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In addition to changes in stroke volume, peripheral oxygen extraction can increase modestly with training. This is due to increased capillary density in the trained skeletal muscles that facilitates very high levels of oxygen extraction across exercising skeletal muscle vascular beds (5, 104, 143, 313, 314).
In parallel with these structural changes in the heart and the increase in skeletal muscle capillarity, there can be up to approximately twofold increases in skeletal muscle mitochondrial content with endurance training (216, 218). In the 1970s, this increase in mitochondrial content was thought to contribute to training-induced increases in V̇o2max. However, subsequent studies in rodents were able to dissociate changes in skeletal muscle mitochondrial content with training and V̇o2max (113, 217, 218). Parenthetically, skeletal muscle mitochondrial content and how it changes with training are major determinants of submaximal endurance performance. These changes also have important implications for substrate metabolism especially during prolonged exercise in both humans and other species (218). In contrast to the heart and skeletal muscle, in most cases the pulmonary system does not show major adaptive changes to endurance exercise training (184, 320, 406). However, lifetime exposure to high altitude can increase both lung volumes and diffusing capacity (87).
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While it is assumed that genetic factors may explain why some individuals have an impressive ability to increase their stroke volume in response to endurance exercise training, the evidence for a single or limited number of DNA variants explaining this phenomenon has not emerged. There is evidence that a suite of genetic markers can explain a significant portion of the variable increase in V̇o2max in response to fitness style training (469); however, it is not clear how these markers influence the stroke volume responses to training. Even less information is available concerning genetic factors that might influence physiological adaptations to the type of prolonged intense training performed by elite endurance athletes. Moreover, success in elite endurance athletics, like most human phenotypes, probably represents a combination of environmental exposures and behavioral factors (e.g., training) that operate in concert with a large number of gene variants and other epigenetic factors (287). There are data suggestive that genetic variability in angiotensin converting enzyme (ACE) might explain at least part of the very high stroke volumes and V̇o2max values seen in elite athletes. However, the evidence is not convincing for common ACE variants. Likewise, gene variants related to mitochondrial function do not explain the very high V̇o2max values seen in elite endurance athletes (372, 378).
The primary effects of endurance exercise training relate to increases in cardiac output driven by an augmented stroke volume due to left ventricular hypertrophy and increases in blood volume. Elite athletes have remarkably high stroke volumes and large blood volumes (94, 95, 414). Cardiac output values of 40 l/min have been seen in elite male endurance athletes (146).
Blood pressure responses to supine and head-down tilt exercise in an individual with autonomic failure as a result of surgical sympathectomy of the thoracolumbar sympathetic chain. The fall in blood pressure during supine exercise highlights the need for the sympathetic nervous system to restrain blood flow to contracting skeletal muscles for the purposes of regulating blood pressure. The fact that this fall in blood pressure also occurred when venous return was maximized by 15% head-down tilt emphasizes this point. The x-axis in the figures represents time with each vertical line representing 10 s. For details, see Ref. 301.
Summary figure of the relationships between the local factors causing blood flow to rise in contracting skeletal muscles, cardiac output, and the need to regulate arterial blood pressure to ensure the perfusion of the central nervous system (CNS) and other vital organs. As emphasized throughout this review, factors released by the contracting muscles act locally to evoke vasodilation and blunt sympathetic vasoconstriction (functional sympatholysis). These events require an increase in cardiac output that is also facilitated by the systemic actions of the muscle pump to augment venous return. At the same time, it operates to increase perfusion pressure and amplify the effects of the vasodilating substances in the skeletal muscles. The cardiac hypertrophy and increases in blood volume caused by training also permit higher levels of muscle blood flow in the trained state. All of these acute and chronic adaptions are balanced by the autonomic nervous system in a way the permits arterial blood pressure to be maintained.
Since the Shinkansen started operation in 1964, the catenary has been developed for more than 50 years, and its suspension type has also been continuously improved. The general trend of catenary development and evolution all over the world is the same, that is, from a complex and high-cost catenary structure to a simple and low-cost catenary structure. The earliest catenary was a composite chain-shaped suspension catenary. Due to its complex structure, high cost and difficult maintenance, it was gradually replaced by the elastic chain-shaped suspension catenary. The catenary with elastic chain-shaped suspension has simple structure and high stability, which can meet the current collection requirements of trains running at high speed. In recent years, the structure of the catenary has been further simplified, and the simple chain-shaped suspension catenary with low cost and simple maintenance is widely used all over the world.
Generally, the frame types include double-arm and single-arm frames. The double-arm frames include the four-wrist diamond double-arm frame and the two-wrist diamond double-arm frame. The four-wrist diamond double-arm frame is composed of four arms, and each arm includes two parts: the upper arm and the lower arm, which are symmetrically arranged in front and back, left and right. This type of frame has the advantages of high strength and good stability, but it has the disadvantages of high cost, large weight, complex structure and difficult adjustment. Later, the four-wrist diamond frame was improved, and the lower frame was changed from the original four arms to two arms, while the upper frame remained unchanged, so it was called two-wrist diamond double-arm frame. Compared with the four-wrist diamond double-arm frame, the two-wrist diamond double-arm frame is simpler in structure, lighter in weight, and less difficult to adjust. The double-arm frame is gradually phased out because of its complex structure, heavy weight and high maintenance costs, and displaced with the single-arm frame. The structure of the single-arm frame is only half of that of the two-wrist diamond double-arm frame, which has the advantages of simple structure, small overall size, light weight, easy adjustment and good dynamic characteristics. Therefore, the single-arm pantograph is widely used in modern electrified trains [31, 32].
With the development of electrified trains, the pantographs have been constantly refined. Although the development histories of electrified railways in various countries are different, the development of pantographs worldwide can be summarized as an evolution process from double-arm pantographs with complex structure and high weight to single-arm pantographs with simple structure, low weight, flexibility and reliability.
Initially, four-wrist diamond-shaped double-arm pantographs were used in the early stage of high-speed railway operation, but they were eliminated due to their complex structure and the large aerodynamic noise generated when the trains were running at high speed. Japan then successfully developed a T-type pantograph, which has a simple structure but a high cost, so they are no longer used at present. Subsequently, Japan developed PS207 and PS208 single-arm pantographs, and in 2011, the maximum running speed of E5 series high-speed trains with PS208 single-arm pantographs reached 320 km/h [35]. France, Germany and China established high-speed railways after Japan, and all of them use single-arm pantographs, which are small, lightweight and reliable and have excellent aerodynamic performance.
In 1991, the German high-speed railway began to operate with a maximum speed of 250 km/h. ICE is one of the most reliable, advanced, and comfortable high-speed railways in Europe. The German electrified railway adopts a traction power supply system of AC 15 kV, 16.67 Hz. ICE trains are characterized by high power, and their main technical parameters are shown in Table 2.
Contact wire is an important part of catenary, which transmits electrical energy to the electric locomotive through pantograph slide. Contact wire is generally built in zig-zag to reduce the wear on pantograph slide. When the train is running at a high speed, the contact wire has to face the extreme working environment such as vibration shock, temperature difference, environmental corrosion, mechanical friction, and arc ablation [48]. Its material properties directly affect the current collection quality and operation safety of the train. There are many types of electrified railway contact wires, which are mainly classified into pure copper contact wires and copper alloy contact wires according to the material [49].
Subsequently, as the train speed increased to 100 km/h, the pure carbon slide plate could no longer be used because of its low mechanical strength, poor impact toughness, and low service life at higher speeds. In order to solve this problem, a P/M slide plate with good impact toughness was developed, which was based on the metal (iron and copper) powder. This approach greatly reduced the wear of contact wire, and the slide plate had a better performance in wear resistance as well. It was therefore used as an ideal solution for trains on 100 km/h trunk lines.
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