were computed by dividing the change in angular rate by the data collection frequency [eg, (angular rate data point 2 – angular rate data point 1)/0.0005 s)]. Reduced linear accelerations, angular rates, and angular accelerations were then exported for subsequent statistical analyses in SAS software (SAS Institute Inc, Cary, North Carolina).

Statistical Analyses

We computed descriptive statistics (mean and 95% confidence intervals) on peak linear acceleration, angular rate, and angular acceleration for each event type. Each rider sustained multiple sudden head movements over the course of their ride. Therefore, the data represent repeated measures on the same group of rodeo athletes over time. To account for the lack of statistical independence due to repeated measures, we used a variation of linear regression known as random intercepts general mixed linear models.19 This model is similar to traditional linear regression but allows each rider to have his own value for the model intercept. A variable for the rider was included in every model. Event type (bare-back versus bull riding) was included as a separate independent variable (in addition to rider) in the statistical models used in this study. All analyses were performed using SAS version 9.2 software. Our alpha level was established at 0.05 a priori.

Results

We recorded a total of 238 sudden head accelerations during our bull and bareback rides for which we were able to compute peak linear acceleration, angular rate, and angular acceleration. Of these sudden head accelerations, 45.8% (109 of 238) occurred during bull riding events, whereas the remaining 54.2% (129 of 238) occurred during bareback riding. We observed a statistically significant higher amount of peak head linear acceleration associated with bareback riding, compared with bull riding. Likewise, we found angular rate during bareback riding to be greater than the head angular rates observed during bull riding. We did not observe a statistically significant difference in angular acceleration between the 2 events. Descriptive and statistical information for these analyses are provided in the Table.

Frequency of Recorded Sudden Head Accelerations, Mean Peak Resultant Linear Acceleration, Angular Rate, and Angular Acceleration Sustained by Event Type

Table: Frequency of Recorded Sudden Head Accelerations, Mean Peak Resultant Linear Acceleration, Angular Rate, and Angular Acceleration Sustained by Event Type

Discussion

We were able to use in-ear accelerometer technology during rough stock rodeo events. Further, we were capable of collecting in-event data and using these data to identify differences between bareback and bull riding. In so doing, we were able to successfully accomplish our primary objective of the study—to use in-ear accelerometer instrumentation to record head acceleration during a nonhelmeted athletic event. This system could be used to study head acceleration in other nonhelmeted sports.

However, we identified a limitation in how in-ear accelerometers compute biomechanics relative to the head center of gravity. We must acknowledge that although the accelerometers are located within the participant’s ear, this is not exactly synonymous with the head center of gravity. Due to this limitation, rotation of the head in any direction will invalidate the linear acceleration data. For example, in the scenario whereby the only movement is rotation about the longitudinal axis (z-axis, represented when expressing “no”), linear acceleration will still be recorded relative to the in-ear accelerometer due to its displacement away from the head center of gravity when, in fact, the only movement is the pure rotational movement. Thus, the linear acceleration measurements will be valid only when the head is fixed with no rotation in any direction. The likelihood of this occurring in the context of athletic participation is near impossible; the limitation does exist and is worthy of discussion in the context of this methodological feasibility study.

In vivo studies of head biomechanics related to sport concussion have largely used an array of helmet-mounted linear accelerometers. A system used in many recent studies of helmeted athletes is the HIT System, which uses an array of 6 helmet-mounted accelerometers.3–7,19 From this system, data computed includes head linear acceleration, angular acceleration, and impact location. Olvey et al9 tested ear-mounted linear accelerometers in Formula race car drivers and reported good correlation between these sensors, a 9-accelerometer helmet-mounted system, and car-mounted accelerometers during driving. However, this system is limited to studying head linear acceleration only. In addition, given the limitations identified in our study, we feel that ear-mounted accelerometer systems need further laboratory validation before further in vivo studies are undertaken.

As for our secondary objective, the data show head linear acceleration and angular rate are significantly greater in bareback riding, compared with bull riding. No differences in head angular acceleration were found between the 2 rough stock events. Studies in helmeted athletes have attempted to define a threshold of linear and angular acceleration that predicts risk of head injury. Broglio et al3 suggested that exposures of 5582.3 rad/sec2 and 96.1 g increases the risk of concussion in football players, and this correlated well with previously published findings.19 However, Guskiewicz et al7 found that concussed football players sometimes experience much less head linear and angular acceleration. Historically, laboratory studies examining traumatic brain injury have measured head angular acceleration, given that the rate of change in head velocity is thought to create shearing forces on the brain necessary for injury.20–22 However, rotational velocity has been shown to be strongly correlated to relative brain motion.23,24 A recent study by Rowson et al25 examined rotational head kinematics in football players using the HIT System and reported a nominal injury risk of 90% with head velocities of 33.2 rad/sec. Although none of the rough stock riders tested were diagnosed with concussion, bareback riders’ heads experienced a mean peak resultant angular rate of 33.2 rad/sec, which could put them at risk for injury. The frequency of recorded events in our study is much smaller than the 300,000 reported by Rowson et al25; their study also examined direct head impacts rather than indirect (ie, nonimpact-related) head motion. Further study is needed with larger samples of rough stock riders to better define the risk of head injury during rough stock events.

Soccer is a nonhelmeted sport in which researchers have evaluated predisposing factors for concussion. Tierney et al26 compared head–neck segment dynamic stabilization during head acceleration in male and female soccer players, finding that female soccer players demonstrated greater head-neck segment angular acceleration than did male soccer players. This was hypothesized to result from factors such as differences in neck muscle mass and girth. However, this research was not performed in vivo and used motion analysis to determine acceleration, not direct measurement.26 In addition, Mansell et al27 found that strength training did not enhance head–neck segment dynamic stabilization in collegiate soccer players, despite increases in isometric strength and girth. Although refinements to the design would be needed, ear-mounted accelerometers could allow researchers to study head biomechanics in vivo among soccer players and other nonhelmeted athletes.

The sport of rodeo presents many challenges for safely and accurately measuring head motion. The rider and animal complete complex and unpredictable movements during a short period of time (ie, a successful attempt must last at least 8 seconds), followed by what is often an abrupt dismount. Fitted earplugs or a mouthpiece were initially considered for testing; however, only a limited number of sensors were available to accommodate this design. Thus, we devised 3 different earplug sizes to be used interchangeably by the rodeo athletes. The Indy Racing League was able to provide only medium and large earplugs for testing, which meant fit and coupling presented a problem for some of the riders. Future studies could use fitted earplugs or a mouthpiece embedded with sensors. Despite these limitations, this is the first attempt at measuring head acceleration in a nonhelmeted sport and can serve as a baseline for future studies in the sport of rodeo and other nonhelmeted sports.

In their current form, we have identified several issues related to the instrumentation used in this study. Given the small size that is required of the data recorder, it became hot to the touch during operation. This was the result of the small surface area, incapable of dissipating the heat produced by the internal circuitry. Although the heat did not bother any of our athletes, given that we had enclosed the data recorder in a commercially available camera case, researchers interested in using similar technology should mount this instrumentation to structures that will conduct heat away from the data recorder. Another helpful modification would involve reinforcement of the connector that attaches the earplug wires to the data recorder unit. One of these wires broke during rodeo testing, likely due to a jerking motion during one of the rides. This equipment failure is likely to be replicated in other nonhelmeted sports in which sudden head movements are typical, and enhancements to their design would be warranted before implementation in future studies. In addition, limitations in battery life prevent data collection for more than 10 minutes. Although only the size of a smartphone, the data recorder is still bulky when worn during a sporting event and requires wires to connect to the ear-mounted sensors. Improvements in miniaturization and wireless technology could improve on this design.

Implications for Clinical Practice

Given the limitations identified and the small sample size studied, it is difficult to make recommendations to clinicians caring for rough stock riders. Although our study suggests that bareback riders may be at a higher risk for nonimpact-related head injury, more study and hardware improvements are needed to better understand this phenomenon. However, clinicians should be particularly diligent about screening for concussion in these athletes. In addition, educating both bareback and bull riders about the forces placed on their heads may encourage them to identify concussion-related symptoms to the medical team, thereby enhancing the sport’s understanding of nonimpact-related head injury.

Conclusion

Our study suggests in-ear accelerometers may be feasibly used to study head acceleration in nonhelmeted sports. In the context of our pilot study, we used this instrumentation in rodeo to maximize the frequency and severity of sudden head movement. We were able to successfully record head acceleration, provide meaningful biomechanical comparisons within rough stock rodeo events, and identify some of the limitations of the current instrumentation that will need to be addressed before researchers use these technologies in other nonhelmeted sports.

References

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Frequency of Recorded Sudden Head Accelerations, Mean Peak Resultant Linear Acceleration, Angular Rate, and Angular Acceleration Sustained by Event Type

FREQUENCY (%)LINEAR ACCELERATION (g)
ANGULAR RATE (rad/s)
ANGULAR ACCELERATION (rad/s2)
MEAN95% CI
PaMEAN95% CI
PaMEAN95% CI
Pa
LULULU
Bareback129 (54.2)15.712.219.2<0.00133.227.139.3<0.0012129.41359.32899.50.387
Bull ridingb109 (45.8)6.14.57.6(Ref)12.08.515.6(Ref)1577.8508.22647.4(Ref)
Total23811.410.112.723.421.225.61876.81310.92442.6
Authors

Dr Mathers is from the NASA/UTMB Aerospace Medicine Residency Program, Preventive Medicine & Community Health, and Dr Watkins is from Preventive Medicine & Community Health, The University of Texas Medical Branch, Galveston; Mr Andrews is from the J. Pat Evans Research Foundation, Dallas; Mr Kiecke is from Justin Sportsmedicine Team, Flower Mound, Texas; Dr Mihalik is from the Matthew A. Gfeller Sport-Related Traumatic Brain Injury Research Center, Department of Exercise and Sport Science, The University of North Carolina, Chapel Hill, North Carolina; and Mr Puzzuto is from Diversified Technical Systems Inc, Seal Beach, California. Dr Mathers is also from the Division of Preventive, Occupational, and Aerospace Medicine, Mayo Clinic, Scottsdale, Arizona.

Address correspondence to Charles H. Mathers, MD, MPH, Division of Preventive, Occupational, and Aerospace Medicine, 13400 E. Shea Boulevard, Scottsdale, AZ 85259; e-mail: mathers.charles@mayo.edu

10.3928/19425864-20120309-01

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