The exercise and environmental physiology of extravehicular activity

Extravehicular activity (EVA), i.e., exercise performed under unique environmental conditions, is indispensable for supporting daily living in weightlessness and for further space exploration. From 1965-1996 an average of 20 h x yr(-1) were spent performing EVA. International Space Station (ISS) assembly will require 135 h x yr(-1) of EVA, and 138 h x yr(-1) is planned for post-construction maintenance. The extravehicular mobility unit (EMU), used to protect astronauts during EVA, has a decreased pressure of 4.3 psi that could increase astronauts' risk of decompression sickness (DCS). Exercise in and repeated exposure to this hypobaria may increase the incidence of DCS, although weightlessness may attenuate this risk. Exercise thermoregulation within the EMU is poorly understood; the liquid cooling garment (LCG), worn next to the skin and designed to handle thermal stress, is manually controlled. Astronauts may become dehydrated (by up to 2.6% of body weight) during a 5-h EVA, further exacerbating the thermoregulatory challenge. The EVA is performed mainly with upper body muscles; but astronauts usually exercise at only 26-32% of their upper body maximal oxygen uptake (VO2max). For a given ground-based work task in air (as opposed to water), the submaximal VO2 is greater while VO2max and metabolic efficiency are lower during ground-based arm exercise as compared with leg exercise, and cardiovascular responses to exercise and training are also different for arms and legs. Preflight testing and training, whether conducted in air or water, must account for these differences if ground-based data are extrapolated for flight requirements. Astronauts experience deconditioning during microgravity resulting in a 10-20% loss in arm strength, a 20-30% loss in thigh strength, and decreased lower-body aerobic exercise capacity. Data from ground-based simulations of weightlessness such as bed rest induce a 6-8% decrease in upper-body strength, a 10-16% loss in thigh extensor strength, and a 15-20% decrease in lower-body aerobic exercise capacity. Changes in EVA support systems and training based on a greater understanding of the physiological aspects of exercise in the EVA environment will help to insure the health, safety, and efficiency of working astronauts.     

Muscle physiology and pathophysiology: magnetic resonance imaging evaluation

Just as the overlying skin hides skeletal muscle from direct assessment in clinical evaluation of muscle disease, so does it hamper studies that probe basic mechanisms underlying muscle use in exercise science. As a test of organ function, magnetic resonance imaging (MRI) of muscle activation expanded the breadth and type of information available to scientists by allowing the noninvasive dissection of muscle activity, merging functional and morphologic information that link directly to classical tests of muscle performance. Extending to sequellae of overexertion, from the self-limited condition of sore muscles that all of us experience to the more burdensome problems of acute muscle injuries and complications, MRI continues to develop as an important tool to unveil hidden mysteries that underlie and limit locomotion. This article reviews a substantial body of data accumulated over the last 10 years in these interesting, albeit slightly unconventional, applications.     

New and old in pediatric exercise physiology

Researchers in pediatric exercise physiology sometimes overlook previously published findings that are similar, or even identical, to their own. The intent of this review is to provide examples for the above pattern. These examples relate to the following topics: 1) is the O2 uptake plateau a necessary criterion for the establishment of maximal O2 uptake in children? 2) children's greater utilization of fat during prolonged exercise, 3) children's lower maximal blood lactate levels, 4) the higher metabolic energy cost of locomotion in children and the possible causes for this phenomenon, and 5) the lower cardiac output and stroke volume in children, at any given O2 uptake.     

Muscle remodeling and the exercise physiology of fish

Fish muscle responds to aerobic exercise training and cold acclimation with a more aerobic muscle phenotype than mammalian muscle but through both conserved and distinct molecular events. Differences from mammals in exercise metabolism and diversity in protein isoforms suggest that the regulation of muscle fuel use is more complex in fish. This review considers fish as powerful models for exercise and muscle physiology.     

Respiratory control in residents at high altitude: physiology and pathophysiology

Highland population (HA) from the Andes, living above 3000 m, have a blunted ventilatory response to increasing hypoxia, breathe less compared to acclimatized newcomers, but more, compared to sea-level natives at sea level. Subjects with chronic mountain sickness (CMS) breathe like sea-level natives and have excessive erythrocytosis (EE). The respiratory stimulation that arises through the peripheral chemoreflex is modestly less in the CMS group when compared with the HA group at the same P(ET(O2)). With regard to CO(2) sensitivity, CMS subjects seem to have reset their central CO(2) chemoreceptors to operate around the sea-level resting P(ET(CO2)). Acetazolamide, an acidifying drug that increases the chemosensitivity of regions in the brain stem that contain CO(2)/H(+) sensitive neurons, partially reverses this phenomenon, thus, providing CMS subjects with the possibility to have high CO(2) changes, despite small changes in ventilation. However, the same type of adjustments of the breathing pattern established for Andeans has not been found necessarily in Asian humans and/or domestic animals nor in the various high altitude species studied. The differing time frames of exposure to hypoxia among the populations, as well as the reversibility of the different components of the respiratory process at sea level, provide key concepts concerning the importance of time at high altitude in the evolution of an appropriate breathing pattern.     

Influence of Scandinavian scientists in exercise physiology

The cradle of Scandinavian exercise physiology was in Copenhagen. Peter Ludwig Panum (1820-1885) was head of a new physiological laboratory and his research included respiration, digestion and metabolism. Christian Bohr (1855-1911) was his most prominent co-worker and became his successor in 1885. Part of Bohr's training was in Leipzig, Germany, with Carl Ludwig, which led to his lifelong interest in problems of pulmonary exchange and blood transport of oxygen and carbon dioxide. He discovered the effect of carbon dioxide on the dissociation curve of hemoglobin (the Bohr effect). He attracted eminent young co-workers, among them A. Krogh, V. Henriques and K. A. Hasselbalch (known for the Henderson-Hasselbalch formula). One of his children, Niels Bohr, became one of the greatest modem nuclear physicists. We could name August Krogh as the father of exercise physiology in Scandinavia. This review will concentrate on some of his and his co-workers' and students' scientific achievements up to the 1940s.     

Respiratory physiology: adaptations to high-level exercise

Most exercise scientists would agree that the physiological determinants of peak endurance performance include the capacity to transport oxygen to the working muscle, diffusion from the muscle to the mitochondria, energy production and force generation, all influenced by signals from the central nervous system. In general, the capacity of the pulmonary system far exceeds the demands required for ventilation and gas exchange during exercise. Endurance training induces large and significant adaptations within the cardiovascular, musculoskeletal and haematological systems. However, the structural and functional properties of the lung and airways do not change in response to repetitive physical activity and, in elite athletes, the pulmonary system may become a limiting factor to exercise at sea level and altitude. As a consequence to this respiratory paradox, highly trained athletes may develop intrathoracic and extrathoracic obstruction, expiratory flow limitation, respiratory muscle fatigue and exercise-induced hypoxaemia. All of these maladaptations may influence performance.     

Compression garments and exercise: garment considerations, physiology and performance

Compression garments (CGs) provide a means of applying mechanical pressure at the body surface, thereby compressing and perhaps stabilizing/supporting underlying tissue. The body segments compressed and applied pressures ostensibly reflect the purpose of the garment, which is to mitigate exercise-induced discomfort or aid aspects of current or subsequent exercise performance. Potential benefits may be mediated via physical, physiological or psychological effects, although underlying mechanisms are typically not well elucidated. Despite widespread acceptance of CGs by competitive and recreational athletes, convincing scientific evidence supporting ergogenic effects remains somewhat elusive. The literature is fragmented due to great heterogeneity among studies, with variability including the type, duration and intensity of exercise, the measures used as indicators of exercise or recovery performance/physiological function, training status of participants, when the garments were worn and for what duration, the type of garment/body area covered and the applied pressures. Little is known about the adequacy of current sizing systems, pressure variability within and among individuals, maintenance of applied pressures during one wear session or over the life of the garment and, perhaps most importantly, whether any of these actually influence potential compression-associated benefits. During exercise, relatively few ergogenic effects have been demonstrated when wearing CGs. While CGs appear to aid aspects of jump performance in some situations, only limited data are available to indicate positive effects on performance for other forms of exercise. There is some indication for physical and physiological effects, including attenuation of muscle oscillation, improved joint awareness, perfusion augmentation and altered oxygen usage at sub-maximal intensities, but such findings are relatively isolated. Sub-maximal (at matched work loads) and maximal heart rate appears unaffected by CGs. Positive influences on perceptual responses during exercise are limited. During recovery, CGs have had mixed effects on recovery kinetics or subsequent performance. Various power and torque measurements have, on occasions, benefitted from the use of CGs in recovery, but subsequent sprint and agility performance appears no better. Results are inconsistent for post-exercise swelling of limb segments and for clearance of myocellular proteins and metabolites, while effects on plasma concentrations are difficult to interpret. However, there is some evidence for local blood flow augmentation with compression. Ratings of post-exercise muscle soreness are commonly more favourable when CGs are worn, although this is not always so. In general, the effects of CGs on indicators of recovery performance remain inconclusive. More work is needed to form a consensus or mechanistically-insightful interpretation of any demonstrated effects of CGs during exercise, recovery or - perhaps most importantly - fitness development. Limited practical recommendations for athletes can be drawn from the literature at present, although this review may help focus future research towards a position where such recommendations can be made.     

高血压患者左心室结构与运动耐量的相关性研究

目的 探讨高血压患者左心室结构与运动耐量的关系.方法 2008年2月-2010年2月连续收集在浙江医院心血管病门诊就诊的339例原发性高血压患者,其中男226例,女113例.所有患者均行超声心动图检查评价左室结构参数并测定6min步行距离和运动峰耗氧量,根据左室重量指数(LVMI)将患者分为高血压伴左室肥厚(LVH)组...     

Running on land and in water: comparative exercise physiology.

The effect of water immersion on cardiorespiratory and blood lactate responses during running was investigated. Wearing a buoyant vest, 10 trained runners (mean age 26 yr) ran in water at four different and specified submaximal loads (target heart rates 115, 130, 145, and 155-160 beats.min-1) and at maximal exercise intensity. Oxygen uptakes (VO2), heart rates, perceived exertion, and blood lactate concentrations were measured. Values were compared with levels obtained during treadmill running. For a given VO2, heart rate was 8-11 beats.min-1 lower during water running than during treadmill running, irrespective of exercise intensity. Both the maximal oxygen uptake (4.03 vs 4.60 1 x min-1) and heart rate (172 vs 188 beats.min-1) were lower during water running. Perceived exertion (legs and breathing) and the respiratory exchange ratio (RER) were higher during submaximal water running than during treadmill running, while ventilation (1 x min-1) was similar. The blood lactate concentrations were consistently higher in water than on the treadmill, both when related to VO2 and to %VO2max. Partly in conformity with earlier cycle ergometer studies, these data suggest that immersion induces acute cardiac adjustments that extend up to the maximal exercise level. Furthermore, both the external hydrostatic load and an altered running technique may add to an increased anaerobic metabolism during supported water running.     

Testosterone physiology in resistance exercise and training: the up-stream regulatory elements

Testosterone is one of the most potent naturally secreted androgenic-anabolic hormones, and its biological effects include promotion of muscle growth. In muscle, testosterone stimulates protein synthesis (anabolic effect) and inhibits protein degradation (anti-catabolic effect); combined, these effects account for the promotion of muscle hypertrophy by testosterone. These physiological signals from testosterone are modulated through the interaction of testosterone with the intracellular androgen receptor (AR). Testosterone is important for the desired adaptations to resistance exercise and training; in fact, testosterone is considered the major promoter of muscle growth and subsequent increase in muscle strength in response to resistance training in men. The acute endocrine response to a bout of heavy resistance exercise generally includes increased secretion of various catabolic (breakdown-related) and anabolic (growth-related) hormones including testosterone. The response of testosterone and AR to resistance exercise is largely determined by upper regulatory elements including the acute exercise programme variable domains, sex and age. In general, testosterone concentration is elevated directly following heavy resistance exercise in men. Findings on the testosterone response in women are equivocal with both increases and no changes observed in response to a bout of heavy resistance exercise. Age also significantly affects circulating testosterone concentrations. Until puberty, children do not experience an acute increase in testosterone from a bout of resistance exercise; after puberty some acute increases in testosterone from resistance exercise can be found in boys but not in girls. Aging beyond 35-40 years is associated with a 1-3% decline per year in circulating testosterone concentration in men; this decline eventually results in the condition known as andropause. Similarly, aging results in a reduced acute testosterone response to resistance exercise in men. In women, circulating testosterone concentration also gradually declines until menopause, after which a drastic reduction is found. In summary, testosterone is an important modulator of muscle mass in both men and women and acute increases in testosterone can be induced by resistance exercise. In general, the variables within the acute programme variable domains must be selected such that the resistance exercise session contains high volume and metabolic demand in order to induce an acute testosterone response.     

Normoxic and acute hypoxic exercise tolerance in man following acetazolamide

The influence of acetazolamide (ACZ) upon the ability to perform and sustain maximal and submaximal exercise bouts under normoxic and hypoxic conditions was examined in four groups of healthy male subjects (N = 27). ACZ (500 mg) or inert placebo (Pla) was administered prior to exercise in a quasi-randomized, double-blind, crossover fashion. ACZ was shown to lower venous pH (ACZ, 7.31 +/- 0.01, vs Pla, 7.35 +/- 0.08) and bicarbonate (ACZ, 22.4 +/- 0.27 mM, vs Pla, 25.4 +/- 0.6 mM) and to elevate urine pH (ACZ, 7.36 +/- 0.06, vs Pla, 5.84 +/- 0.19) and tended to elevate VE (P = 0.07) at rest. Peak VO2 measured using a continuous incremental protocol was unaltered in normoxia, while peak VCO2 and RER were lowered by ACZ. No significant effect of ACZ upon VO2, VCO2, RER, or heart rate (HR) was observed during submaximal exercise (75% of peak VO2) although VE was increased by 14% and time to exhaustion (EXHt) was reduced by 29%. During acute hypoxia at a simulated altitude of 4,270 m (Pbar = 446 mm Hg), no significant differences were noted in VE, VO2, VCO2, RER, HR, or arterial saturation (SaO2) at rest. Prior to exercise, venous pH (ACZ, 7.39 +/- 0.04, vs Pla, 7.44 +/- 0.007) and bicarbonate were lower with ACZ (ACZ, 21.6 +/- 0.46 mM, vs Pla, 24.2 +/- 0.25 mM), while urine pH was higher (ACZ, 7.6 +/- 0.07, vs Pla, 5.9 +/- 0.25). Other than a higher PCO2 and lower venous lactate with ACZ, no significant differences were identified at peak VO2.(ABSTRACT TRUNCATED AT 250 WORDS)     

Growth hormone and exercise tolerance in patients with cystic fibrosis

Cystic fibrosis (CF) is a life-limiting inherited disorder characterised by pulmonary disease, pancreatic dysfunction and symptoms of malnutrition that are all interrelated with low exercise capacity and poor survival rate. Therapy with growth hormone (GH) may improve the reduced dimensional and functional capacity associated with poor nutritional status and catabolism and therefore improve exercise tolerance, quality of life and survival rate in patients with CF. The literature about GH treatment and its effect on exercise tolerance are rather limited, not always consistent and methodological concerns restrict further analysis. GH treatment may have beneficial effects on both growth and exercise tolerance without serious complications in prepubertal children with CF. The observed dimensional changes of the muscular, cardiovascular and pulmonary system seem to improve aerobic exercise capacity and respiratory and peripheral muscle strength. The physiological background of the observed changes is not yet fully understood, therefore, larger-scale studies with an optimised design are required.     

穴位埋线对大鼠运动耐力及非特异性免疫的影响

目的探讨穴位埋线刺激足三里、脾俞、太冲穴位对抗运动性疲劳的作用及其机制。方法选取wistar成年雄性大鼠24只,适应性游泳后随机分为对照组、穴位埋线组、电针组,每组8只。除对照组(与其他组一样负载游泳,不做任何处置)外,电针组采用电针刺激足三里穴、脾俞穴和太冲穴,隔日1次,持续以频率15 Hz、电流2 mA的连续波电刺激10 min,共7次;穴位埋线组采用穴位埋线方法刺激足三里穴、脾俞穴和太冲穴,于实验的第2、7、13天实施。实验第15天记录末次力竭运动的游泳时间数值。并在行为学观察结束后,计算吞噬百分率与吞噬指数。结果 (1)负载游泳试验:实验第15天,穴位埋线组和电针组游泳时间较针刺前均有所增加,与对照组比较差异有统计学意义(P<0.05)。而穴位埋线组、电针组组间比较差异无统计学意义。(2)吞噬百分率和吞噬指数:实验第15天,穴位埋线组和电针组巨噬细胞吞噬百分率、吞噬指数明显高于对照组,且差异有统计学意义(P<0.05),而穴位埋线组与电针组之间比较差异无统计学意义。结论穴位埋线能够有效地提高大鼠的运动耐力及抗疲劳能力,其机制可能与提高大鼠的非特异性免疫反应有关。穴位埋线与电针治疗相比在治疗运动性疲劳上同样有效,对临床治疗具有一定的指导意义。