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Results for “Acceleration”

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At least 523 records · Page 29Linked to original sources

PubMed2003

An accidental exposure by a medical linear accelerator under construction.

A radiation accident occurred at a medical linear accelerator facility under construction in Japan. The radiation source was a 3- and 6-MV potential drop accelerator designed to produce X-rays for radiation therapy. This accelerator was also capable of producing a 5 to14-MV swept electron beam. During setting up, an operator turned on the accelerator to test the beam not knowing that a man was working on the ceiling above the accelerator. Thus, an X-ray beam was emitted against the ceiling and the man was exposed to 10-MV of X-ray irradiation. However, no obvious physical symptoms were noted. Dose estimation was made from reconstruction of the accident and clinical examinations including chromosome analysis. Mean dose of the whole body ranged from 70 to 180 mSv. Estimated dose from his right foot to hand was between180 to 900 mSv.

Humans↗

PubMed2005

The effects of resisted sled-pulling sprint training on acceleration and maximum speed performance.

AIM: The purpose of the present study was to examine the effects of resisted (RS) and un-resisted (US) sprint training programs on acceleration and maximum speed performance. METHODS: Twenty-two male students (age 20.1+/-1.9 y, height 1.78+/-7 cm, and weight 73+/-2 kg) completed RS (n=11) or US (n=11) sprint training programs. The RS group followed a sprint-training program with 5 kg sled pulling and the US group followed a similar sprint-training program without sled pulling. The training program consisted of 4x20 m and 4x50 m maximal runs, and was applied 3 times/week for 8 weeks. Before and after the training programs the subjects performed a 50 m run and the running velocity of 0(-1)0 m, 10(-2)0 m, 20-40 m and 40-50 m was measured. In addition, stride length and stride frequency were evaluated at the 3(rd) stride in acceleration phase and between 42-47 m in maximum speed phase. RESULTS: The RS improved running velocity in the run sections 0(-1)0 m and 0(-2)0 m, while in US group the running velocity in all run sections in acceleration phase remained unchanged (p>0.05). In contrast, RS training had no effect on running velocity in maximum speed phase, whereas US improved running velocity in 20-40 m, 40-50 m, and 20-50 m run sections (p<0.05). Stride rate increased only after RS in acceleration phase (+7.1+/-2.9%; p<0.05), whereas stride length increased only after US in maximum speed phase (+5.5+/-2.5%; p<0.05). CONCLUSION: Sprint training with 5 kg sled pulling for 8 weeks improves acceleration performance (0(-2)0), while un-resisted sprint training improves performance in maximum speed phase (20-40) in non-elite athletes. It appears that each phase of sprint run demands a specific training approach.

Acceleration↗

PubMed2006

Role of translational and rotational accelerations on brain strain in lateral head impact.

Translational and rotational accelerations from blunt head impact can induce excessive brain strain and cause traumatic brain injuries. However, it is not clear which acceleration plays a major role in the mechanism. The current study used the SIMon human finite element head model (FEHM) and delineated the contributions of these accelerations using post mortem human subject (PMHS) lateral head impact experimental data. Results indicated that rotational acceleration contributes more than 90% of total strain, and translational acceleration produces minimal strain. Therefore, the rotational component is a more important biomechanical metric in this study.

Acceleration↗

PubMed1991

Contribution of angular motion and gravity to tibial acceleration.

A bone-mounted accelerometer and high-speed cinematography were used to compare the axial tibial acceleration caused by ground impact with the total tibial axial acceleration as measured by a transducer. Due to the effects of gravity and tibial angular motion, the magnitude of the peak acceleration at foot strike was 43% below and 18% above the peak axial acceleration due to impact for running and walking, respectively. Depending on the distance of the accelerometer from the tibial center of rotation which is located at the ankle joint, different axial acceleration signals should be expected during comparable locomotor activities.

Acceleration↗

PubMed1989

The effect of sustained acceleration and noise on workload in human operators.

Biodynamic stressors such as acceleration, vibration, heat, and cold can affect pilot performance. The objective of this research was to determine the individual effect of two of these stressors, sustained acceleration or high intensity pink noise, on workload in human operators. Combined stressors were not investigated. A total of 13 workload measures, including 1 subjective, 4 performance, and 8 physiological, were recorded on subjects (N = 9) performing a dual psychomotor task in the human centrifuge. Increasing noise stress (max 100 dB-A weighted) had a significant effect on the Subjective Workload Assessment Technique or SWAT scores (p = 0.0001) and reaction times (p = 0.0189); acceleration stress (average peak, 3.75 Gz) had a significant effect on SWAT (p = 0.0001), heart rate (p = 0.0001), total eye blinks (p = 0.0184), blink duration (p = 0.0017), and the standard deviation of the EMG on the tracking forearm (p = 0.0452). Although many of the workload measures were affected by the noise or acceleration, subjects were able to maintain their performance on the primary tracking task. It is concluded that biodynamic stressors, such as noise and acceleration can adversely affect subjective operator workload without affecting objective task performance.

Acceleration↗

PubMedDate not supplied

[Thresholds of long-latency evoked potentials and of the sensations of motion occurring during human exposure to linear accelerations].

Forty-seven healthy test subjects were exposed several times each to linear acceleration of varying value. Depending on the acceleration value, they reported subjectively three types of sensations at the threshold level: nondiscriminated in terms of direction (P1, a = 8.0 +/- 2.5 cm/sec2), inverted (P2, a = 12.4 +/- 2.5 cm/sec2) and discriminated (P3, a = 16.1 +/- 3.5 cm/sec2). The acceleration value at which P2 was recorded caused most typical responses in different test subjects whereas that at which P3 was recorded induced greatest individual variations. Evoked potentials in response to linear acceleration were recorded in such a manner which excluded potential instrumental artefacts or possible contribution of eye movements, excitation of the hearing organ and muscle tone changes. The data obtained indicate that acceleration-induced evoked potentials are predominantly of vestibular origin and consist of three peaks, viz. P1, N1, and P2, the latencies of which are equal to 31.3 +/- 7.2, 69.1 +/- 9.1 and 157.6 +/- 10.5 msec at the threshold where they emerge. The peak P1 is most variable, the peak N1 is most stable and the peak P2 is characterized by the largest ratio of the interindividual variation to the individual variation.

Acceleration↗

PubMed1985

Effect of physical training in cool and hot environments on +Gz acceleration tolerance in women.

Rectal temperature (Tre), sweat rate, plasma volume (PV), peak oxygen uptake (peak VO2), and relaxed +Gz acceleration tolerance (0.5 G X min-1 linear to grayout) were measured in 15 healthy women 21-41 years old before and after submaximal isotonic exercise training for 2 h X d-1 on a cycle ergometer. The women had 2 weeks of acceleration runs and Vo2 testing, followed by 8 d of exercise training, post-training acceleration runs on day 9, and peak Vo2 tests on day 10. They were divided into three groups: an exercise (heat) group, ambient temperature (Ta) 40.6 degrees C, relative humidity (rh) 42%, and a peak Vo2 of 52%; an exercise (cool) group, Ta = 18.7 degrees, rh = 48%, and Vo2 peak = 55%; and a sedentary control (cool) group. There was no change in peak ventilation, peak heart rate (HR), peak Vo2, or in resting PV in any group after training. Heart rate and Tre were significantly lower after training in both cool and hot environments; HR by 17 b X min-1 (p less than 0.05) and 27 b X min-1 (p less than 0.05), respectively, and Tre by 0.4 degrees C (p less than 0.05) and 0.4 degrees C (p less than 0.05), respectively. Sweat rates were not different in any group. In all groups, acceleration tolerances were not different after training; they ranged from 3.5 to 3.8 G (373 - 410 s). The loss (shift) in PV during acceleration ranged from -5.8% to -10.3% (nonsignificant).(ABSTRACT TRUNCATED AT 250 WORDS)

Acceleration↗

PubMed1983

[Operating characteristics and potential use of the LUEV-15M1 linear electron accelerator].

The manufacture of a series of medical electron accelerators LUEV-15M1 is in line with a tendency to equip radiological units in our country with electron linear accelerators. Clinical and dosimetric characteristics of a linear accelerator of this design that are distinguished by a high penetrating power, a high dose gradient along the beam edge, were studied in the Central Research Roentgeno-Radiological Institute. Its main distinctive feature is an ability to exercise programmed control over a depth dose field in the static and moving irradiation regimens by measuring the dose rate. Computed depth dose fields demonstrated an opportunity to use effectively the accelerator for irradiation of deep foci. Ten patients with head, chest and pelvic tumors were irradiated. Short-term positive results were obtained. Radiation complications hampering radiotherapy, were absent. The shortcomings are as follows: a small rotation angle of the irradiator, small movement rates of the diaphragm blocks and the adjustment of a light field, absence of a functionally fixed shaft. The accelerator has been shown to meet medical and technical requirements.

Adenoma↗

PubMed1984

Prediction of arterial wall failure under acceleration stress in high-performance aircraft.

Experimental results indicate that the passive mechanics (no smooth muscle action) of the larger arteries, such as the aorta and pulmonary, in humans are closely related to the elastin content at low strain and to the collagen content at high strain. In an aerial combat maneuver, the acceleration stress typically rises to a maximum, remains there for several seconds, and then falls off. The arterial wall viscoelastic property relevant here is hysteresis. The strain remaining when the stress is removed is called "permanent set." This so-called permanent set will decay away exponentially with a comparatively long time constant on the order of several hundred seconds. A first approximation to the permanent set, after a hysteresis loop has been executed, has been derived. Collagen can only stretch by 20% before failing. Choosing combat scenarios in which the maneuver is repeated at 1-min intervals so that the decay in permanent sets can be ignored, the number of maneuvers required to damage or rupture the collagen is computed for ranges of acceleration (g) values equivalent to increases in the arterial intraluminal pressures. The calculations are done for the external carotid, abdominal aorta and pulmonary arteries. For example, applying the continual recruitment of collagen fibers theory to the pulmonary artery produces the following result. The permanent set at net acceleration value of 5 g is 0.085 and the number of sequential maneuvers required to damage 2.5% of the collagen is 2.4. Based on discontinuous recruitment of 100% collagen at a strain of 1.7 in the pulmonary artery, the number of maneuvers required to rupture the collagen would be 58 at an acceleration of 5 g. At 10 g, this number falls to about 10 maneuvers. The results emphasize the importance of present protective measures, such as the use of G suits and valsalva maneuvers. But these measures can only reduce the effects predicted here, not eliminate them. The results also highlight the necessity for servo-controlled seat orientation, and for the introduction of integrating accelerometers for all aircrew to record acceleration exposure history.

Acceleration↗

PubMed1983

Evoked potential studies of the effects of impact acceleration on the motor nervous system.

The initial results of a continuing investigation into the effects of various levels of impact acceleration on the functional integrity of the motor nervous system are summarized. The results are based on the measurement of alterations in neural transmission along the motor pathway of the Rhesus monkey as revealed by latency and amplitude changes in the motor pathway evoked potential (EP) following the delivery of various levels of impact acceleration to a test vehicle. The EPs were produced by electrical stimulation of and recording from the motor pathway of experimental animals subjected to -Y (lateral impact) acceleration and animals subjected to -X (frontal impact) acceleration. High resolution latency and amplitude measures of the EP recorded from these animals before and after impact were tracked so that the time course of recovery of nerve propagation following impact could be accurately assessed. Analysis of these EP measures revealed that the time course of recovery to preimpact values is directly related to the intensity of the acceleration impulse delivered to the test vehicle.

Acceleration↗

PubMed1982

Human endocrine responses to acceleration stress.

Five healthy male volunteers were subjected to accelerations of up to +6 Gz for 1 min in order to investigate the effect of acceleration stress upon the concentration of various hormones in peripheral blood. No significant changes were seen in the levels of GH, PRL, TSH, LH, and FSH compared to control values obtained at +1 Gz (normal gravity). Changes in serum cortisol levels were significant (p less than 0.001) with peak values occurring 20 min after acceleration stress. A significant effect (p less than 0.001) of acceleration was also observed on plasma volume with maximum reductions occurring at the end of acceleration. The unusual specificity of the hormone responses is discussed.

Acceleration↗

PubMed1980

The efffect of sustained +Gz acceleration on extravascular lung water content in domestic fowl.

In order to examine the effects of sustained acceleration on pulmonary fluid balance, chickens were centrifuged at low sustained +G acceleration (LSG) intensities of +3Gz or +4Gz or high sustained +G acceleration (HSG) at +6Gz for varying periods. Animals were exposed to acceleration on a hydraulic-driven centrifuge. Controls (Group I) were not centrifuged. The vascular lungs were rapidly excised after centrifugation, and the extravascular lung water content (EVLW) was measured. EVLW in Group I animals = 2.19 +/- 0.38 g/g. Results indicate that positive acceleration elevated in animals exposed to +4Gz for 60 min (Group III). In this group, EVLW = 3.01 +/- 1.07 g/g. This increase is largely attributable to an elevated EVLW in those animals which died as a result of centrifugation, in which EVLW = 3.90 +/- 1.04 g/g. Exposure to +3Gz for 120 min (Group II) or +6Gz for periods up to 50 min (Group IV) did not produce significant elevations in EVLW in either group as a whole, but EVLW in those Group II animals which died as a result of centrifugation was significantly elevated. EVLW in these animals = 2.87 +/- 0.21 g/g. We conclude that pulmonary edema caused by centrifugation is dependent on both magnitude and duration of G force.

Acceleration↗

PubMed1994

Perceptual and eye movement responses elicited by linear acceleration following spaceflight.

Responses of four crewmembers were measured before and after a 9-d Spacelab Mission during inter-aural (y-axis) and longitudinal (z-axis) linear acceleration. During one test, eye movements were recorded with simultaneous sinusoidal acceleration and constant-velocity optokinetic stimulation. Postflight responses indicated changes in the horizontal sinusoidal response during the y-axis trials. In another test, subjects indicated direction of acceleration during a series of low acceleration steps. More incorrect direction indications were observed postflight, and the y-axis sensitivity increased while the z-axis sensitivity decreased for three subjects. In a third test, subjects used a joystick to null pseudo-random linear motion. Not one subject showed a significantly enhanced ability to null linear motion in the y-axis, while two subjects showed an enhanced ability during z-axis testing postflight. Oculomotor and perceptual responses to linear acceleration are modified by exposure to microgravity and become more variable postflight. The observed changes may be due to an altered interpretation of inertial cues in space that is inappropriate postflight.

Acceleration↗

PubMed1994

The effect of head and body position on +Gz acceleration tolerance.

It has been suggested there is a relationship between acceleration-induced loss of consciousness (G-LOC) and head/body position. A two-part investigation was conducted to determine whether head and body position affects acceleration tolerance. A retrospective analysis of high-G training data (N = 1,914) compared G-LOC occurrence during straight-ahead exposure to a "check-6" exposure [10 s at +9 Gz; 6 G/s onset rate; G-suit inflated; anti-G straining maneuver (AGSM) performed]. A prospective study (N = 12) was conducted with acceleration exposures using light loss criteria with subjects in straight-ahead, above, over-the-right shoulder, or over-the-left shoulder positions. Profiles consisted of 0.1 G/s onset-rate runs (no G-suit inflation; relaxed) to a maximum of +9 Gz and 0.5 G/s onset-rate runs (G-suit inflated; AGSM performed) to +9 Gz for up to 26 s. In the retrospective study, no significant difference existed between G-LOC occurrence during straight-ahead (22/1914) and check-6 (32/1914) positions. During the prospective study with AGSM runs, there was no significant difference in the time at maximum G among any of the positions. During the relaxed runs, several comparisons yielded significant differences in peak G attained. These results indicate there may be an underlying physiologic effect of head and body position on acceleration tolerance; however, the AGSM and the G-suit overcame this effect. Although task saturation and distraction may compromise performance of the AGSM and subsequently predispose acceleration-related hazards, a proper AGSM, combined with effective protective systems, remains essential components of a protection strategy.

Acceleration↗

PubMed1996

[Study of human orientation with respect to vertical under effect of lateral and combined longitudinal-lateral accelerations].

Six test subjects were subjected to NZ and Ny/Nz accelerations in centrifuge cabin having rotation radius of 6.55 m. During rotation, test subjects have determined the position of the subjective visual vertical. The performed studies have demonstrated that during exposure to NZ and Ny/NZ accelerations the image of subjective vertical the direction of which with rise of lateral component approaches the direction of the vector of resultant acceleration. The expressivity of this effect decreases with an increase of the longitudinal component of acceleration. It was suggested that pulling of high lateral accelerations (up to 2-3 units of g) in the flight of advanced highly manoeuvrable aircrafts can hinder in-flight spatial orientation of a pilot due to appearing of the stable illusions of spatial position.

Acceleration↗

PubMed1975

Accelerated human renal allograft rejection.

A series of 125 renal transplants were analyzed in order to ascertain the characteristics of accelerated allograft rejection. An intense accelerated rejection could be identified within the first five days in 12 of 67 transplants (18%) with good immediate renal function. Accelerated rejection differed from the usual acute rejection reaction by higher fever, increased duration and intensity of the rejection, and increased difficulty in reversing the reaction. Accelerated rejection was reversible and associated with satisfactory renal function one year posttransplant in 58% of patients. Lymphocytotoxic and heterophil antibodies in preoperative serum and eluates of removed kidneys were not present. Contrary to recent reports, accelerated renal allograft rejection is a potentially reversible process and not necessarily due to humoral antibody presensitization.

Antibodies↗

PubMed1983

Wound healing accelerated by Staphylococcus aureus.

While comparing the effects on wound healing of a heated scalpel with those of the cold scalpel, we discovered that inoculation of rat skin incisions with a strain of Staphylococcus aureus dramatically accelerated the gain in wound strength. The accelerating effect was evident four days postoperatively, was maximal at seven to ten days, and was still present at 28 days. The accelerating effect was correlated with the number of S aureus organisms introduced into the wound, and was found in conventional rats and rats germ free up to the time of monocontamination with S aureus. There was no evidence of infection on gross examination; on histologic examination an occasional microabscess was seen in some rats. There may be both local and systemic mechanisms underlying the S aureus accelerating effect. Seven strains of S aureus with varying characteristics demonstrated the wound-healing accelerating effect. In sharp contrast, Staphylococcus epidermidis (three strains), Staphylococcus hominis (one strain), and Pseudomonas aeruginosa (two strains) did not show this effect. The increases in wound healing due to S aureus were substantially greater than reported previously for any nutritional supplement, drug, or other chemical or physical agent.

Animals↗

PubMed1999

Evolution of morphology in UHMWPE following accelerated aging: the effect of heating rates.

Accelerated aging methods are used to evaluate the oxidative stability of UHMWPE components for total joint replacements. In this study, we traced the evolution of the crystalline morphology during accelerated thermal aging of UHMWPE in air with the intent of explaining previous, counterintuitive heating rate effects. GUR4150HP extruded rod stock material was machined into miniature (0.5 mm thick) specimens that were either gamma irradiated in air or in nitrogen (27 +/- 3 kGy) or left unirradiated (control). Accelerated aging in an air furnace (at 80 degrees C, atmospheric pressure) was performed on half of the test samples at a heating rate of 0.1 degrees C/min and at 5 degrees C/min for the remaining half. Although the initial heating rate, as measured by changes in density, did influence the absolute degradation rate by up to 214%, the heating rate effect did not appear to influence the relative ranking of UHMWPE in terms of its oxidative stability. The heating rate effect is more consistent with a kinetic mechanism of the oxidation process than it is with a previously hypothesized diffusion mechanism. UHMWPE morphology, as characterized using a transmission electron microscope (TEM), demonstrated considerable rearrangement of the crystalline regions as a result of the accelerated aging. The stacking of the lamellae observed after accelerated aging was not consistent with the morphology of naturally aged UHMWPE components. The observed differences in crystalline morphology likely result from the enhanced mobility of the polymer chains due to thermal aging and may be analogous to an annealing process.

Algorithms↗