Angular Acceleration Of Head Research Articles

Lightweight low-profile nine-accelerometer package to obtain head angular accelerations in short-duration impacts

Despite recognizing the importance of angular acceleration in brain injury, computations using data from experimental studies with biological models such as human cadavers have met with varying degrees of success. In this study, a lightweight and a low-profile version of the nine-accelerometer system was developed for applications in head injury evaluations and impact biomechanics tests. The triangular pyramidal nine-accelerometer package (PNAP) is precision-machined out of standard aluminum, is lightweight (65 g), and has a low profile (82 mm base width, 35 mm vertex height). The PNAP assures accurate orthogonal characteristics because all nine accelerometers are pre-aligned and attached before mounting on a human cadaver preparation. The feasibility of using the PNAP in human cadaver head studies is demonstrated by subjecting a specimen to an impact velocity of 8.1 m/s and the resultant angular acceleration peaked at 17 krad/s 2. The accuracy and the high fidelity of the PNAP device at high and low angular acceleration levels were demonstrated by comparing the PNAP-derived angular acceleration data with separate tests using the internal nine-accelerometer head of the Hybrid III anthropomorphic test device. Mounting of the PNAP on a biological specimen such as a human cadaver head should yield very accurate angular acceleration data.

Kinematics of head movement and onset of sternocleidomastoid muscle activity in simulated low-velocity rear-end impacts

Objectives. Rear-end impacts and whiplash injuries are highly associated. More than a million whiplash injuries occur each year in the US.1Malleson A. Chronic whiplash syndrome. Psychosocial epidemic.Canadian Family Physician. 1994; 40: 1906-1909Google Scholar The kinematics of head movement in simulated rear-end impacts has been described. However, the influence of impact awareness remains controversial. In addition, the suggestion that cervical muscles activate too late in rear-end impacts may be incorrect.2Siegmund G.P. Brault J.R. Wheeler J.B. The relationship between clinical and kinematic responses from human subject testing in rear-end automobile collisions.Accident Analysis & Prevention. 2000; 32: 207-217Google Scholar The goal of this study was to analyze the kinematics of head movement in rear-end impacts related to 2 impact magnitudes, impact awareness and onset of sternocleidomastoid (SCM) muscle response. Study design. The Human Research Ethics Board at the University of Alberta approved this study. Thirty subjects completed the study. Each subject underwent 3 impacts: 1 slow unexpected (4.5 m/sec/sec), 1 fast unexpected (10.0 m/sec/sec), and 1 fast expected impact of the same magnitude as the fast unexpected impact. Kinematics of head movement was simultaneously recorded with a custom accelerometer board and the use of video cameras. Angular head displacement, and angular and linear head acceleration were recorded. Normalized SCM surface electromyography (EMG) was obtained. Onset and peak time for the EMG and the kinematics were determined. Onset time was defined as the time in which 5% of the magnitude value of the peak occurred. Peak time was defined as the time in which the maximum value of the variable was reached. Repeated-measures statistical method was used to analyze the head movement behavior associated with impact velocity and expectation. The same statistical method was used to determine whether muscle activity or head movement was initiated first. Results. The video camera and accelerometer data presented a good agreement. Subjects presented an initial rearward movement in which the head moved backward followed by a forward movement of the head. The rearward angular head displacement almost doubled (∼13 degrees) with increased impact magnitude (P < .05). The peak magnitude of rearward and forward angular acceleration increased 2 to 3 times with increased impact magnitude (P < .05). Rearward and forward linear displacement was 2.5-3.5 times higher for the fast than the slow unexpected impacts (P < .05). There were no significant differences in the magnitude of angular head acceleration, angular head displacement, and linear head acceleration regarding awareness (P>0.05). The onset time of SCM peak EMG ranged from 78-114 ms later than peak of linear head acceleration for all groups (P < .05). The onset of the SCM peak EMG ranged from 136-188 ms earlier than the peak rearward angular head displacement in all impacts (P < .05). Onset time of peak SCM EMG was significantly earlier (30 ms) than the peak angular head acceleration for the fast unexpected impact (P < .05). Conclusions. Kinematics of head movement increased with increased impact magnitude. Magnitude of angular head displacement was within physiologic limits and do not support the hyperextension theory. Temporal and amplitude awareness did not produce different magnitude in kinematics of head movement. The temporal relationship between the SCM and angular head acceleration is different from the temporal relationship between the SCM and linear head acceleration.

Gender Differences in Head???Neck Segment Dynamic Stabilization during Head Acceleration

Recent epidemiological research has revealed that gender differences exist in concussion incidence but no study has investigated why females may be at greater risk of concussion. Our purpose was to determine whether gender differences existed in head-neck segment kinematic and neuromuscular control variables responses to an external force application with and without neck muscle preactivation. Forty (20 females and 20 males) physically active volunteers participated in the study. The independent variables were gender, force application (known vs unknown), and force direction (forced flexion vs forced extension). The dependent variables were kinematic and EMG variables, head-neck segment stiffness, and head-neck segment flexor and extensor isometric strength. Statistical analyses consisted of multiple multivariate and univariate analyses of variance, follow-up univariate analyses of variance, and t-tests (P < or = 0.05). Gender differences existed in head-neck segment dynamic stabilization during head angular acceleration. Females exhibited significantly greater head-neck segment peak angular acceleration (50%) and displacement (39%) than males despite initiating muscle activity significantly earlier (SCM only) and using a greater percentage of their maximum head-neck segment muscle activity (79% peak activity and 117% muscle activity area). The head-neck segment angular acceleration differences may be because females exhibited significantly less isometric strength (49%), neck girth (30%), and head mass (43%), resulting in lower levels of head-neck segment stiffness (29%). For our subject demographic, the results revealed gender differences in head-neck segment dynamic stabilization during head acceleration in response to an external force application. Females exhibited significantly greater head-neck segment peak angular acceleration and displacement than males despite initiating muscle activity earlier (SCM only) and using a greater percentage of their maximum head-neck segment muscle activity.

Variability in Head Movement during Gait Transitions

Predictions from a model of coupled oscillators, which was based on stationary interlimb coordination, were examined in locomotor intralimb coordination. The present study measured head linear and angular accelerations and coordination relationships of the head and torso, e.g., using relative phase measures, during walking and running on a treadmill. Head linear acceleration increased from walking to running and decreased from running to walking, and head pitch/torso pitch relative phase decreased from walking to running and increased from running to walking. No other measures were significantly different for either walking or running (head angular acceleration, mean head vertical displacement/head pitch relative phase, SD head vertical displacement/head pitch, and SD head pitch/torso pitch). This information will aid further understanding of the many aspects involved in locomotor coordination.

Verification of biomechanical methods employed in a comprehensive study of mild traumatic brain injury and the effectiveness of American football helmets

Concussion, or mild traumatic brain injury, occurs in many activities, mostly as a result of the head being accelerated. A comprehensive study has been conducted to understand better the mechanics of the impacts associated with concussion in American football. This study involves a sequence of techniques to analyse and reconstruct many different head impact scenarios. It is important to understand the validity and accuracy of these techniques in order to be able to use the results of the study to improve helmets and helmet standards.Two major categories of potential errors have been investigated. The first category concerns error sources specific to the use of crash test dummy instrumentation (accelerometers) and associated data processing techniques. These are relied upon to establish both linear and angular head acceleration responses. The second category concerns the use of broadcast video data and crash test dummy head–neck–torso systems. These are used to replicate the complex head impact scenarios of whole body collisions that occur on the football field between two living human beings.All acceleration measurement and processing techniques were based on well-established practices and standards. These proved to be reliable and reproducible. Potential errors in the linear accelerations due to electrical or mechanical noise did not exceed 2% for the three different noise sources investigated. Potential errors in the angular accelerations due to noise could be as high as 6.7%, due to error accumulation of multiple linear acceleration measurements. The potential error in the relative impact velocity between colliding heads could be as high as 11%, and was found to be the largest error source in the sequence of techniques to reconstruct the game impacts. Full-scale experiments with complete crash test dummies in staged head impacts showed maximum errors of 17% for resultant linear accelerations and 25% for resultant angular accelerations.

Head-trunk coordination in elderly subjects during linear anterior-posterior translations.

This study examined whether the head of elderly subjects was less stable in space when the trunk was free to move than when the trunk was fixed to a linearly moving platform. Fourteen healthy elderly subjects were seated on a linear sled with their trunk either fixed to the seat or free to move. Subjects received 10 cm, 445 cm/s2 anterior-posterior ramps and 0.35-4.05 Hz sum-of-sines translations while performing a mental distraction task in the dark. Kinematics of the head and trunk were derived from an Optotrak motion analysis system and a linear accelerometer placed on the head. Electromyographic (EMG) signals were collected for neck and paraspinal muscles. Data were tested for significance with paired t-tests corrected for multiple testing and compared (Mann-Whitney U-test) with previously published data from 12 healthy young adults (Keshner 2003). Linear acceleration trajectories of the head corresponded to the direction of sled linear acceleration when the trunk was fixed and countered the direction of the sled when the trunk was free. Angular head accelerations countered the sled or the trunk when the trunk was fixed or free, respectively. Peak amplitudes of head angular acceleration in space were greater with a fixed trunk. With the trunk free, amplitudes of head linear peak acceleration, angular accelerations, and response gains exceeded those of the young adults. Muscle EMG response latencies did not vary with the timing of head acceleration onset but the neck muscles were activated more frequently in a direction consistent with a vestibulocollic or cervicocollic reflex. Differences in angular motion of the head could be explained by the biomechanical constraints of the two tasks having one freely moving mass (head) with the trunk fixed and two freely moving masses (head and trunk) with the trunk free. These data suggest that elderly subjects rely upon active trunk mechanics in order to coordinate their head and trunk motion; however, a contribution by vestibular and ascending segmental inputs cannot be completely ruled out. A less flexible trunk in the elderly could explain why they were not as successful as young adults in stabilizing their heads in space when the trunk was free to move.

Theoretical Study of the Effect of Ball Properties on Impact Force in Soccer Heading

The objective of this study was to theoretically model, based on the Hertz contact theory, the impact force and contact time, as well as the linear and angular head accelerations during heading in children using two neck stiffness conditions (infinite and negligible stiffness). The following mathematical model inputs were obtained: elastic modulus and mass of size three, four, and five balls at inflation pressures of 10, 12, and 14 psi, head modulus, head mass, head length, head and trunk moment of inertia, and the precontact ball velocity. The model outputs consisted of linear and angular head acceleration, impact force, contact time between the ball and head, and head impact criteria (HIC) all at the point of impact. Head mass and length were obtained as a percentage of body weight and height, respectively, based on age. With an increase in head mass, there is a decrease in the linear and angular head acceleration. With an increase in ball size, for the same head mass, there is an increase in the contact time between the head and the ball. Changing ball inflation pressure has little effect on the impact characteristics. Infinite neck stiffness decreased linear and angular head acceleration and HIC. Head mass and ball size have an effect on linear and angular head acceleration and contact time, respectively, whereas ball inflation pressure has a minimal effect on the impact characteristics. These results indicate that children should be restricted to using the appropriate ball for their age. Smaller head size within an age group is an underemphasized though important identifier of a player's injury risk.

Can shaking alone cause fatal brain injury? A biomechanical assessment of the Duhaime shaken baby syndrome model.

A biomechanical model of a one-month old baby was designed and tested by Duhaime and co-workers in 1987 in an attempt to assess the biomechanics of the shaken baby syndrome (SBS). The study implied that pure shaking alone cannot cause fatal head injuries, a factor which has been applied in criminal courts. In an attempt to test the validity of the model a preliminary study was undertaken in which a replica was constructed and tested. The broad description of the design and construction of the Duhaime model allowed for variations and therefore uncertainties in its reproduction. It was postulated therefore that differences in certain parameters may increase angular head accelerations. To further investigate this observation, an adjustable replica model was developed and tested. The results indicated that certain parameter changes in the model did in fact lead to an increase in angular head acceleration. When these parameter changes were combined and an injurious shake pattern was employed, using maximum physical effort, the angular head acceleration results exceeded the original Duhaime et al. (1987) results and spanned two scaled tolerance limits for concussion. Additionally, literature suggests that the tolerance limits used to assess the shaking simulation results in the original study may not be reliable. Results from our study were closer to the internal head injury, subdural haematoma, tolerance limits. A series of end point impacts were identified in the shake cycles, therefore, an impact-based head injury measure (Head Injury Criterion - HIC) was utilized to assess their severity. Seven out of ten tests conducted resulted in HIC values exceeding the tolerance limits (critical load value, Stürtz, 1980) suggested for children. At this present stage the authors conclude that it cannot be categorically stated, from a biomechanical perspective, that pure shaking cannot cause fatal head injuries in an infant. Parameters identified in this study require further investigation to assess the accuracy of simulation and increase the biofidelity of the models before further conclusions can be drawn. There must now be sufficient doubt in the reliability of the Duhaime et al. (1987) biomechanical study to warrant the exclusion of such testimony in cases of suspected shaken baby syndrome.

Head-trunk coordination during linear anterior-posterior translations.

The purpose of this study was to evaluate the relative contributions of inputs from the vestibular system and the trunk to head-trunk coordination. Twelve healthy adults and 6 adults with diminished bilateral labyrinthine input (LD) were seated with their trunk either fixed to the seat or free to move. Subjects received 10-cm, 445-cm/s(2) anterior-posterior ramps and 0.35- to 4.05-Hz sum-of-sines translations while performing a mental distraction task in the dark. Kinematics of the head and trunk were derived from an Optotrak motion analysis system and a linear accelerometer placed on the head. EMG signals were collected from neck and paraspinal muscles. Data were tested for significance with multivariate ANOVA (MANOVA) and Bonferroni post hoc analyses. Initial linear and angular head acceleration directions differed in healthy subjects when the trunk was fixed or free, but did not differ in LD subjects. Peak head angular accelerations were significantly greater with the trunk fixed than when free, and were greater in LD than in control subjects. EMG response latencies did not differ when the trunk was fixed or free. Low-frequency phase responses in the healthy subjects were close to 90 degrees and had a delayed descent as frequency increased, suggesting some neural compensation that was absent in the LD subjects. Results of this study revealed a strong initial reliance on system mechanics and on signals from segmental receptors. The vestibular system may act to damp later response components and to monitor the position of the head in space secondary to feedback from segmental proprioceptors rather than to generate the postural reactions.

Flexibility of vestibulo-ocular reflex adaptation to modified visual input in human

The vestibulo-ocular reflex (VOR) serves to keep images relatively stable on the retina. To maintain appropriate performance and minimize image slip throughout life, VOR is subjected to long-term adaptive regulation by visual input. It has been reported that adaptive changes in VOR gain (eye velocity/head velocity) are evoked either by fitting subjects with magnifying, miniaturizing, or reversing spectacles during normal behavior, or by moving a large visual field in or out of phase relative to the subject's head movement. The changes in VOR gain are frequency selective. Here, we examine the extent of VOR gain flexibility by causing VORs of similar direction to undergo different behavioral gain changes. Nine healthy adults participated in the study, ranging in age from 24 to 38 years (mean: 26 years) and with no history of neurotological symptoms. All subjects were clinically normal according to a screening battery that included combined neurologic and otologic physical examinations. Horizontal and vertical eye positions were recorded by bitemporal DC-coupled electro-oculography (EOG). The subject sat in a rotating chair. The axis of rotation of the body was always earth-vertical, with the interaural axis crossing the axis of rotation of the chair. The head was pointed 20° downwards in all experiments and stabilized in this position using a chin rest. The chair was surrounded by a half-cylindrical optokinetic screen (78 cm in diameter) placed in front of the subject, onto which random dot patterns were projected. Goggles were used to ensure that the subject was in complete darkness during both pre- and postadaptation periods. The chair was rotated sinusoidally at maximum amplitude of 30° or 60°: for 30° the stimulation was at 0.1, 0.2, 0.3, and 0.4 Hz; for 60° it was at 0.1, 0.2, and 0.3 Hz. VOR adaptation was obtained by inducing a retinal slip velocity by short-term alteration of the visual input of the large field; this change was produced by a combination of sinusoidal head rotation and the random dot patterns. In each adaptation session, the sinusoidal head rotation was either at 0.1 or 0.3 Hz and the amplitude was 30°. The random dot pattern was synchronized with sinusoidal head rotation in the same direction (x0 experiment) to make the retinal slip zero, and in the opposite direction to make the retinal slip twofold (x2 experiment). Therefore, a total of four adaptation protocols were tested. The subjects were asked to fixate on a single dot with the eyes straight-ahead in the x0 experiment, and follow it within random dot patterns in the x2 experiment. Each adaptation session lasted for 30 min. Two adaptation experiments were performed on each subject once per day. The averages of VOR gain and phase lag were calculated using Fourier analysis. Seven of the subjects who participated in the x2 adaptation experiment at 0.3 Hz and 30° amplitude showed a steady increase of VOR gain within a couple of trials. One of the remaining two subjects showed a decrease in VOR gain in all the three trials, and the other subject showed a VOR gain increase in three trials and decrease in two trials. In the x2 adaptation experiment at 0.3 Hz and 30° amplitude (peak velocity: 28°/s), the percentage gain change ((post−pre)/pre) was 133% at the same stimulation and 100% at 0.4 Hz (peak velocity: 37°/s). The gain change was 65% at 0.1 Hz and 60° amplitude (peak velocity: 18°/s), and 64% at 0.2 Hz and 60° amplitude (peak velocity: 37°/s). In the x1 adaptation experiment (30° at 0.3 Hz), the percentage gain change was −62% under the same conditions, −50% at 0.1 Hz and 60° amplitude, and −30% at 0.2 Hz and 60° amplitude. No change of VOR gain was observed at other frequencies, and the subject was not adapted at 0.1 Hz and 30° amplitude. The larger VOR gain increase in the x2 adaptation experiment for the faster head angular acceleration leads to the conclusion that the VOR gain mechanism can set very different gain states for the VOR that are quite similar in terms of the head acceleration.

The vestibulo-ocular reflex in three dimensions.

The purpose of this paper is to review the kinematics and dynamics of the vestibulo-ocular reflex (VOR) in three dimensions. We give a brief, didactic tutorial on vectors and matrices and their importance as representational schemes for describing the kinematics and dynamics of the angular and linear accelerations that activate the vestibular system. We show how the vectors associated with angular and linear head accelerations are transformed by the peripheral and central vestibular systems to drive the oculomotor system to produce eye movements in three-dimensional space. We also review critical questions and controversies related to the compensatory and orientation behavior of the VOR. One such question is how the central vestibular system distinguishes tilts of the head, which generate interaural linear acceleration from translations along the interaural axis. Another question is how the velocity-position integrator is implemented centrally. The review has been placed in the context of a model that explains the behavior of the VOR in three dimensions. Model processes have been related to peripheral and central neural behavior in order to gain insight into the nature of the three-dimensional organization and the controversial questions that are addressed.

Vestibulo-Ocular Reflex Modification after Virtual Environment Exposure

Immersion in an illusory world is possible by means of virtual reality (VR), where environmental perception is modified by artificial sensorial stimulation. The application of VR for the assessment and rehabilitation of pathologies affecting the vestibular system, in terms of both diagnosis and care, could represent an interesting new line of research. Our perception of reality is in fact based on static and dynamic spatial information perceived by our senses. During head movements in a virtual environment the images on the display and the labyrinthine information relative to the head angular accelerations differ and therefore a visuo-vestibular conflict is present. It is known that mismatches between visual and labyrinthine information may modify the vestibulo-oculomotor reflex (VOR) gain. We studied the post-immersion modifications in 20 healthy subjects (mean age 25 years) exposed to a virtual environment for 20 min by wearing a head-mounted display. VOR gain and phase were measured by means of harmonic sinusoidal stimulation in the dark before, at the end of and 30 min after VR exposure. A VOR gain reduction was observed in all subjects at the end of VR exposure which disappeared after 30 min. Our data show that exposure to a virtual environment can induce a temporary modification of the VOR gain. This finding can be employed to enable an artificial, instrumental modification of the VOR gain and therefore opens up new perspectives in the assessment and rehabilitation of vestibular diseases.

Central Versus Peripheral Origin of Vestibuloocular Reflex Recovery Following Semicircular Canal Plugging in Rhesus Monkeys

We have previously shown that there is a slowly progressing, frequency-specific recovery of the gain and phase of the horizontal vestibuloocular reflex (VOR) in rhesus monkeys following plugging of the lateral semicircular canals. The adapted VOR response exhibited both dynamic and spatial characteristics that were distinctly different from responses in intact animals. To discriminate between adaptation or recovery of central versus peripheral origin, we have tested the recovered vestibuloocular responses in three rhesus monkeys in which either one or both coplanar pairs of vertical semicircular canals had been plugged previously by occluding the remaining semicircular canals in a second plugging operation. We measured the spatial tuning of the VOR in two or three different mutually orthogonal planes in response to sinusoidal oscillations (1.1 Hz, +/-5 degrees, +/-35 degrees /s) over a period of 2-3 and 12-14 mo after each operation. Apart from a significant recovery of the torsional/vertical VOR following the first operation we found that these recovered responses were preserved following the second operation, whereas the responses from the newly operated semicircular canals disappeared acutely as expected. In the follow-up period of up to 3 mo after the second operation, responses from the last operated canals showed recovery in two of three animals, whereas the previously recovered responses persisted. The results suggest that VOR recovery following plugging may depend on a regained residual sensitivity of the plugged semicircular canals to angular head acceleration.

Early components of the human vestibulo-ocular response to head rotation: latency and gain.

To characterize vestibulo-ocular reflex (VOR) properties in the time window in which contributions by other systems are minimal, eye movements during the first 50-100 ms after the start of transient angular head accelerations ( approximately 1000 degrees /s(2)) imposed by a torque helmet were analyzed in normal human subjects. Orientations of the head and both eyes were recorded with magnetic search coils (resolution, approximately 1 min arc; 1000 samples/s). Typically, the first response to a head perturbation was an anti-compensatory eye movement with zero latency, peak-velocity of several degrees per second, and peak excursion of several tenths of a degree. This was interpreted as a passive mechanical response to linear acceleration of the orbital tissues caused by eccentric rotation of the eye. The response was modeled as a damped oscillation (approximately 13 Hz) of the orbital contents, approaching a constant eye deviation for a sustained linear acceleration. The subsequent compensatory eye movements showed (like the head movements) a linear increase in velocity, which allowed estimates of latency and gain with linear regressions. After appropriate accounting for the preceding passive eye movements, average VOR latency (for pooled eyes, directions, and subjects) was calculated as 8.6 ms. Paired comparisons between the two eyes revealed that the latency for the eye contralateral to the direction of head rotation was, on average, 1.3 ms shorter than for the ipsilateral eye. This highly significant average inter-ocular difference was attributed to the additional internuclear abducens neuron in the pathway to the ipsilateral eye. Average acceleration gain (ratio between slopes of eye and head velocities) over the first 40-50 ms was approximately 1.1. Instantaneous velocity gain, calculated as Veye(t)/Vhead(t-latency), showed a gradual build-up converging toward unity (often after a slight overshoot). Instantaneous acceleration gain also converged toward unity but showed a much steeper build-up and larger oscillations. This behavior of acceleration and velocity gain could be accounted for by modeling the eye movements as the sum of the passive response to the linear acceleration and the active rotational VOR. Due to the latency and the anticompensatory component, gaze stabilization was never complete. The influence of visual targets was limited. The initial VOR was identical with a distant target (continuously visible or interrupted) and in complete darkness. A near visual target caused VOR gain to rise to a higher level, but the time after which the difference between far and near targets emerged varied between individuals.

Influence of surgical plugging on horizontal semicircular canal mechanics and afferent response dynamics.

Mechanical occlusion of one or more of the semicircular canals is a surgical procedure performed clinically to treat certain vestibular disorders and used experimentally to assess individual contributions of separate canals and/or otoliths to vestibular neural pathways. The present experiments were designed to determine if semicircular canal afferent nerve modulation to angular head acceleration is blocked by occlusion of the endolymphatic duct, and if not, what mechanism(s) might account for a persistent afferent response. The perilymphatic space was opened to gain acute access to the horizontal canal (HC) in the oyster toadfish, Opsanus tau. Firing rate responses of HC afferents to sinusoidal whole-body rotation were recorded in the unoccluded control condition, during the process of duct occlusion, and in the plugged condition. The results show that complete occlusion of the duct did not block horizontal canal sensitivity; individual afferents often exhibited a robust firing rate modulation in response to whole-body rotation in the plugged condition. At high stimulus frequencies (about >8 Hz) the average sensitivity (afferent gain; spikes/s per degrees /s of head velocity) in the plugged condition was nearly equal to that observed for unoccluded controls in the same animals. At low stimulus frequencies (about <0.1 Hz), the average sensitivity in the plugged condition was attenuated by more than two orders of magnitude relative to unoccluded controls. The peak afferent firing rate for sinusoidal stimuli was phase advanced approximately 90 degrees in plugged canals relative to their control counterparts for stimulus frequencies approximately 0.1-2 Hz. Data indicate that afferents normally sensitive to angular velocity in the control condition became sensitive to angular acceleration in the plugged condition, whereas afferents sensitive to angular acceleration in the control condition became sensitive to the derivative of acceleration or angular jerk in the plugged condition. At higher frequencies (>8 Hz), the phase of afferents in the plugged condition became nearly equal, on average, to that observed in controls. A three-dimensional biomechanical model of the HC was developed to interpret the residual response in the plugged condition. Labyrinthine fluids were modeled as incompressible and Newtonian; the membranous duct, osseous canal and temporal bone were modeled as visco-elastic materials. The predicted attenuation and phase shift in cupular responses were in close agreement with the observed changes in afferent response dynamics after canal plugging. The model attributes the response of plugged canals to labyrinthine fluid pressure gradients that lead to membranous duct deformation, a spatial redistribution of labyrinthine fluids and cupular displacement. Validity of the model was established through its ability to predict: the relationship between plugged canal responses and unoccluded controls (present study), the relationship between afferent responses recorded during mechanical indentation of the membranous duct and physiological head rotation, the magnitude and phase of endolymphatic pressure generated during HC duct indentation, and previous model results for cupular gain and phase in the rigid-duct case. The same model was adjusted to conform to the morphology of the squirrel monkey and of the human to investigate the possible influence of canal plugging in primates. Membranous duct stiffness and perilymphatic cavity stiffness were identified as the most salient model parameters. Simulations indicate that canal plugging may be the most effective in relatively small species having small labyrinths, stiff round windows, and stiff bony perilymphatic enclosures.

Technique for the assessment of vestibular compensation--clinical observation/quantitative neuro-otometry?

Reliable evaluation of vestibular compensation in indispensable to determine whether labyrinth surgery or vestibular neurectomy is indicated. It is also important for postoperative follow-up. Vestibular compensation can manifest itself differently in distinct frequency ranges. Twenty volunteers were examined in five distinct situations of daily vestibular stimulation. Measurement of angular and linear head acceleration was performed using accelerometers fixed on the volunteers' head. A 200 Hz AD fed data to a PC database. FFT was used for data analysis. Stimulus frequency of the vestibular system varies between 0.01 and 2 Hz. Most of the patients suffering from vestibular lesions showed a reduced vestibulo ocular reflex (VOR) below 0.1 Hz. In all cases of unilateral vestibular function loss, there was a correlation between the symptoms during movement and the corresponding frequency range of the distinct motion pattern. Rotatory vestibular pendular testing was used to document vestibular disorders in patients who had normal findings in routine vestibular testing. Clinical use of rotatory vestibular pendular testing results must be performed using broad stimulus frequency spectra (0.01-0.06 Hz). This method must be used in preoperative examination before labyrinth surgery as well as in estimating individual tolerance for vestibular stimulation in daily situations.

Protection of the side of the head

The side of the head is a frequent impact site for pedal cyclists, motorcyclists, and industrial workers. It is vulnerable to impact, yet many helmet standards do not have impact tests at the side of the helmet. Laboratory impact tests should reproduce the phenomena in real impacts, but usually they do not allow the headform to rotate, or ignore the effect of the neck on the motion of the head. The authors designed a linkage, simulating the flexibility of the neck, for use with anthropomorphic dummies in helmet testing. Impacts at the side of the helmet, normal to the helmet surface, were made with flat and hemispherical strikers. Head rotation during impacts caused the impact site to move, in some cases to below the level protected by the helmet. The peak angular head acceleration correlated with the peak linear accelerations. Neck flexibility means that helmets can protect the head from higher kinetic energy impacts than those specified in standards using immovable headforms or impact anvils. The load spreading efficiency of polypropylene foam, used in soft shell bicycle helmets, is better than that of polystyrene foam. The authors conclude that the side of the head can be protected by a suitable helmet design.

Determinants of semicircular canal afferent response dynamics in fish.

Present results separate the relative contributions of semicircular canal biomechanics from hair cell/afferent biophysics in determining the amplitude and phase of afferent responses to sinusoidal motion of the head. Separation was achieved by combining electrical polarization of the endolymph with mechanical indentation of the canal limb to modulate the instantaneous firing rate of horizontal semicircular canal afferents. The electrical stimulus drives hair cell transduction currents via modulation of the Nernst-Planck potential, whereas the mechanical stimulus mimics head rotation and modulates the open probability of the transduction channels. Responses for electrical polarization therefore reflect post-transduction-current (PTC) mechanisms, and responses for mechanical stimulation include the additional influence of canal mechanics. Linear transfer functions defining individual afferent response dynamics were obtained for low levels of each stimuli and are reported in Bode form providing gain (spikes/s per micron or mV) and phase (deg re: peak stim) over the frequency range from 0.02 to 40 Hz. Combined results for electrical and mechanical stimuli distinguish the component of sensory signal processing carried out by canal mechanics from that carried out by the hair cell/afferent complexes. Individual afferents were categorized according to their response to the mechanical stimuli as low-gain velocity (LG), high-gain velocity (HG) or acceleration (A) sensitive, groups as originally defined by Boyle and Highstein to describe interafferent diversity present within the population. In contrast to the results for mechanical stimuli, all afferent groups exhibit nearly equal increases in gain and phase for increasing frequencies of electrical stimulation. Comparison of individual afferent responses for the two stimuli leads to the conclusion that the LG, HG, and A groups are distinguished primarily by diversity in the mechanical activation of associated hair cells and not by PTC mechanisms. Even though PTC processing does not contribute significantly to determining these groups, it is the primary determinant underlying high-frequency gain and phase enhancements observed in the population average. Comparison of mechanical and electrical responses also reveals the mechanical lower-corner responsible for phase enhancements and gain decreases in all afferents at low frequencies of mechanical stimulation (< 0.05 Hz). Results imply that LG afferents encode angular head velocity by canceling a phase lag and gain attenuation due to the mechanics with a phase lead and gain enhancement due to PTC mechanisms above approximately 0.2 Hz. In contrast, A group afferents encode angular head acceleration by combining high-frequency phase leads and gain enhancements present in both the mechanics and PTC mechanisms across the physiological frequency spectrum. HG afferents fall between these two extremes, and, other than the influence of the mechanical lower-corner, their response primarily reflects PTC processing.

Evaluation of four processing methods of the VCM ankle joint model

Despite improvements in vehicle design and safety technologies, frontal automotive collisions continue to result in a substantial number of injuries and fatalities each year. Although a considerable amount of research has been performed on PMHSs and ATDs, matched dynamic whole-body frontal testing with PMHSs and the current ATD aimed at quantifying both kinetic and kinematic data in a single controlled study is lacking in the literature. Therefore, a total of 4 dynamic matched frontal sled tests were performed with three male PMHSs and a Hybrid III 50th percentile male ATD (28.6 g, Δv = 40 kph). Each subject was restrained using a 4 kN load limiting, driver-side, 3-point seatbelt. Belt force was measured for the lap belt and shoulder belt. Reaction forces were measured at the seat pan, seat back, independent foot plates, and steering column. Linear head acceleration, angular head acceleration, and pelvic acceleration were measured for all subjects. Acceleration of C7, T7, T12, both femurs, and both tibias were also measured for the PMHSs. A Vicon motion analysis system, consisting of 12 MX-T20 2 megapixel cameras, was used to quantify subject 3D motion (±1 mm) at a rate of 1 kHz. Excursions of select anatomical regions were normalized to their respective initial positions and compared by test condition and between subject types. Notable discrepancies were observed in the responses of the PMHSs and the ATD. The reaction forces and belt loading for the ATD, particularly foot plate, seat back, steering column, and lap belt forces, were not in agreement with those of the PMHSs. The forward excursions of the ATD were consistently within those of the PMHSs with the exception of the left upper extremity. This could potentially be due to the known limitations of the Hybrid III ATD shoulder and chest. The results presented herein demonstrate that there are some limitations to the current Hybrid III ATD under the loading conditions evaluated in the current study. Overall, this study presents a comprehensive data set of belt forces, reaction forces, accelerations, and bilateral displacement data that can be used to evaluate the performance of ATDs and validate computational models.

Inertial representation of angular motion in the vestibular system of rhesus monkeys. I. Vestibuloocular reflex

1. The spatial organization of the vestibuloocular reflex (VOR) was studied in six rhesus monkeys by applying fast, short-lasting, passive head and body tilts immediately after constant-velocity rotation (+/- 90 degrees/s) about an earth-vertical axis. Two alternative hypotheses were investigated regarding the reference frame used for coding angular motion. 1) If the vestibular system is organized in head-centered coordinates, postrotatory eye velocity would decay invariably along the direction of applied head angular acceleration. 2) Alternatively, if the vestibular system codes angular motion in inertial, gravity-centered coordinates, postrotatory eye velocity would decay along the direction of gravity. 2. Horizontal VOR was studied with the monkeys upright. Pitch (roll) tilts away from upright elicited a transient vertical (torsional) VOR and shortened the time constant of the horizontal postrotatory slow phase velocity. In addition, an orthogonal torsional (after pitch tilts) or vertical (after roll tilts) response gradually built up. As a result, the eye velocity vector transiently deviated in the roll (pitch) plane and then gradually rotated in the same direction as gravity in the pitch (roll) head plane until the orthogonal component reached a peak value. Subsequently, the residual postrotatory eye velocity decayed along a line parallel to gravity. 3. The time constant of the horizontal postrotatory response was maximal in upright position (21.5 +/- 5.7 s, mean +/- SD) and minimal after tilts to prone (3.8 +/- 0.7 s), supine (4.5 +/- 1.2 s), and ear-down (5.2 +/- 1.6 s) positions. A similar dependence on head orientation relative to gravity characterized the dynamics of the resultant eye velocity vector in the pitch and roll planes. 4. Torsional VOR was studied with the monkeys in supine or prone position. Pitch (yaw) tilts from the supine or prone position toward upright (ear-down) position elicited a transient vertical (horizontal) VOR and shortened the time constant of the torsional postrotatory response while a horizontal (vertical) orthogonal component slowly built up. As a result the eye velocity vector gradually rotated in the pitch (yaw) plane until the orthogonal component reached a peak value. Subsequently residual postrotatory eye velocity decayed along a line parallel to gravity. 5. The time constant of the torsional postrotatory response in supine/prone positions was 16.5 +/- 6.8 s. After tilts from supine/prone positions toward upright position, time constants decreased and were minimal after tilts to upright position (2.7 +/- 0.7 s).(ABSTRACT TRUNCATED AT 400 WORDS)