Comparing Head Impact Power with Resultant Head Kinematics as a Metric to Evaluate Soccer Header Biomechanics

Introduction: Soccer is the fastest-growing youth sport in the United States with the number of high school participants increasing by 32% since 2002 [1]. Concussion rates in soccer are often comparable, and even exceed, contact sports, such as football and rugby, with girls having a higher rate of concussion than boys [2], [3]. Heading in soccer is a fundamental part of the game; and is a major contributor to an athletes’ overall exposure to repetitive head impacts [4]. Repetitive head impacts have been associated with structural brain changes, visual memory impairment, and postural control deficits, even in the absence of clinically diagnosed concussion [5]. Previous studies have characterized head impact exposure in soccer using peak resultant head kinematics [4], [6]; however, these metrics alone do not account for the six degrees of freedom (DOF), time dependent biomechanics of the head during an impact, which may be important in understanding brain strain [7]. Head Impact Power (HIP) is a head injury assessment function which uses a summation of the power terms in all six DOF to find a maximum power index utilizing linear acceleration and velocity and rotational acceleration and velocity in one metric [8]. The objective of this study was then to characterize HIP as a metric accounting for the linear and rotational kinematics that occur during heading in soccer, and evaluate its relationship with existing peak kinematic metrics.

Materials and

Methods: Eight female youth soccer players (13.5 ± 0.11 years) on a U15 club soccer team participated in this study during two seasons in the fall and spring of 2019. Study athletes were instrumented with a custom-fit mouthpiece device utilizing a tri-axial accelerometer and gyroscope embedded in acrylic which has been previously validated for bare and helmeted impacts [9]. An event was recorded when the accelerometer exceeded a 5g threshold (in season 1) and 3.5 g (in season 2) in any direction for at least 3ms. 15ms of pre-trigger data and 45ms of post-trigger data were recorded for each event. The kinematic data recorded by the mouthpiece was transformed to the head center of gravity for analysis. True-positive head acceleration events (HAEs; i.e., the mouthpiece sensor recorded an event and the event was observed on video) were identified using time-synchronized film collected at each practice and game. HAEs were categorize by contact scenario (e.g., header, dive, head-to-head contact). Peak resultant kinematics and HIP were then calculated for each event in a sample of validated headers for analysis. HIP was then compared with each peak resultant kinematic using a linear regression.

Results, Conclusions, and Discussions:

Results: Head impact data was recorded over two seasons totaling 110 sessions (72 practices and 38 games). A total of 4,465 head impacts were observed from film review, with 2,457 (55%) true-positive events recorded by the mouthpiece. Of these, a total of 878 (36%) true-positive events were excluded from the current analysis; 535 (22%) because no head contact occurred or the impact was not a header and 343 (14%) due high frequency gyroscope measurements or the entire header not being captured. The final data set contained 1579 (64%) headers. The median (95th percentile) linear acceleration, rotational acceleration, rotational velocity, and HIP were 8.0g (14.1g), 504.8 rad/s2 (1208.2 rad/s2), 3.8 rad/s (6.6 rad/s), and 149.2W (365.6W) respectively. Linear regressions between HIP and linear acceleration, rotational acceleration, and rotational velocity yielded R2 values of 0.81, 0.57, and 0.45 respectively.

Discussion and

Conclusion: The results of this study describe the relationship between kinematics of the head and HIP of headers during female youth soccer. HIP showed positive correlations with head kinematics with the metric being most correlated with linear acceleration and least correlated with rotational velocity. The metric takes into account linear and rotational head motion, and could better describe the biomechanics of soccer heading in future characterization studies and evaluations. HIP and other metrics related to impact power and energy transfer that take into account more of the biomechanics of the head can be enhanced to better characterize HAEs, and could yield sport- or impact-specific metrics that better relate to brain strain [10]. Future research should seek to evaluate the relationship between HIP and brain strain experienced during heading to inform its utility as a measure to evaluate future interventions designed to reduce the risk of brain injury in soccer players.

Acknowledgements (Optional):

References:
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