Fidel Hernandez

Evaluation of a laboratory model of human head impact biomechanics

This work describes methodology for evaluating laboratory models of head impact biomechanics. Using this methodology, we investigated: how closely does twin-wire drop testing model head rotation in American football impacts? Head rotation is believed to cause mild traumatic brain injury (mTBI) but helmet safety standards only model head translations believed to cause severe TBI. It is unknown whether laboratory head impact models in safety standards, like twin-wire drop testing, reproduce six degree-of-freedom (6DOF) head impact biomechanics that may cause mTBI. We compared 6DOF measurements of 421 American football head impacts to twin-wire drop tests at impact sites and velocities weighted to represent typical field exposure. The highest rotational velocities produced by drop testing were the 74th percentile of non-injury field impacts. For a given translational acceleration level, drop testing underestimated field rotational acceleration by 46% and rotational velocity by 72%. Primary rotational acceleration frequencies were much larger in drop tests (~100 Hz) than field impacts (~10 Hz). Drop testing was physically unable to produce acceleration directions common in field impacts. Initial conditions of a single field impact were highly resolved in stereo high-speed video and reconstructed in a drop test. Reconstruction results reflected aggregate trends of lower amplitude rotational velocity and higher frequency rotational acceleration in drop testing, apparently due to twin-wire constraints and the absence of a neck. These results suggest twin-wire drop testing is limited in modeling head rotation during impact, and motivate continued evaluation of head impact models to ensure helmets are tested under conditions that may cause mTBI.

Erratum to: Six Degree-of-Freedom Measurements of Human Mild Traumatic Brain Injury.

FIDEL HERNANDEZ, LYNDIA C. WU, MICHAEL C. YIP, KAVEH LAKSARI, ANDREW R. HOFFMAN, JAIME R. LOPEZ, GERALD A. GRANT, SVEIN KLEIVEN, and DAVID B. CAMARILLO Department of Mechanical Engineering, Stanford University, Stanford, CA, USA; Department of Bioengineering, Stanford University, Stanford, CA, USA; Department of Medicine, Stanford University, Stanford, CA, USA; Department of Neurology, Stanford University, Stanford, CA, USA; Department of Neurosurgery, Stanford University, Stanford, CA, USA; and Department of Neuronic Engineering, KTH Royal Institute of Technology, Stockholm, Sweden

Comparing In Vivo Head Impact Kinematics From American Football With Laboratory Drop and Linear Impactors

Roughly 5% of all collegiate and high school American football players suffer a concussion each season [1]. Concussions and repetitive sub-concussive trauma can have measurable effects on brain function and neurophysiological changes [2]. Several studies have suggested that a combination of linear and angular kinematic measures may be predictive of concussion [3, 4]. Presently, laboratory testing and analysis of purely linear kinematics is used to design and assess the safety of protective headgear. However, it is not known how well existing laboratory tests recapitulate angular kinematics. In this study, we analyze combinations of linear and angular head kinematics experienced by players on the field. This study sought to answer the question: how well do the twin-wire drop test apparatus and a spring-driven linear impactor reproduce the combination of linear and angular head impact kinematics experienced in vivo by players of American football?Copyright

Voluntary Head Rotational Velocity and Implications for Brain Injury Risk Metrics

We investigated whether humans could sustain high head rotational velocities without brain injury. Rotational velocity has long been implicated for predicting concussion risk, and has recently been used to develop the rotational velocity-based Brain Injury Criterion (BrIC). To assess the efficacy of rotational velocity and BrIC for predicting concussion risk, we instrumented 9 male subjects with sensor-laden mouthguards and measured six-degree-of-freedom head accelerations for 27 rapid voluntary head rotations. The fastest rotations produced peak rotational velocities of 12.6, 17.4, and 25.0 rad/s in the coronal, sagittal, and horizontal planes, respectively. All of these exceeded the corresponding medians from padded sports impacts (8.9, 10.7, and 8.4 rad/s, respectively) and, in the case of sagittal and horizontal rotation, were within 1 standard deviation of published concussion averages. In the horizontal plane, four voluntary rotations exceeded the concussive impact median BrIC. The area under the precision-recall curve was lower in BrIC (0.49) than just using horizontal rotational acceleration (0.8), which distinguished concussive and subconcussive motions better. Voluntary motions produced less than 4% max principal strain (MPS) in finite element simulation, 5 times below predictions from dummy impacts used to develop BrIC. Despite having the highest critical velocity in BrIC, coronal rotation produced more tract-oriented strain in the corpus callosum than other planes. Baseline and post-experiment neurological testing revealed no significant deficits. We find that the head can tolerate high-velocity, low-acceleration rotational inputs too slow to produce substantial brain deformation. These findings suggest that the time regime over which angular velocities occur must be carefully considered for concussion prediction.

Dependency of head impact rotation on head-neck positioning and soft tissue forces

Objective: Humans are susceptible to traumatic brain injuries from rapid head rotations that shear and stretch the brain tissue. Conversely, animals such as woodpeckers intentionally undergo repetitive head impacts without apparent injury. Here, we represent the head as the end effector of a rigid linkage cervical spine model to quantify how head angular accelerations are affected by the linkage positioning (head-neck configuration) and the soft tissue properties (muscles, ligaments, tendons). Methods: We developed a two-pivot manipulator model of the human cervical spine with passive torque elements to represent soft tissue forces. Passive torque parameters were fit against five human subjects undergoing mild laboratory head impacts with tensed and relaxed neck muscle activations. With this representation, we compared the effects of the linkage configuration dependent end-effector inertial properties and the soft tissue resistive forces on head impact rotation. Results: Small changes in cervical spine positioning (<5 degrees) can drastically affect the resulting rotational head accelerations (>100%) following an impact by altering the effective end-effector inertia. Comparatively, adjusting the soft tissue torque elements from relaxed to tensed muscle activations had a smaller (<30%) effect on maximum rotational head accelerations. Extending our analysis to a woodpecker rigid linkage model, we postulate that woodpeckers experience relatively minimal head impact rotation due to the configuration of their skeletal anatomy. Conclusion: Cervical spine positioning dictates the head angular acceleration following an impact, rather than the soft tissue torque elements. Significance: This analysis quantifies the importance of head positioning prior to impact, and may help us to explain why other species are naturally more resilient to head impacts than humans.