Lyndia Wu

Soccer players experience repetitive subconcussive impacts during heading, which may cause long-term neurological damage. Neck muscle strength and activation have been suggested to mitigate head impacts and decrease the risk of brain injury. Specifically, increasing neck muscle activation before and during head impacts may decrease head kinematics parameters related to brain injury. However, studies have primarily investigated muscle activation in laboratory environments, where head impacts are typically of lower magnitude than real-life events. We recruited 10 participants with previous soccer heading experience and instrumented them with wearable sensors to collect neck muscle activation and head impact kinematics during soccer headers. Before conducting participant experiments, we created a custom wearable sensor system and designed a circuit to time synchronize muscle activity and kinematics. We characterized neck muscle activation, timing, magnitude, and co-contraction patterns using electromyography (EMG) and peak head kinematics using a custom instrumented mouthguard (iMG). We found that the neck muscles were actively engaged around the time of impact, and that peak activation magnitude increased when soccer balls were delivered at higher speeds. However, the timing and magnitude of the activation were not always maximized for head stabilization. Our findings suggest players may benefit from training to improve the timing of muscle engagement to maximize their stabilization potential and reduce head kinematics.

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Instrumented mouthguards measure on-field sports head impact and concussion biomechanics. While an increasing number of studies have applied mouthguard sensors, potential kinematic error associated with on-field mouthguard coupling variations have not been quantified. In addition, past research has been heavily biased in study population, lacking sex and sport diversity. Studies of concussion mechanisms have also primarily concentrated on single severe impacts with limited investigation of repeated impact exposure mechanisms. This research aims to assess and apply instrumented mouthguards in on-field settings to investigate head impact biomechanics and their injury influence in sports. We deployed mouthguard sensors to university women’s rugby and men’s hockey athletes during competitive seasons. Coupling categories were defined using on-field proximity sensor data before comparing signal characteristics between categories. Additionally, impacts from women’s rugby and men’s hockey were characterized using head kinematics- and brain deformation-based injury criteria. The cumulative effect of head impacts on concussion risk was investigated by modifying and applying a time-weighted exposure injury criterion to concussion and non-concussion datasets. Results showed that poorly coupled mouthguards led to higher angular kinematics and high-frequency noise. In our impact biomechanics characterization, women’s rugby impacts showed lower angular kinematics. Our modified time-weighted exposure metric showed increased sensitivity in differentiating concussion and non-concussion days compared to the same-day impact frequency, timing, and magnitudes separately. From our findings, we demonstrate the potential for using proximity sensors in instrumented mouthguards to assess on-field head impact coupling and data quality. Characterization of women’s rugby and men’s hockey head impacts and comparison with existing literature revealed that impact kinematics may vary by sensor type, age, sport, and sex. These differences must be accounted for when developing injury mitigation techniques, particularly in women’s sports. Finally, considering head impact history may allow for increased sensitivity in predicting concussions. In summary, this thesis contributes novel insights into on-field instrumented mouthguard data quality, potential sport- and sex-based differences in head impact exposure, and the role of repeated impact exposure in concussion risk. The findings from this work may be applied in future research for more rigorous sensor-based investigation of sports head impacts and concussions.

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Electroencephalography (EEG) is a common neuroimaging technique used in clinical, research, and consumer technology due to its non-invasiveness, high time resolution, and sensitivity. Traditional EEG systems consist of wet electrodes and a desktop amplifier for measuring brain activity. While this system provides high signal quality in lab settings, it limits real-world EEG use, such as for ambulatory EEG monitoring, or EEG measurements during exercise. This is due to the training and set up time required for wet electrode application and the bulky, wired traditional amplifier. While some wearable EEG devices exist on the market, EEG measurements during motion requires further signal quality validation to confirm clinical or research utility due to motion artifacts. The objectives of this thesis are to develop custom dry flexible electrodes with improved usability for wearable systems and to compare signal quality of the custom dry electrodes with traditional wet electrodes in mobile measurements. We have developed comb-shaped, dry electrodes for measuring scalp EEG. This electrode incorporates embedded silver thread for a novel yet simple to fabricate design, which allows for a flexible electrode providing impedances similar or lower to those in literature. Participants were instrumented with an EEG cap containing gold cup, gel, and dry electrodes for brain activity measurement as well as an inertial measurement unit (IMU) mouthguard for head kinematics measurements. Data were collected from 8 participants, and during nearly all trials, dry electrodes showed worse signal correlation with gold cup electrodes compared with gel electrodes. For motion artifact characterization, EEG-IMU coherence was found to be higher for dry electrodes in low motion scenarios (e.g., walking), but similar between the 3 electrode types for high motion scenarios (e.g., jumping). This indicates that dry electrodes may be more prone to motion artifacts at lower levels of activity, but with large enough activity all 3 types of electrodes could be equally affected. In summary, we developed custom flexible dry electrodes with improved usability for wearable systems. However, we showed that dry electrode signal quality may be substantially affected by motion artifacts, and wet electrodes may be more suitable for measurements during motion.

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Whiplash injuries remain the most common injury associated with rear-end low-speed collisions despite improvements in head restraint designs and the introduction of innovative anti-whiplash seats. To address this problem, our research group developed an active car seat (RoboSeat) that controls seat hinge rotation and seatback cushion deformation. Preliminary experiments showed that the RoboSeat could reduce the kinematic and kinetic responses of an anthropometric test device. The aim of this study was to compare the performance of an actively controlled experimental anti-whiplash (RoboSeat) seat to passive control anti-whiplash seats in human volunteers: General Motor’s High Retention seat (GMHR) & Volvo’s Whiplash Protection Seat (WHIPS). Twelve healthy participants were exposed to a whiplash-like perturbation (4km/h speed change) while seated on each of the three seats. We recorded the electromyographic activity of sternocleidomastoid, neck paraspinals, splenius capitis, and multifidus along with the head/torso kinematics to quantify the participants’ responses to the whiplash-like perturbations. Given that excessive strain in cervical facet capsules is suggested as a cause of whiplash injury, we quantified cervical multifidus activation while head retraction occurred because both factors can potentially increase strains in the capsular ligaments. We hypothesized that the dynamic seatback rotation and cushion deformation of the RoboSeat prevents simultaneous activation of the neck multifidus muscle and head retraction. The RoboSeat reduced the combined multifidus activation and head retraction by 76% and 43% as well as 12/16 and 6/16 kinematic variables compared to the GMHR and WHIPS seats, respectively. Overall, our results suggest that the active seat can lower some head/torso kinematic responses compared to the current anti-whiplash seats and minimize combined activation of the multifidus muscle and head retraction. Our active RoboSeat represents a promising approach to potentially decrease the risk of whiplash injuries following low-speed rear-end collisions.

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Soccer heading is a common technique where players use their head to pass, shoot or clear the ball. The ball-to-head impact involved in this technique has raised concern for risk of head and brain injury. However, there are inconclusive findings on the effect of soccer headers in the literature. The objective of this study was to investigate, in a controlled environment, whether mild soccer head impacts result in immediate neurophysiological changes in the brain, and if yes, whether the changes are affected by impact level and header direction. Controlled soccer headers were simulated at 2 impact levels in 3 directions, representative of the mildest headers experienced on the field, using a custom pendulum impactor with a soccer ball attachment. Participants were instrumented with an inertial measurement unit (IMU) to record the head kinematics of head impacts and an electroencephalography (EEG) device to measure neurophysiological changes. The EEG changes were evaluated by four metrics (absolute and relative power, magnitude squared and imaginary coherence) for common brain wave frequency bands. With data from 8 participants (6 males, 2 females), the study provided statistically significant evidence of the immediate neurophysiological changes after mild soccer headers. We found a surge in normalized absolute power right after heading across all the frequency bands with a larger increase for headers at the higher impact level. In addition, we observed an increase in delta band relative power, along with a decrease in the higher frequency bands, indicating slowing of activity in the brain. These findings are consistent with those observed in patients with traumatic brain injury or post-concussive syndrome. Aside from changes in power, we also found evidence of significant changes in coherence with different patterns between magnitude squared and imaginary coherence, suggesting effects of heading on the brain’s functional connectivity. Finally, we found evidence that these changes diminished over time and participant’s neurocognitive performance remained unchanged. Our findings suggest that even mild soccer headers could lead to immediate, transient neurophysiological changes and highlight the importance of further investigation of the effects of long-term head impact accumulation in sports.

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