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排序方式: 共有69条查询结果,搜索用时 15 毫秒
1.
An estimation of drag in front crawl swimming 总被引:3,自引:0,他引:3
A J van der Vaart H H Savelberg G de Groot A P Hollander H M Toussaint G J van Ingen Schenau 《Journal of biomechanics》1987,20(5):543-546
Propulsive arm forces of twelve elite male swimmers during a front crawl swimming-like activity were measured. The swimmers pushed off against grips which are attached to a 23 m tube at 0.8 m under the water surface. The tube was fixed to a force transducer. Since at constant speed, mean propulsive force equals mean drag force this method also provides the mean active drag on a moving swimmer. The mean propulsive force at a speed of v = 1.48 m s-1 appeared to be 53.2 +/- 5.8 N which is two to three times smaller than what is reported by other authors for active drag but which is in agreement with values reported for passive drag on a (towed) swimmer who is not moving. Discrepancies with indirect active drag measurements are discussed. 相似文献
2.
Coordination in vertical jumping 总被引:5,自引:0,他引:5
The present study was designed to investigate for vertical jumping the relationships between muscle actions, movement pattern and jumping achievement. Ten skilled jumpers performed jumps with preparatory countermovement. Ground reaction forces and cinematographic data were recorded. In addition, myoelectric activity (EMG) was recorded from seven leg muscles. EMG-signals were rectified and low-pass filtered to obtain EMG-levels. The latter, which were assumed to reflect activation levels, rose to a plateau in the sequence m. semitendinosus, long head of m. biceps femoris, m. gluteus maximus, m. vastus medialis, m. rectus femoris, m. soleus, m. gastrocnemius. It was attempted to link the EMG-pattern to the purpose of the push-off, namely to maximize the effective energy (Ey) of the mass center of the body (MCB). The term Ey designates the sum of the potential energy of MCB and the kinetic energy due to the vertical velocity of MCB. One of the requirements for maximization of Ey is that the mono-articular extensor muscles release as much energy as possible before toe-off occurs. It is argued that this requirement can only be satisfied if the vertical velocity differences between the proximal and distal ends of body segments reach their peaks in a sequence. The sequence that is realized by the pattern of muscular activation is upper body, upper legs, lower legs, feet. Another important requirement is that the mechanical energy released by the muscles is optimally used. This requirement can be satisfied by transportation of energy via the biarticular m. rectus femoris and m. gastrocnemius. 相似文献
3.
Toussaint H. M.; Beelen A.; Rodenburg A.; Sargeant A. J.; de Groot G.; Hollander A. P.; van Ingen Schenau G. J. 《Journal of applied physiology》1988,65(6):2506-2512
In this study the propelling efficiency (ep) of front-crawl swimming, by use of the arms only, was calculated in four subjects. This is the ratio of the power used to overcome drag (Pd) to the total mechanical power (Po) produced including power wasted in changing the kinetic energy of masses of water (Pk). By the use of an extended version of the system to measure active drag (MAD system), Pd was measured directly. Simultaneous measurement of O2 uptake (VO2) enabled the establishment of the relationship between the rate of the energy expenditure (PVO2) and Po (since when swimming on the MAD system Po = Pd). These individual relationships describing the mechanical efficiency (8-12%) were then used to estimate Po in free swimming from measurements of VO2. Because Pd was directly measured at each velocity studied by use of the MAD system, ep could be calculated according to the equation ep = Pd/(Pd + Pk) = Pd/Po. For the four top class swimmers studied, ep was found to range from 46 to 77%. Total efficiency, defined as the product of mechanical and propelling efficiency, ranged from 5 to 8%. 相似文献
4.
F J Avis H M Toussaint P A Huijing G J van Ingen Schenau 《European journal of applied physiology and occupational physiology》1986,55(5):562-568
Negative and positive work performed during leg extension movements of 53 well trained subjects was measured with the help of a special dynamometer. The subjects performed four maximal push off trials against five different loads (25-105 kg): two two-legged extensions from a squatting position (SM) with a knee angle of 70 degrees and two trials with a preliminary counter movement (CM) but with the same extension range as in the SM. Positive work differed only by about 4% between CM and SM in spite of large differences in initial forces at the onset of concentric contraction. Based on simulations, it is suggested that in CM the advantage of stored elastic energy can almost completely be nullified by the disadvantage of a limited shortening distance of the contractile elements. It is hypothesised that elastic energy in CM can only cause considerable extra work during concentric contraction compared to the maximal positive work done in SM if the total range of lengthening and shortening of the muscle(s) involved is larger in CM than in SM. 相似文献
5.
Membrane particle changes attending the acrosome reaction in guinea pig spermatozoa 总被引:15,自引:11,他引:4 下载免费PDF全文
To examine the freeze-fracture appearance of membrane alterations accompanying the preparation of sperm membranes for fusions-the first preparatory stage occurring before physiological release of the acrosomal content, the second afterward-we induced the acrosome reaction in capacitated guinea pig spermatozoa by adding calcium to the mixture. The most common features observed before fusion of the acrosomal and plasma membranes were the deletion of fibrillar intramembranous particles from the E-fracture faces of both membranes, and the clearance of globular particles from the P face of the plasma membrane-events taking place near the terminus of the equatorial segment. Large particles, >12nm, remained not far from the cleared E-face patches. The P face of the outer acrosomal membrane is virtually clear from the outset. In addition, when fusion was completed, occasional double lines of large particles transiently embossed the P face of the plasma membrane (postacrosomal) side of the fusion zone. Behind the line of fusion, another series of particle-cleared foci emerged. We interpreted these postfusion membrane clearances as a second adaptation for sperm-egg interaction. Induction of the acrosome reaction in media containing phosphatidylcholine liposomes resulted in their apparent attachment, incorporation, or exchange in both the originally and secondarily cleared regions. Our observations support the concepts that membranes become receptive to union at particle- deficient interfaces, and that the physiologically created barren areas in freeze-fracture replicas may herald incipient membrane fusion. 相似文献
6.
Modifications of anionic-lipid domains preceding membrane fusion in guinea pig sperm 总被引:19,自引:15,他引:4 下载免费PDF全文
The relationship between anionic-lipid concentration and the functional properties of plasma-membrane domains was explored using the guinea-pig sperm membrane as a model, with polymyxin B (PXB) as a probe. Areas of plasmalemma specialized for fusion during the acrosome reaction had a higher affinity for the probe than adjacent nonfusigenic regions. In addition, capacitation--a process preceding acrosome:plasma-membrane fusion--markedly enlarged the area susceptible to PXB binding over the acrosomal cap. Protease treatment mimicked capacitation by increasing the acrosome-reaction incidence as well as PXB binding, at enzyme concentrations not affecting the surface coat nor altering filipin/sterol localization. Both proteolytic digestion and capacitation failed to augment PXB- or filipin-affinity in nonfusigenic zones, such as the post-acrosomal segment, including its particle-free maculae. Incubation of sperm in capacitating medium supplemented with 32P-labeled phosphate, followed by lipid extraction, thin-layer chromatography, and autoradiography, revealed a radioactive band comigrating with cardiolipin and phosphatidic acid. Vermiform protrusions elicited by PXB in the outer lamellae of cardiolipin- phosphatidylcholine liposomes resembled those seen in fusional regions of sperm membrane. We conclude that (a) differing concentrations of anionic lipids are found in adjacent domains of the sperm plasma membrane; (b) these domains mirror the functional regions of the membrane, with higher anionic-lipid concentrations localized over fusional zones; (c) the surface coat does not participate in the maintenance of such domains; (d) anionic-lipid synthesis may contribute to their formation; and (e) anionic-lipid concentrations increase as the membrane becomes fusionally competent, indicating that cellular modulation of lipid domains accompanies regulation of membrane function. 相似文献
7.
Cytochrome P-448 (mol wt 55,000 Daltons) from rabbit liver was purified to a specific content of 16.6 nmol/mg. Mice were immunised with this preparation, their spleens removed and dissociated lymphocytes hybridised with myeloma cells. Four monoclonal antibodies against cytochrome P-448 were raised and partially characterised. All four antibodies interacted with cytochrome P-448 in intact microsomal fractions and selectively immunoadsorbed cytochrome P-448 from solubilised microsomal preparations. One of the antibodies inhibited benzo[a] pyrene hydroxylase activity in a reconstituted system, one had no effect on activity and two increased activity. The possible applications of such antibodies are discussed. 相似文献
8.
Nanocrystalline SrS phosphors doped with Ce3+ ions at different concentrations (0.5, 1, 1.5 and 2 mol%) are synthesized via the solid‐state diffusion method (SSDM), which is suitable for the large‐scale production of phosphors in industrial applications. The as‐prepared samples are characterized using an X‐ray diffraction (XRD) technique, field emission scanning electron microscopy (FESEM), high‐resolution transmission electron microscopy (HRTEM) and energy‐dispersive X‐ray (EDX) analysis. The optical properties of these phosphors are analyzed using reflectance spectra, photoluminescence spectra and afterglow decay curves. The cubic structure of the SrS phosphor is confirmed by XRD analysis and the crystallite size calculated by Scherer's formula using XRD data shows the nanocrystalline nature of the phosphors. No phase change is observed with increasing concentrations of Ce3+ ions. The surface morphology of the prepared phosphors is determined by FESEM, which shows a sphere‐like structure and good connectivity of the grains. The authenticity of the formation of nanocrystalline phosphors is examined by HRTEM analysis. Elemental compositional information for the prepared phosphors is gathered by EDX analysis. Photoluminescence studies reveal that the emission spectra of the prepared phosphor shows broad band emission centered at 458 and 550 nm due to the transition of electrons from the 5d → 4f energy levels. The afterglow decay characteristics of different as‐synthesized SrS:Ce3+ nanophosphors are conceptually described. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
9.
S Patyar R Joshi DS Prasad Byrav A Prakash B Medhi BK Das 《Journal of biomedical science》2010,17(1):21
Resistance to conventional anticancer therapies in patients with advanced solid tumors has prompted the need of alternative
cancer therapies. Moreover, the success of novel cancer therapies depends on their selectivity for cancer cells with limited
toxicity to normal tissues. Several decades after Coley's work a variety of natural and genetically modified non-pathogenic
bacterial species are being explored as potential antitumor agents, either to provide direct tumoricidal effects or to deliver
tumoricidal molecules. Live, attenuated or genetically modified non-pathogenic bacterial species are capable of multiplying
selectively in tumors and inhibiting their growth. Due to their selectivity for tumor tissues, these bacteria and their spores
also serve as ideal vectors for delivering therapeutic proteins to tumors. Bacterial toxins too have emerged as promising
cancer treatment strategy. The most potential and promising strategy is bacteria based gene-directed enzyme prodrug therapy.
Although it has shown successful results in vivo yet further investigation about the targeting mechanisms of the bacteria are required to make it a complete therapeutic approach
in cancer treatment. 相似文献
10.
Jim M Dunwell Mike J Wilkinson Stephen Nelson Sri Wening Andrew C Sitorus Devi Mienanti Yuzer Alfiko Adam E Croxford Caroline S Ford Brian P Forster Peter DS Caligari 《BMC plant biology》2010,10(1):1-25