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PubMed2004

Effects of seat-back angle and accelerometer height at the seat-back on seat-back x axis r.m.s. acceleration in filed experiments according to the ISO2631-1 standard.

The effects of seat-back angle and accelerometer height at the seat-back on seat-back x-axis frequency-weighted root-mean-square (r.m.s.) acceleration have been investigated in field experiments. Experiment 1 investigated the range and variability, of frequency-weighted r.m.s. acceleration at the same measurement position, where the seat-back angle was 24 degrees from vertical on the floor and the accelerometer height was 38 cm up from the seat cushion surface. Experiment 2 investigated ranges of frequency-weighted r.m.s. acceleration between the reference position, which was the position used in experiment 1, and test positions at different seat-back angles, 4 degrees ahead of and 4 degrees behind the reference position. Experiment 3 investigated the range of frequency-weighted r.m.s. acceleration between the reference position which was the same as in experiments 1 and experiment 2, and test positions at different accelerometer heights at the seat-back from a seat cushion surface, 2.5 cm higher, 2.5 cm lower and 5 cm lower than the reference position. This investigation clarifies that different seat-back angles and accelerometer heights at the seat-back affect the frequency-weighted r.m.s. acceleration at these measurement positions, which is beyond the exposure values at which people are able to distinguish different vibration acceleration magnitudes.

Acceleration↗

PubMed1994

Significance of acceleration period in a dynamic strength testing study.

The acceleration period that occurs during isokinetic tests may provide valuable information regarding neuromuscular readiness to produce maximal contraction. The purpose of this study was to collect the normative data of acceleration time during isokinetic knee testing, to calculate the acceleration work (Wacc), and to determine the errors (ERexp, ERwork, ERpower) due to ignoring Wacc during explosiveness, total work, and average power measurements. Seven male and 13 female subjects attended the test by using the Cybex 325 system and electronic stroboscope machine for 10 testing speeds (30-300 degrees/sec). A three-way ANOVA was used to assess gender, direction, and speed factors on acceleration time, Wacc, and errors. The results indicated that acceleration time was significantly affected by speed and direction; Wacc and ERexp by speed, direction, and gender; and ERwork and ERpower by speed and gender. The errors appeared to increase when testing the female subjects, during the knee flexion test, or when speed increased. To increase validity in clinical testing, it is important to consider the acceleration phase effect, especially in higher velocity isokinetic testing or for weaker muscle groups.

Acceleration↗

PubMed1991

[Experimental study of the whole body response in rabbits by linear oscillatory acceleration and/or optokinetic stimulation].

Motion sickness may be defined as an abnormal body response caused to certain kinds of motion in normal subjects. Although we know etiologic factors and symptoms of motion sickness, the mechanism of motion sickness remains unknown. To clarify some characteristics of motion sickness in rabbits, we observed responses of the autonomic nervous system consisting of a change in heart rate (HR), coefficient of variation of R-R interval (CV-RR), and serum adrenaline (AD) -and free fatty acid (FFA) concentrations during linear oscillatory acceleration and/or optokinetic stimulation. The following results were obtained as 1. Although transverse and vertical linear oscillatory accelerations affected little the body responses, longitudinal linear oscillatory acceleration diminished HR and increased CV-RR and FFA. 2. Since injection of atropine or labyrinthectomy reduced the changes of HR and CV-RR induced by longitudinal linear oscillatory acceleration the responses may originate from the vestibulo-parasympathetic nerve reflex. 3. An increase of FFA during longitudinal linear oscillatory acceleration indicates that sympathetic nerve as well as parasympathetic nerve activities are elevated. 4. In this study optokinetic stimulation was not so influential as linear oscillatory acceleration for provoking motion sickness in rabbits. 5. Concomitant stimulations of optokinetic and vestibular inputs enlarged variations of these responses.

Acceleration↗

PubMed1999

Elicitation of otolithic nystagmus by step and sinusoidal modes of linear acceleration (Gy) in humans.

The vestibular otolith organ, a sensor of linear acceleration, is known to be important not only for posture control and spatial orientation, but also for eye movement control. Niven et al. (1966) have reported that linear acceleration along the Y (left-right, interaural) head axis by using a linear track induces compensatory horizontal eye movements including nystagmus. This was confirmed by several authors using a linear accelerator and a parallel swing. We have already found that a step mode of Gy linear acceleration is useful to elicit otolithic nystagmus and that nystagmus have directional preponderance (DP) (Mori and Katayama, 1998). In the present report, we attempted to compare two modes of acceleration, step and sinusoidal, in nystagmic elicitation induced by linear acceleration.

Acceleration↗

PubMedDate not supplied

[Effect of accelerations, hypergravity and hypokinesia on protein metabolism in Japanese quail. I. Effect on muscle composition].

The effect of hypokinesia, hypergravity achieved by centrifugation and additional weight load on the content and composition of proteins and nucleic acids in the chest and pelvic muscles of four groups of quails (Coturnix coturnix japonica) was studied. The first group was used as controls, the second included hypokinetic birds, the third was made of birds with an additional weight load (the load was a double weight of the animal) and the fourth group included birds exposed to acceleration of 3 g. The birds were exposed to the above effects for 1 to 6 hours during 8 days. They were given identical food through forced feeding. The content of total proteins, sarcoplasmatic proteins, DNA and RNA, cholesterol and esterified fatty acids was measured in chest and pelvic muscles. The composition of total lipids was examined in pelvic muscles. The level of corticosterone was determined in the blood plasma. The above experimental variants made it possible to discriminate individual contributions of acceleration, additional weight load and hypo-inesia to the effect. Weight load and acceleration decreased and hypokinesia increased the content of total proteins in the pelvic muscles. During an exposure to acceleration and hypokinesia the content and the portion of sarcoplasmatic proteins decreased and during an exposure to weight load increased significantly. Acceleration did not exert a significant effect on the RNA and DNA content in muscles. The content of esterified fatty acids increased under the influence of acceleration and hypokinesia and decreased significantly under the influence of additional weight.

Acceleration↗

PubMed1998

Sympathetic nerve response to muscle during anteroposterior acceleration in humans.

This study aimed to elucidate the effects of linear acceleration on muscle sympathetic nerve activity (MSNA) in humans. Eight healthy young male volunteers were seated in a linear accelerator (sled) during the recording of their electrocardiogram, blood pressure with the Finapres, thoracic impedance and respiration curve. MSNA was recorded from the tibial nerve by microneurography. At a fixed distance of sled movements in an anteroposterior direction, eight modes of stimulation with peak accelerations at 0.05, 0.10, 0.15, and 0.20 G (gravity) in sinusoidal or step mode were applied to each subject. Each movement was repeated for 5 cycles. Both the total activity and the burst rate of MSNA decreased during acceleration, and the level of the decrease was proportional to the level of the acceleration, whereas the average heart rate, thoracic impedance and mean arterial pressure did not change significantly. These results suggests that moderate linear acceleration may suppress MSNA in humans.

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PubMed2003

Function of otolith organ in goldfish revealed from analysis of eye movement induced by acceleration.

An otolith organ on ground behave as a detector of both gravity and linear acceleration, and play an important role in controlling posture and eye movement for tilt of the head or translational motion. On the other hand, a gravitational acceleration ingredient to an otolith organ disappears in microgravity environment. However, linear acceleration can be received by otolith organ and produce a sensation that is different from that on Earth. It is suggested that in microgravity signal from the otolith organ may cause abnormality of posture control and eye movement. Therefore, the central nervous system may re-interprets all output from the otolith organ to indicate linear motion. A study of eye movement has been done a lot as one of a reflection related to an otolith organ system. In this study, we examined function of otolith organ in goldfish revealed from analysis of eye movement induced by linear acceleration or the tilt of body. We analyzed both torsional and vertical eye movements from video images frame by frame. For tilting stimulation, torsional eye movements induced by head down was larger than that induced by head up for larger tilt angle than 30 degrees. In the case of linear acceleration below 0.4 G, however, no clear differences were observed in both torsional and vertical eye movement. These results suggest that body tilt and linear acceleration may not be with equivalent stimulation to cause eye movement on the ground.

Acceleration↗

PubMed2006

The quick and the fast: the evolution of acceleration capacity in Anolis lizards.

Although of prime ecological relevance, acceleration capacity is a poorly understood locomotor performance trait in terrestrial vertebrates. No empirical data exist on which design characteristics determine acceleration capacity among species and whether these design traits influence other aspects of locomotor performance. In this study we explore how acceleration capacity and sprint speed have evolved in Anolis lizards. We investigate whether the same or different morphological traits (i.e., limb dimensions and muscle mass) correlate with both locomotor traits. Within our sample of Anolis lizards, relative sprint speed and acceleration capacity coevolved. However, whereas the variation in relative acceleration capacity is primarily explained by the variation in relative knee extensor muscle mass, the variation in relative sprint speed is correlated to the variation in relative femur, tibia, and metatarsus length as well as knee extensor muscle mass. The fact that the design features required to excel in either performance trait partly overlap might explain the positive correlation between the variation in relative sprint speed and acceleration capacity. Furthermore, our data show how similar levels of sprint performance can be achieved through different morphological traits (limb segment lengths and muscle mass) suggesting that redundant mapping has potentially played a role in mitigating trade-offs.

Acceleration↗

PubMed1989

[Injury tolerance of the human skull in relation to simulation of impact acceleration].

A mathematical torso-neck-head model was used to simulate frontal and lateral head impact. The model consisted of nine rigid bodies representing the torso, the seven vertebrae, and the head. The external force acting during impact was described by a triangular force-time function. The impact conditions were varied, i.e., the mass of the head (1.5 and 5.5 kg), the mass (0.1 and 1.0 kg) and velocity (10, 30, and 50 m/s) of the impacting body, and the elasticity (0.5 and 1.0) of the impact. The computer simulations produced linear and angular head accelerations, which were compared with the tolerance level for injury and used to determine the tolerance curves which discriminate between "safe" and "unsafe" acceleration. For the linear head acceleration, the tolerance level was 1000 as defined by the Head Injury Criterion, and for the angular head acceleration the tolerance level was 1800 rad/s2. Our results showed that the risk of head injury was lower for linear head acceleration than for angular head acceleration, and it was lower for frontal impact than for lateral impact.

Acceleration↗

PubMed1978

Prediction of head/neck dynamic response of selected military subjects to -Gx acceleration.

Eighteen young male subjects with NAMRL sled test experience to 15 G in --Gx acceleration were measured for physical characteristics of the head and neck and general body anthropometry. Measurements taken include head/neck range of motion, neck muscle stretch reflex time, neck muscle isometric strength capabilities, and seated and standard anthropometry. Data from these tests were tabulated and five subjects whose physical characteristics were most similar were selected for use in simulations. Experimental data from NAMRL sled tests were obtained for the five subjects in 6- and 15-G test runs. Measurements data from the five subjects were used to establish a data set for the MVMA-2D Crash Victim Simulator and acceleration profiles for 6- and 15-G sled runs were used as input to the model. Simulation results for head angular acceleration, head angular velocity, head angular position, head resultant acceleration, and T1 resultant acceleration were compared with the averaged experimental curves for the five subjects. In general, excellent agreement between simulation and experimental results was obtained although some consistent differences were noted. Effects of varying levels of muscle activation were investigated. Variations in muscle tension level were found to have significant effects on simulation results at both 6 and 15, G, especially on head angular position. The effects were noticeably greater at lower acceleration's, however. The model was also used to investigate some of the biomechanical mechanisms behind observed response characteristics.

Acceleration↗

PubMed1984

Tolerance of the human cervical spine to high acceleration: a modelling approach.

A sagittal plane mathematical model for the cervical spine has been used to simulate the neck's response to loads due to high acceleration. The model is capable of simulating the muscular response of the cervical spine and the stress distribution between the joint levels. In order to obtain conservative estimates of the maximum acceleration that the neck can support, the neck was simulated using the assumption that the inertial load is supported primarily by the muscles. It was found that accelerations of up to 30 g can be supported with the appropriate posture and direction of acceleration. Estimates were also obtained using experimental results to approximate the role that the ligaments of the spine play in supporting the load. It was found that accelerations of up to 40 g can be supported for the appropriate posture and acceleration direction.

Acceleration↗

PubMed1994

Accelerated intravenous dosing of recombinant tissue-type plasminogen activator causes rapid but unstable reperfusion in a canine model of acute myocardial infarction.

BACKGROUND: Accelerated intravenous dosing is currently the preferred regimen for administering recombinant tissue-type plasminogen activator (rt-PA) in acute myocardial infarction (AMI). This regimen is recommended on the basis of clinical angiographic studies that reported superior 60 and 90 min patency rates. However, continuous infarct-vessel flow characteristics after reperfusion following this regimen are poorly described. The aim of our study was an improved definition of the characteristics of infarct-vessel flow induced by intravenous accelerated rt-PA using an animal model of acute coronary artery thrombosis. METHODS: We studied the characteristics of reperfusion induced by an accelerated intravenous rt-PA dosing regimen in a canine coronary artery thrombosis model that simulates AMI. We created a critical stenosis in the left anterior descending coronary artery (LAD) of 24 open-chest dogs. Thrombosis was caused immediately proximal to the stenosis by injection of a blood and thrombin mixture into a segment of the LAD which had intimal damage. Hemodynamics and distal LAD blood flow velocity were recorded continuously. Six animals did not complete the protocol. Animals in the treatment group (n = 10) received an intravenous accelerated rt-PA regimen (1.25 mg/kg total dose) and were observed for 150 min. Eight animals served as controls. RESULTS: None of the control animal arteries reperfused. In the treatment group, reperfusion occurred 31.5 +/- 7.3 min after starting rt-PA dosing in two general patterns. In one pattern, reperfusion onset was gradual and linear; in the other pattern, reperfusion was sudden. LAD flow was cyclical in all animals, with marked variations in magnitude compared with baseline (0-173% of baseline). Reocclusion was common and occurred 30.3 +/- 6.3 min after the initial reperfusion. There were 1.9 +/- 0.4 reocclusions per animal, and two distinct patterns were noted. In one, flow ceased abruptly and, in the second, flow declined gradually in a linear fashion before stopping. Infarct-vessel flow was evident for 103.1 +/- 14.1 min of the 150 min observation period. CONCLUSIONS: Reperfusion occurs rapidly after an accelerated dosing regimen of rt-PA. However, the infarct-vessel flow resulting from this therapy is unstable and is associated with early reocclusion and marked variation in flow magnitude. Analysis of continuous infarct-vessel flow velocity patterns suggests at least two different mechanisms for the onset of reperfusion and reocclusion after this therapy. Recognizing several mechanisms of arterial opening and closing may have important implications for methods of stabilizing infarct vessels after reperfusion by accelerated dosing of rt-PA.

Acceleration↗

PubMed1994

[Accelerated idioventricular rhythm].

The term accelerated idioventricular rhythm describes an ectopic ventricular rhythm with 3 or more consecutive ventricular premature beats with a rate faster than the normal ventricular intrinsic escape rate of 30 to 40 beats per minute, but slower than ventricular tachycardia. Accelerated idioventricular rhythm differs from ventricular tachycardia by additional features such as the onset with a long coupling interval, the end by a gradual decrease of the ventricular rate or increase of the sinus rate and, last but not least, by a good prognosis. Clinically, accelerated idioventricular rhythm can occur in any form of structural heart disease and occasionally in adults or children without structural heart disease. Accelerated idioventricular rhythm most often can be seen in patients with coronary artery disease. Its occurrence after thrombolysis during acute myocardial infarction is a marker of successful reperfusion. Since accelerated idioventricular rhythm is usually hemodynamically well tolerated and not associated with malignant ventricular tachycardias; as a rule, no specific treatment other than care of the underlying heart disease is necessary. The present overview discusses electrocardiographic criteria, possible mechanisms, and the clinical significance of accelerated idioventricular rhythms.

Accelerated Idioventricular Rhythm↗

PubMed1994

Perceptual thresholds of radial accelerations as indicated by visually perceived eye level.

The purpose of this study was to determine whether very low gravito-inertial forces produced by centrifugation affect the visually perceived eye level (VPEL) in the same way as the oculagravic illusion. Eleven subjects in total darkness were instructed to set a luminous target to the VPEL, either while they were motionless or undergoing very low centrifugation. Results showed a significant effect on VPEL at 0.01 m/s2 radial acceleration, which corresponds to a resultant gravito-inertial equal to 9.81001 m/s2. This radial acceleration value is lower than the lowest perception thresholds previously measured for a linear acceleration (about 0.05 m/s2). Thus, as previous results have shown that the oculogyral illusion indirectly decreases perceptual thresholds for the perception of angular acceleration in darkness, the lowering of the VPEL indirectly decreases thresholds for perception of radial acceleration produced by centrifugation. Moreover, there is a logarithmic relationship between very low centrifugation and the positioning of the VPEL at a lower level. This relationship is explained as a direct and sole effect on the sensory utricular otolithic membrane by the radial acceleration of centrifugation.

Acceleration↗

PubMed1996

The severity of nauseogenic effect of cross-coupled rotation is proportional to gyroscopic angular acceleration.

METHOD AND RESULTS: Subjects underwent cross-coupled rotation of the head (i.e., Coriolis stimuli) during which the upper body was tilted from side to side during horizontal rotation of the whole body about the Earth-vertical axis. In Experiment 1, the angle between the two axes of cross-coupled rotation was changed to vary the magnitude of gyroscopic angular acceleration without wide variations in Coriolis linear acceleration. The severity of nausea evoked by cross-coupled rotation stimuli varied relative to the magnitude of gyroscopic angular acceleration. It is noteworthy that nausea was not evoked unless gyroscopic angular acceleration was generated, even though Coriolis linear acceleration was equally induced. In Experiment 2, subjects tilted the upper body with or without restriction of head movement to a vertical plane during Earth-vertical axis rotation of the body at various angular velocities. The severity of evoked nausea was in direct proportion to angular velocity of body rotation irrespective of the restriction. CONCLUSION: These results indicated that the severity of nauseogenic effect of cross-coupled rotation is directly proportional to gyroscopic angular acceleration.

Acceleration↗

PubMedDate not supplied

Three-dimensional baselines for perceived self-motion during acceleration and deceleration in a centrifuge.

Three-dimensional motion trajectories were computed, representing the motions that would be perceived by a perfect processor of acceleration information during the acceleration and deceleration stages of a centrifuge run. These motions serve as "baselines" for perceived self-motion in a centrifuge, and depend on the initial perception of orientation and velocity immediately preceding the acceleration and immediately preceding the deceleration. The baselines show that a perfect processor of acceleration information perceives self-motion during centrifuge deceleration significantly differently from self-motion during centrifuge acceleration, despite the fact that the angular accelerations have equal magnitude (with opposite direction). At the same time, the baselines can be compared with subjects' reported perceptions to highlight limitations of the nervous system; limitations and peculiarities of the nervous system are identified as deviations from a baseline. As a result, peculiarities of the nervous system are held responsible for any perception of pitch or roll angular velocity or change in tilt of the body-horizontal plane of motion during the centrifuge run. On the other hand, baselines explain perception of tilt position during deceleration, linear velocity, possible lack of significant linear velocity during deceleration, and yaw angular velocity, including on-axis angular velocity during centrifuge deceleration. The results lead to several experimental questions.

Acceleration↗

PubMed1997

Female acceleration tolerance: effects of menstrual state and physical condition.

INTRODUCTION: The literature contains a paucity of information on female tolerance to high sustained acceleration. With women now flying high-performance aircraft, gender-specific factors that may affect female acceleration tolerance have become increasingly important. The purpose of this investigation was to determine how menstrual state and physical condition affect acceleration tolerance. We hypothesized the menstrual cycle would have no effect on acceleration tolerance and that a positive correlation would exist between physical fitness level and tolerance to high sustained acceleration. METHODS: Centrifuge exposures on 8 female subjects consisted of a relaxed gradual-onset run (0.1 G.s-1) to the visual endpoint, a rapid-onset run (6 G.s-1) to +5 GZ for 15 s, and a +4.5 to +7 GZ simulated aerial combat maneuver (SACM) to physical exhaustion. Acceleration tolerance data were collected at onset of menstruation and 1, 2 and 3 weeks following the onset for two complete menstrual cycles. On separate days, body composition, anaerobic power output and peak oxygen uptake were determined. Retrospective data from 10 male subjects who had performed the +4.5 to +7 GZ SACM were analyzed and compared to these data. RESULTS: Analysis of variance revealed no significant difference in relaxed tolerance or SACM duration between the four selected menstrual cycle time points. Time-to-fatigue on the +4.5 to +7 GZ SACM was positively (p < or = 0.05) correlated with absolute fat-free mass (r = 0.87) and anaerobic power production (r = 0.76) in female subjects. However, when these variables were adjusted for total body mass, the significant correlations no longer existed. No correlation was found between SACM duration and absolute (L min-1) nor relative (ml.kg-1.min-1) aerobic fitness. Time-to-fatigue during the SACM was not significantly different between male and female subjects (250 +/- 97 and 246 +/- 149 s, respectively).

Acceleration↗

PubMed1993

Acceleration of axonal outgrowth in rat sciatic nerve at one week after axotomy.

Following injury of sciatic motor axons in the rat, the rate of axonal outgrowth is faster if there has been a prior "conditioning" axotomy. The acceleration of outgrowth is due to an acceleration of SCb, the rate [slow (SC)] component of axonal transport that carries cytomatrix proteins; this occurs throughout the axon by 7 days after the conditioning axotomy (Jacob and McQuarrie, 1991a, J. Neurobiol. 22:570-583). To further characterize the conditioning lesion effect (CLE), it is important to know (1) the minimum effective conditioning interval (time between conditioning and testing lesions), (2) whether the cell body reaction is required, and (3) whether outgrowth accelerates after a single axotomy. Outgrowth distances were measured by radiolabeling all newly synthesized neuronal proteins and detecting those carried to growth cones by fast axonal transport. When the conditioning and testing lesions were made simultaneously (0 day conditioning interval), there was no CLE. With a conditioning interval of 3 days, there was a shortening of the initial delay (before the onset of outgrowth) without a change in outgrowth rate. With conditioning intervals of 7, 14, and 21 days, the rates of outgrowth were increased by 8%, 22%, and 11%, respectively. To determine whether the cell body reaction to axotomy is necessary for the CLE, a nonaxotomizing stimulus to axonal growth (partial denervation) was used in place of a conditioning axotomy. This had no effect on the rate of outgrowth from a testing lesion made 14 days later. Finally, we examined the possibility that outgrowth accelerates after a single lesion. Outgrowth was faster at 6-9 days after axotomy than at 3-6 days (p < 0.001), and accelerated further at 9-12 days (p < 0.001). We conclude that (1) the shortest effective conditioning interval is 3 days; (2) the cell body reaction is necessary for the CLE; (3) axonal outgrowth from a single axotomy accelerates in concert with the anabolic phase of the cell body reaction. The SCb motor is, in turn, upregulated by this reaction. This suggests that the SCb motor responds to a fast-transported signal that is a product of the cell body reaction.

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