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PubMed1981

The threshold of perception of angular acceleration as a function of duration.

The threshold for rotation about the yaw axis was determined for constant acceleration stimuli as a function of their duration in the range from 3 to 25 s. From the torsion-swing model the following theoretical equation can be derived: alpha thr = C/[1--exp(-ts/tau 1)], where alpha thr = acceleration amplitude at threshold, ts = duration of the acceleration, tau 1 = time constant, C = threshold for very long stimuli. According to this formula the Mulder product (i.e. the product of the threshold acceleration amplitude and the duration of the stimulus) is constant for durations up to 0.3 tau 1. The best fit of this theoretical function to the somatosensory data is found for tau 1 = 14.5 s, and C = 0.220/s2. The time within the Mulder product is constant (about 5 s) is doubtless due to the mechanics of the semicircular canals. For the oculogyral data a lower value of tau 1 is found. We do not have any explanation for this lower value.

Acceleration↗

PubMed1995

Maximal power and torque-velocity relationship on a cycle ergometer during the acceleration phase of a single all-out exercise.

Seven subjects pedalled on a Monark cycle ergometer as fast as possible for approximately 7 s against four different resistances which corresponded to braking torques (TB) equal to 19, 38, 57 and 76 N.m at the crank level. Exercise periods were separated by 5-min recovery periods. Pedal velocity was recorded every 50 ms by means of a disc with 360 slots fixed on the flywheel, passing in front of a photo-electric cell linked to a microcomputer which processed the data. Every 50 ms, the time necessary to perform half a pedal revolution (t1/2) was computed by adding the 50-ms periods necessary to reach 669 slots (the number of slots corresponding to half a pedal revolution). To measure t1/2 to an accuracy better than 50 ms, this time was computed by a linear interpolation of the time-slot number relationship. Power (P) was averaged during t1/2 by adding the power dissipated against braking torque and the power necessary to accelerate the flywheel. The torque-velocity (T-v) relationship was studied during the acceleration phase of a sprint against a single TB by computing every 50 ms the relationship between v and T (N.m), equal to the sum of TB and the torque necessary to accelerate the flywheel at the same time. The T-v relationships calculated from the acceleration phase of a single all-out exercise were linear and similar to the previously described relationships between peak velocity and braking force.(ABSTRACT TRUNCATED AT 250 WORDS)

Acceleration↗

PubMed1996

The course of the traumatising force in acceleration head injury: CT evidence.

Since brain damage in closed head injury of acceleration-deceleration type depends on the site of impact and on the course of the traumatising force, a reconstruction of these was attempted using CT. The study included 45 standard CT studies of adults with closed acceleration-deceleration head injuries. We selected 32 patients exclusively on the basis of CT findings of soft tissue contusion, skull fractures and cerebral contusions (coup and/or contrecoup, as well as other parenchymal lesions), while in the remaining 13 patients clinical findings were also used. On CT, the axial section of the skull was divided into 12 sections imitating the clock-face, to permit computerised graphic presentation of the direction of the traumatising force. Analysis of cerebral contusions on different CT studies in each case allowed location of "dominant coup" and "dominant contrecoup" lesions to be determined. The site of impact and the course of the traumatising force were reconstructed and graphically presented on the basis of these findings, supplemented with data on soft tissue contusions (present in 71% of cases), skull fractures (in 36%) and sometimes on other brain lesions. Comparison of the computerised graphic presentation of the site of impact and direction of the traumatic force and the location of lesions revealed a high correlation between them. In 80% of cases, the site of impact could be visualised only by CT. The acceleration force acted along the longer axis of the head (centroaxial or semioblique) in 87% and along the shorter axis in 13% of cases. Multiple lesions were found in 87% of cases. The lesions were most frequent in the frontal (51%) and temporal (26%) lobes. CT was very useful for reconstruction of the site of impact and of the course of the traumatising force in acceleration head injury. Data obtained by this procedure may have far-reaching prognostic and forensic implications.

Acceleration↗

PubMedDate not supplied

Noninvasive acceleration measurements to characterize knee arthritis and chondromalacia.

Devising techniques and instrumentation for early detection of knee arthritis and chondromalacia presents a challenge in the domain of biomedical engineering. The purpose of the present investigation was to characterize normal knees and knees affected by osteoarthritis, rheumatoid arthritis, and chondromalacia using a set of noninvasive acceleration measurements. Ultraminiature accelerometers were placed on the skin over the patella in four groups of subjects, and acceleration measurements were obtained during leg rotation. Acceleration measurements were significantly different in the four groups of subjects in the time and frequency domains. Power spectral analysis revealed that the average power was significantly different for these groups over a 100-500 Hz range. Noninvasive acceleration measurements can characterize the normal, arthritis, and chondromalacia knees. However, a study on a larger group of subjects is indicated.

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PubMed1990

The human horizontal vestibulo-ocular reflex in response to high-acceleration stimulation before and after unilateral vestibular neurectomy.

The normal horizontal vestibulo-ocular reflex (HVOR) is largely generated by simultaneous stimulation of the two horizontal semicircular canals (HSCCs). To determine the dynamics of the HVOR when it is generated by only one HSCC, compensatory eye movements in response to a novel vestibular stimulus were measured using magnetic search coils. The vestibular stimulus consisted of low-amplitude, high-acceleration, passive, unpredictable, horizontal rotations of the head with respect to the trunk. While these so called head "impuses" had amplitudes of only 15-20 degrees with peak velocities up to 250 deg/s, they had peak accelerations up to 3000 deg/s/s. Fourteen humans were studied in this way before and after therapeutic unilateral vestibular neurectomy; 10 were studied 1 week or 1 year afterwards; 4 were studied 1 week and 1 year afterwards. The results from these 14 patients were compared with the results from 30 normal control subjects and with the results from one subject with absent vestibular function following bilateral vestibular neurectomy. Compensatory eye rotation in normal subjects closely mirrored head rotation. In contrast there was no compensatory eye rotation in the first 170 ms after the onset of head rotation in the subject without vestibular function. Before unilateral vestibular neurectomy all the patients' eye movement responses were within the normal control range. One week after unilateral vestibular neurectomy however there was a asymmetrical bilateral HVOR deficit. The asymmetry was much more profound than has been shown in any previous studies. The HVOR generated in response to head impulses directed away from the intact side largely by ampullofugal disfacilitation from the single intact HSCC (ignoring for the moment the small contribution to the HVOR from stimulation of the vertical SCCs), was severely deficient with an average gain (eye velocity/head velocity) of 0.25 at 122.5 deg/sec head velocity (normal gain = 0.94 +/- 0.08). In contrast the HVOR generated in response to head impulses directed toward the intact side, largely by ampullopetal excitation from the single intact HSCC, was only mildly (but nonetheless significantly) deficient, with an average gain of 0.80 at 122.5 deg/sec head velocity. At these accelerations there was no significant improvement in the average HVOR velocity gain in either direction over the following year. These results indicate that ampullopetal excitation from one HSCC can, even in the absence of ampullofugal disfacilitation from the opposite HSCC, generate a near normal HVOR in response to high-acceleration stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Acceleration↗

PubMed2001

Responses of rostral fastigial neurons to linear acceleration in an alert monkey.

Vestibular-only neuronal responses to angular acceleration have been systematically characterized in the rostral fastigial nucleus (FN) by several studies. However, responses of these neurons to linear acceleration have not been examined. In this study, we recorded single-unit activity of vestibular-only neurons in an alert monkey during pure sinusoidal linear acceleration along different directions in the horizontal plane. Spatiotemporal response properties were quantified by computing two-dimensional response ellipses in the horizontal plane. Based on this analysis, neurons were classified as narrowly or broadly tuned. About 29% (5/17) of neurons were broadly tuned. The other 71% (12/17) were narrowly tuned. Unlike vestibular nuclei neurons, all recorded FN neurons exhibited irregular resting discharge rates (CV*0.2). Based on studies of linear motion-sensitive neurons in the vestibular nuclei, the data suggest that irregular neurons in the rostral FN and the vestibular nuclei have similar responses to linear acceleration in behaving monkeys.

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PubMed2001

Neck muscle responses to abrupt vertical acceleration in the seated human.

The control of neck muscle during marked changes in "g" loading must be to protect head-trunk orientation. However, little is known about the organization of reflexes. We therefore investigated the shortest latency in neck muscle evoked by abrupt ascending and descending vertical acceleration with a stroke of 20 cm and peak acceleration 0.4xg in six healthy subjects. The subjects were seated upright and restrained on a chair driven by a hydraulic servo system. Ascent induced small responses in sternocleidomastoid muscles (SCMs) with a latency of 24.0 ms (SE 1.7) from onset of head acceleration, followed by larger responses of 100 ms in duration. In comparison the responses in SCMs evoked by descent were significantly delayed, with greater interindividual variations at 44.1 ms (SE 3.1) and smaller in amplitude than responses in ascent. Latencies were consistent and showed no habituation. In order to eliminate stretch components, we measured the neck responses to vertical acceleration with the head fixed by cervical collar. The latency of the fast response of SCM evoked by ascent was not significantly different from the latency of SCMs without the collar. These results may indicate that the fast responses of SCMs to sudden ascent may be composed of a vestibular-collic reflex for making the neck and head rigid for defense to sudden gravitational change.

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PubMed2005

Modeling the acceleration sensitive neurons in the pigeon optokinetic system.

Recent physiological findings revealed that about one-third of motion-sensitive neurons in the pigeon's pretectal nucleus encoded the acceleration of visual motion. Here we propose a microcircuit hypothesis, in which the slow adaptive depressions play a significant role in response generating, to account for the origin of the three important properties of the acceleration-sensitive neurons: the plateau-shaped speed-tuning curves, the opposite-signed after-responses (OSARs) and the acceleration sensitivities. The flat plateau within the speed-tuning curves and the OSARs to motion offset observed in experiments are reproduced successfully in simulations, and the simulative responses of the acceleration-sensitive neurons to step changes, ramp changes in stimulus speed and sine wave modulations of stimulus speed are qualitatively consistent with physiological observations. Thus, a biologically plausible substrate for the neurons' classification and the origin of the three properties are provided.

Acceleration↗

PubMed1986

Analysis of blood flow through a model of the human arterial system under periodic body acceleration.

The human system may be subjected to a body acceleration deliberated for example by making subjects lie down on vibrating tables or more frequently unintentionally, for example during travel in water and land or in air and space. The present study is concerned with the effects of externally imposed body accelerations on blood flow in a branched system of arteries. A finite-element model of flow in the arterial system subject to periodic body accelerations is presented. Computational results on the flow rates through selected arteries and the corresponding inlet and outlet pressures under different conditions (magnitude, frequency and direction) of applied acceleration are presented.

Acceleration↗

PubMed1995

Transfer function between tibial acceleration and ground reaction force.

The purpose of the present study was to capture the relationship between ground reaction force (GRF) and tibial axial acceleration. Tibia acceleration and GRF were simultaneously recorded from five subjects during running. The acceleration of the bone was measured with a transducer mounted onto an intracortical pin. The signals were analyzed in the frequency domain to characterize the relationship between GRF and tibial acceleration. The results confirmed that for each subject this relationship could be represented by a frequency transfer function. The existence of a more general relationship for all five subjects was also confirmed by the results. The transfer functions provided information about transient shock transmissibility for the entire impact phase of running.

Acceleration↗

PubMed1989

Pulsed linear acceleration as a vestibular stimulus in electrophysiological investigations.

A simple method of generating precisely defined pulsed linear acceleration stimuli was developed and used to study vestibular responses to cranial acceleration. Electromechanical shakers were coupled to a stimulus platform. The platform, in turn, was used to couple stimuli to the skull. Movements were controlled by a voltage waveform (V(t] applied to shakers. Software algorithms were used to synthesize voltage functions (V(t]. The resulting acceleration was monitored [a(t)] and used to generate velocity [v(t)] and position [x(t)] functions (first and second integrals of a(t), respectively). The characteristics of stimuli used in physiological experiments are described. The system provides precise control of the timing and amplitude of acceleration, velocity and position pulses to the cranium and has proved to be valuable in developing noninvasive electrophysiological measures of peripheral and central vestibular function.

Acceleration↗

PubMedDate not supplied

Short latency vestibular responses to pulsed linear acceleration.

Far-field vestibular responses to pulsed linear cranial acceleration have not been reported in detail for any species. In this study, precisely defined pulsed linear accelerations were used to elicit vestibular neural responses recorded from the surfaces of the skulls of 23 White Leghorn chicks. Traditional signal averaging techniques were used to resolve responses. At moderate intensities, responses consisted of a series of four to seven dominant peaks occurring within a period of 8 ms, having amplitudes between 0.3 and 20 microV peak-to-peak. The mean response threshold was 0.120 +/- 0.045 g. Latencies and amplitudes varied systematically as a function of stimulus intensity. Hypothermia prolonged response latencies. Response peaks did not invert on stimulus inversion, were present in response to cranial but not trunk acceleration, were not attenuated by broad-band auditory masking or by ambient light conditions, and disappeared with complete bilateral destruction of the labyrinth. The results rule out major contributions from auditory, somatosensory, and visual modalities and support the hypothesis that the responses reflect bilateral neural activity in the vestibular system. The findings suggest that direct noninvasive assessment of peripheral vestibular function can be achieved using pulsed linear acceleration stimuli.

Acceleration↗

PubMed1990

Estimation of complications for linear accelerator radiosurgery with the integrated logistic formula.

Radiosurgery techniques permit high doses of single fraction irradiation to be administered to small volumes of tumor with relative sparing of surrounding brain tissue. The tolerance of surrounding normal brain tissue to dose distributions from linear accelerator radiosurgery with different collimator sizes is an important factor that must be estimated by anyone using these treatment techniques. The exponential and linear quadratic versions of the integrated logistic formula were used to estimate the probability of brain necrosis at different doses for radiosurgical dose distributions administered by a 6 MV linear accelerator with a 5 arc technique for collimator sizes from 12.5 to 30 mm in diameter. Dose-volume isoeffect curves for a 3% risk of brain necrosis from linear accelerator radiosurgery were then calculated. These curves approximate those calculated for gamma knife radiosurgery and a published 1% dose-volume isoeffect line predicted for proton beam irradiation. Similar dose-volume isoeffect curves were calculated for single fraction radiosurgery boosts administered after 30 Gy of whole brain irradiation in 12 fractions. The integrated logistic formula appears to be a useful tool for estimating tolerance and providing guidelines for prescribing radiation doses for linear accelerator radiosurgery.

Brain Diseases↗

PubMed2005

The fluid-dynamic disturbances induced on the ISS, based on the first acceleration measurements on board the space station.

The different acceleration components on the ISS that are responsible for the generation of convective motions in a fluid cell either in the presence of density gradients or in quasi-isodense processes, are analyzed. The NASA measurements of the quasi-steady and periodic acceleration on the ISS are considered and their effects on fluid-dynamic experiments are computed and discussed under different assumptions. In particular, numerical simulations are carried out to identify the relative importance of linear and pendular accelerations, due to possible rotations of the P/L around its center of mass. The effects caused by variable accelerations created by an isolation mount that exhibits an attenuation factor not constant within the payload volume, caused by the reaction forces of the umbilicals, are computed and analyzed.

Acceleration↗

PubMed2005

Regional myocardial velocities and isovolumic contraction acceleration before and after device closure of atrial septal defects: a color tissue Doppler study.

BACKGROUND: The study analyzed the effect of atrial septal defect (ASD) device closure on regional wall motion in the right (RV) and left ventricles (LV) using color tissue Doppler imaging (TDI). Atrial septal defect closure results in acute volume unloading of the RV. For unknown reasons, some patients develop acute left-sided heart failure postintervention. METHODS: Color TDI was performed in 39 pediatric ASD and 75 age-matched controls. Regional wall motion in 5 LV and 1 RV segment were analyzed before, immediately after, and 24 hours after interventional ASD closure. Off-line postprocessing of echocardiographic data was used to determine myocardial velocities and acceleration during isovolumic contraction (IVA). Isovolumic contraction acceleration is the slope of the upstroke of the isovolumic contraction wave (IVA = peak velocity/acceleration time). RESULTS: At baseline, patients with ASD had significantly higher RV systolic velocities than controls. Isovolumic contraction acceleration was similar in patients with ASD and controls. In the catheterization laboratory postintervention, conventional function parameters remained stable but systolic myocardial velocities decreased significantly in all segments. Diastolic velocities fell in LV segments but not in the RV. In contrast to velocities, IVA was stable during ASD device closure. On follow-up at 24 hours, myocardial velocities had normalized. CONCLUSIONS: Device closure of ASD results to an acute transient decrease of regional myocardial velocities in the LV and RV, whereas the load-insensitive marker isovolumic acceleration remained stable. Therefore, the velocity changes may represent a response to altered left and right ventricular loading conditions. Color TDI is a sensitive tool to analyze ventricular mechanics.

Acceleration↗

PubMed2005

Significant spinal injury resulting from low-level accelerations: a case series of roller coaster injuries.

OBJECTIVES: To describe a cohort of significantly injured roller coaster riders and the likely levels of acceleration at which the injuries occurred, and to compare these data with contemporary efforts to define a lower limit of acceleration below which no significant spinal injury is likely to occur. DESIGN: A retrospective case series of roller coaster ride-induced significant spinal injuries. SETTING: Injury incident records and emergency medical service records for the Rattler roller coaster in San Antonio, TX, were evaluated for a 19-month period in 1992 and 1993. Medical records for the more significant injuries were also reviewed and the specific injuries were tabulated, along with the demographics of the cohort. PARTICIPANTS: There were 932,000 riders of the Rattler roller coaster, estimated to represent between 300,000 and 600,000 individual riders. INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: Injury incident reports and medical record review. RESULTS: It is estimated that there were a total of 656 neck and back injuries during the study period, and 39 were considered significant by the study inclusion criteria. Seventy-two percent (28/39) of the injured subjects sustained a cervical disk injury; 71% of these injuries were at C5-6 (15 disk herniations, 5 symptomatic disk bulges) and 54% were at C6-7 (11 disk herniations, 4 symptomatic disk bulges). In the lumbar spine, the most frequent injury was a symptomatic disk bulge (20% of the cohort), followed by vertebral body compression fracture (18%), and L4-5 or L5-S1 disk herniation (13%). Accelerometry testing of passengers and train cars indicated a peak of 4.5 to 5g of vertical or axial acceleration and 1.5g of lateral acceleration over approximately 100ms (0.1s) on both. CONCLUSIONS: The results of this study suggest that there is no established minimum threshold of significant spine injury. The greatest explanation for injury from traumatic loading of the spine is individual susceptibility to injury, an unpredictable variable.

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PubMed2006

Surface electromyography as a tool to assess the responses of car passengers to lateral accelerations. Part II: Objective comparison of vehicles.

The purpose of this study was to objectively assess the response of car passengers to lateral accelerations. Surface EMG signals were collected bilaterally from the cervical erector spinae (CES), latissimus dorsi (LD), erector spinae (ES), external oblique (EO), and vastus lateralis (VL) muscles of 10 subjects. Lateral acceleration was also recorded. Three chassis-seat configurations AA, BA and BB were tested, with the first letter denoting the chassis and the second the seat. SEMG signals were often contaminated by noise, and were, therefore, denoised using the methods explained in part I. Reciprocal phasic activity was observed for all muscles except for the EO, and the reaction of passengers to lateral accelerations was interpreted as a bust torsion. The RMS of EMG segments was used as an indication of muscle activity. Muscle activation of VL and ES were significantly affected by the configuration tested (p<0.05), with greater activation levels observed for the chassis A than for the chassis B. Such a finding implies that greater roll requires greater muscle activity, thus resulting in less comfortable vehicles. Therefore, SEMG can be used to provide an objective measure of discomfort in passengers subjected to lateral accelerations in a car seat.

Acceleration↗

PubMed2004

The stability of driver acceleration behavior, and a replication of its relation to bus accidents.

The reliability over time of a method for measuring driver acceleration behavior was tested on bus drivers in regular traffic. Also, a replication of an earlier finding of a correlation between driver acceleration behavior and accident frequency for the individual drivers was made. It was found that the split-half correlation is probably around 0.50 for the mean (of accelerations) of a 30-min drive, and similar for the test-retest of 2.5h measured about a month apart. With such reliability, the sample was probably too small to reliably determine any association with accidents, but some significant correlations were found. Some ways of holding constant the differences in exposure and driving environment were tried with mixed success. Alternate ways of analyzing the data and several methodological problems were briefly discussed. It was concluded that the measurements of acceleration behavior, for bus drivers, are fairly reliable over at least a few months. However, some strange discrepancies between samples make all interpretations concerning the link to accidents tentative.

Acceleration↗