New physiological insights into exercise-induced diaphragmatic fatigue

Data on the dynamic process and time-point of manifestation of exercise-induced diaphragmatic fatigue (DF) are lacking. Therefore, this study was aimed assessing dynamic changes of diaphragmatic strength during exercise and determining the time-point of DF manifestation. Fourteen trained subjects (maximal oxygen uptake (VO2(max)) 59.3+/-5.5 ml/min/kg) performed standardized exercise protocols (maximal workload: 85% VO2(max)) followed by recovery (6 min). Ergospirometric data and twitch transdiaphragmatic pressure (TwPdi) were consecutively assessed. DF was induced (TwPdi-rest: 2.34+/-0.26 versus TwPdi-end-recovery 2.01+/-0.21 kPa, p<0.01). TwPdi progressively increased during exercise (TwPdi-rest: 2.34+/-0.26 versus TwPdi-maximal-workload: 3.28+/-0.38 kPa, p<0.001). DF was detectable immediately after exercise-termination (TwPdi-maximal-workload: 3.28+/-0.38 versus TwPdi-early-recovery 2.55+/-0.34 kPa, p<0.001). TwPdi during exercise was highly correlated to workload, VO2(max) and dyspnea (r=0.96/r=0.92/r=0.97; all p<0.0001). In conclusion, diaphragmatic strength progressively increases with increasing workload, and DF manifests after - rather than during - exercise. In addition, TwPdi is highly correlated to key-measures of ergospirometry, approving the physiological thesis that muscle strength is progressively enhanced and escapes fatiguing failure during high-intensity exercise performance.     

Post-exercise diaphragm shielding: a novel approach to exercise-induced diaphragmatic fatigue

Based on the "post-exercise diaphragm shielding" hypothesis this study tested whether both diaphragmatic force-generation (DFG) and diaphragmatic fatigue (DF) remain unchanged during consecutive exercise-trials. Twelve subjects ( [Formula: see text] 58.4+/-6.6mlkg(-1)min(-1)) performed three consecutive exercise-trials (T(alpha)/T(beta)/T(gamma); workload(max) 85% [Formula: see text] ) each followed by recovery (6min). Twitch transdiaphragmatic pressure during supramaximal magnetic phrenic nerve stimulation (TwPdi, every 30s), ratings of perceived exertion (RPE, every 90s) and ergospirometric data (continuously) were assessed throughout the entire protocol (46.5min). DFG and DF did not differ among all trials (TwPdi-baseline: 2.2+/-0.7kPa; TwPdi-peak: T(alpha)/T(beta)/T(gamma) 3.1+/-0.7kPa vs 3.0+/-0.8kPa vs 3.2+/-0.8kPa; TwPdi-bottom: T(alpha)/T(beta)/T(gamma) 1.9+/-0.6kPa vs 2.0+/-0.7kPa vs 1.8+/-0.5kPa, both p>0.4, RM-ANOVA). Furthermore, TwPdi revealed close relationships with RPE (r=0.91, p<0.0001) and oxygen uptake (r=0.94, p<0.0001) during exercise. In conclusion, both DFG (baseline-to-peak) and DF (baseline-to-bottom) achieve similar magnitudes during and after consecutive exercise-trials and are closely linked to RPE and oxygen uptake. This suggests that DF neither reflects impaired diaphragmatic function nor impairs exercise performance; rather it is likely to reflect post-exercise diaphragm shielding.     

低氧与膈肌疲劳

膈肌疲劳是导致呼吸衰竭发生的重要的病理生理机制之一,低氧对膈肌疲劳的发生发展起着重要的作用。对低氧造成膈肌疲劳的机制和机体对低氧后的适应性改变进行简要的介绍。     

对中枢与外周运动性疲劳的再认识

背景：在运动性疲劳的研究工作中,不少学者多采用力竭的动物模型,混淆了筋疲力尽与运动性疲劳。 目的：正确的认识运动性疲劳,了解其发生机制,掌握合理有效的防治措施从而消除疲劳对增进大众健康、提高运动成绩、预防运动伤病的发生,对运动性疲劳的概念、研究现状以及存在问题进行综合分析。 方法：以“运动性疲劳、外周疲劳;中枢疲劳、exercise-induced fatigue”为检索词,计算机检索Medline、EMbase、中国生物医学文献数据库、中国期刊全文数据库1966至-2014年与运动性疲劳或与运动疲劳密切相关的文献,对其中49篇进行归纳总结。 结果与结论：运动性疲劳研究的标准和条件控制存在需要再认识的问题,关于中枢疲劳和外周疲劳发生部位、原因与物质变化依旧存在很大的争议。外周疲劳发生在神经肌肉接头、横管系统、肌浆网、线粒体及肌细胞膜等部位,并引起相应的物质变化。运动性中枢疲劳时中枢神经系统产生的神经递质、调质、神经激素会发生相应的变化,这些改变与运动性中枢疲劳的产生有密切关系。 中国组织工程研究杂志出版内容重点：组织构建;骨细胞;软骨细胞;细胞培养;成纤维细胞;血管内皮细胞;骨质疏松;组织工程     

急性缺氧对膈肌疲劳的影响

本文观察急性缺氧对犬膈肌疲劳的影响，发现急性缺氧条件下，膈肌疲劳的耐受时间为１８．００±３．７０ｍｉｎ，较对照组４１．３３±５．３０ｍｉｎ明显缩短（Ｐ＜０．０５），而且疲劳膈肌恢复过程中产生跨膈压（ｔｒａｎｓｄｉａｐｈｒａｇｍａｔｉｃｐｒｅｓｓｕｒｅ，Ｐｄｉ）明显低于对照组（Ｐ＜０．０５），证实急性缺氧使膈肌疲劳的耐受性明显下降，且不利于疲劳膈肌肌力的恢复。     

Physical activity,smoking, and exercise-induced fatigue

This study determined whether persons with coronary risk factors have increased fatigue during or after exercise. Ratings of perceived exertion were first shown to be a valid measure of fatigue; i.e., ratings of perceived exertion correlated with heart rate both during and after exercise and at each of three exercise tests (all within-subjects r>0.88). Physical inactivity and smoking were associated with increased fatigue. Inactive men and smokers had higher levels of fatigue during both exercise and recovery conditions and at each of three exercise tests. The increased fatigue of men who were inactive and smoked was not entirely due to their lower level of fitness. The risk factors of age, Type A behavior pattern, blood pressure, serum cholesterol, serum high-density lipoprotein, and obesity were not associated with increased fatigue. The increased fatigue experienced by inactive persons and smokers may account for their decreased compliance to exercise programs.This research was supported by NHLBI Training Grant 5T32 HL07328-04 (Dr. Hughes), NHLBI Research Career Development Award 5K04 HL00287 (Dr. Jacobs), and University of Minnesota Grant BRSG 2-507-RR-05448-20 (Dr. Jacobs).     

Influence of environmental temperature on exercise-induced inspiratory muscle fatigue

Exercise in the heat has detrimental effects on circulation that might negatively influence endurance performance. If blood is diverted away from the inspiratory muscles to the skin during exercise in the heat, exercise-induced inspiratory muscle fatigue might be exacerbated. Thus, we hypothesised that prolonged heavy endurance exercise in the heat would impair exercise performance and exacerbate inspiratory muscle fatigue compared to exercise in a thermo-neutral environment. Using a crossover design, seven male endurance trained subjects [mean (SEM) maximum oxygen uptake = 62.2 (1.5) ml·kg^–1·min^–1] were assigned at random to either a group that exercised in the heat at an ambient temperature of 35°C (H) or a group that exercised in the cool at 15°C (C). Maximum inspiratory mouth pressure at zero flow (P ₀), pressure normalised maximum relaxation rate (MRR/P ₀), time constant for the pressure decay (), and maximum inspiratory flow at 30% P ₀ ( ₃₀) were assessed immediately before and reassessed within 2, 30, and 60 min of completing a pre-loaded time trial [40 min at 65% peak power, plus ~30 min time trial] on a cycle ergometer . Group H completed the time trial 432 (135) s slower than group C [2,285 (180) vs 1,852 (122) s, respectively; =24 (8)%, P=0.0094]. Repeat measurements within 2 min post-exercise revealed significant declines in P ₀, MRR/P ₀, , and ₃₀ from baseline values, but no between-group differences were observed. In conclusion, heavy sustained exercise in the heat impaired subsequent time-trial performance but did not exacerbate inspiratory muscle fatigue in endurance-trained subjects.     

Acute hypoxic ventilatory response and exercise-induced arterial hypoxemia in men and women

Recent studies claim a higher prevalence of exercise-induced arterial hypoxemia (EIAH) in women relative to men and that diminished peripheral chemosensitivity is related to the degree of arterial desaturation during exercise in male endurance athletes. The purpose of this study was to determine the relationship between the acute ventilatory response to hypoxia (AHVR) and EIAH and the potential influence of gender in trained endurance cyclists and untrained individuals. Healthy untrained males (n = 9) and females (n = 9) and trained male (n = 11) and female (n = 10) cyclists performed an isocapnic AHVR test followed by an incremental cycle test to exhaustion. Oxyhemoglobin saturation (Sa(O(2)) was lower in trained men (91.4 +/- 0.9%) and women (91.3 +/- 0.9%) compared to their untrained counterparts (94.4 +/- 0.8% versus 94.3 +/- 0.7%) (P < 0.05). AHVR and maximal O(2) consumption were related for all subjects (r = -0.46), men (r = -0.45) and women (r = -0.53) (P < 0.05) but AHVR was unrelated to Sa(O(2)) for any groups (P > 0.05). We conclude that resting AHVR does not have a significant role in maintaining Sa(O(2)) during sea-level maximal cycle exercise in men or women.     

Characteristics of diaphragmatic fatigue during exhaustive exercise until task failure

The diaphragm was postulated to fatigue relatively early during exhaustive whole body exercise without further loss in contractility as exercise proceeds towards task failure. Diaphragmatic contractility was investigated prior/during/after exhaustive whole body exercise until task failure by using lung volume corrected twitch transdiaphragmatic pressure (TwPdi(c)) during magnetic phrenic nerve stimulation (every 45s). Eleven cyclists exercised to exhaustion (workloads ≥85% maximal oxygen uptake; 20.7±9.8min). Individual post hoc calculation of TwPdi(c) was conducted (diaphragmatic contractility versus lung volume). Diaphragmatic fatigue (i.e. TwPdi reduction baseline/recovery ≥10%) occurred in 9/11 subjects (82% "fatiguers"; baseline/recovery TwPdi(c) -16±13%, p<0.01). Fatiguers TwPdi(c) was: baseline: 2.99±0.40kPa, exercise-onset: 2.98±0.41kPa, initial third: 2.80±0.67kPa, second third: 2.54±0.55kPa, final third-task failure: 2.51±0.44kPa, recovery: 2.50±0.52kPa. Diaphragmatic contractility and lung volume (rest) were strongly related (r(2)=0.98, mean TwPdi(c) gradient 0.78kPa/l). To conclude, diaphragmatic contractility (lung volume corrected) decreases relatively early (initial two thirds) during exhaustive exercise and remains preserved towards task failure. This confirms previous assumptions postulating that respiratory performance is sustained without further fatigue of the primary inspiratory muscle.     

Effect of exercise-induced fatigue on position sense of the knee in the elderly

Previously, data on the effects of muscle fatigue on joint position sense (JPS) have been provided. However, to our knowledge, no studies have been conducted so far to assess the effects of local muscle fatigue on elderly proprioception. Thus, the purpose of the present study was to determine the effects of local muscle fatigue on knee JPS in old-age-subjects. Sixteen male volunteers (mean age ± SD: 69.81 ± 3.92 years) participated in this study. Each subject completed all of the data collection in one morning session; JPS measures were obtained prior to and immediately after the fatigue protocol. JPS was evaluated by the technique of open-kinetic chain and active knee positioning, and was reported using absolute, relative and variable angular errors. The fatigue protocol applied to the lower extremity consisted of 30 maximum concentric repetitions of the knee extensors and flexors muscles on an isokinetic dynamometer at an angular velocity of 120 s⁻¹ (2.09 rad s⁻¹). The results showed that peak torque of knee extensor and flexor muscles was significantly decreased from rest to post exercise-induced fatigue. After local load to the knee muscles, a significant increase of absolute angular error was observed (2.56°). The relative error showed the directional bias in the extension movement. However, the reliability and accuracy of estimating knee angles as showed by the variable error is similar at both times. It can be concluded that exercise-induced local muscle fatigue alters knee JPS in old age adults.     

Effect of exercise-induced muscle damage on ventilatory and perceived exertion responses to moderate and severe intensity cycle exercise

This study examined the effect of exercise-induced muscle damage (EIMD) on ventilatory and perceived exertion responses to cycle exercise. Ten healthy, physically active men cycled for 6 min at moderate intensity and to exhaustion at severe intensity before and 48 h after eccentric exercise (100 squats with a load corresponding to 70% of body mass). Changes in ventilation and ratings of perceived exertion (RPE) were calculated for each individual and expressed against time (moderate and severe exercise) and as a percentage of time to exhaustion (severe exercise). Ventilation increased during moderate exercise at 48 h ( [(V)\dot]_\textE \dot{V}_{\text{E}} ; 34.5 ± 5.0 to 36.3 ± 3.8 l min⁻¹, P < 0.05) but increases in RPE were not significant. During severe exercise at 48 h, time to exhaustion (TTE) was reduced and [(V)\dot]_\textE \dot{V}_{\text{E}} (87.1 ± 14.1 to 93.8 ± 11.7 l min⁻¹) and RPE (15.5 ± 1.3 to 16.1 ± 1.4) were elevated (P < 0.05). When expressed as a percentage of TTE, the differences in ventilation and RPE values disappeared. Findings indicate that the augmented ventilatory response to cycle exercise following EIMD may be an important cue in informing effort perception during high-intensity exercise but not during moderate-intensity exercise.     

Central and peripheral components of diaphragmatic fatigue during inspiratory resistive load in cats

The development of fatigue was investigated in the diaphragm of anaesthetized, tracheostomized, spontaneously breathing cats during restricted air flow. Ventilation, transdiaphragmatic pressure (P_di), integrated electrical activity of diaphragm (E_di) and phrenic nerve (E_ph) were measured simultaneously and expressed as a percentage of values at unloaded breathing. Inspiratory loads were 60, 70 and 80% of P_{di max}. The P_{di max} was measured by airway occlusion at functional residual capacity. The duration of loads was 40–60 min. The diaphragmatic fatigue developed only during heavy inspiratory loading (80% P_{di max}). During the first 10 min of heavy load P_di, E_di and E_ph increased to 905 ± 60%, 248 ± 20% and 229 ± 24%, respectively (P < 0.01), and then began to fall gradually. Ventilation declined to 39 ± 3% after 60 min of heavy load (P < 0.01), resulting in acute hypercapnia and hypoxia. Initial fatigue appeared as a decrease in P_di (to 781 ± 63%) and parallel decline in E_di (to 233 ± 21%) after 30 min of load (P < 0.05). Phrenic nerve activity did not change during this stage. These data suggest a peripheral basis of diaphragmatic fatigue, related to disorders in neuromuscular transmission. After 60 min of heavy load, P_di fell to 675 ± 49%, E_di declined to 209 ± 28% and E_ph decreased to 189 ± 25%. We interpret the decrease in phrenic nerve activity as a weakening of central inspiratory drive and development of the central component of diaphragmatic fatigue in the last stage.     

Safe cooling limits from exercise-induced hyperthermia

We evaluated the cooling rate of hyperthermic subjects, as measured by three estimates of deep core temperatures (esophageal, rectal and aural canal temperatures), during immersion in a range of water temperatures. The objective of the study was to compare the three indices of core temperature and define safe cooling limits when using rectal temperature to avoid the development of hypothermia. On 4 separate days, seven subjects (four males, three females) exercised for 45.4±4.1 min at 65% at an ambient temperature of 39°C, RH: 36.5%, until rectal temperature (T _re) increased to 40.0°C (39.5°C for two subjects). Following exercise, the subjects were immersed in a circulated water bath controlled at 2, 8, 14 and 20°C until T _re returned to 37.5°C. When T _re reached normothermia during the cooling period (37.5±0.05°C), both esophageal (T _es) (35.6±1.3°C) and aural canal (T _ac) (35.9±0.9°C) temperatures were approaching or reaching hypothermia, particularly during immersion in 2°C water (T _es=34.5±1.2°C). On the basis of the heat loss data, the heat gained during the exercise was fully eliminated after 5.4±1.5, 7.9±2.9, 10.4±3.8 and 13.1±2.8 min of immersion in 2, 8, 14 and 20°C water, respectively, with the coldest water showing a significantly faster cooling rate. During the immersion in 2°C water, a decrease of only 1.5°C in T _re resulted in the elimination of 100% of the heat gained during exercise without causing hypothermia. This study would therefore support cooling the core temperature of hyperthermic subjects to a rectal temperature between 37.8°C (during immersion in water >10°C) and 38.6°C (during immersion in water <10°C) to eliminate the heat gained during exercise without causing hypothermia.     

针刺运动性疲劳模型大鼠不同穴位后心肌和肝脏组织的相关变化

背景：中医针灸研究发现刺激某些穴位对运动后机体抗氧化能力确有一定的影响,而穴位不同配伍对机体的影响有所不同。 目的：对大鼠力竭游泳后不同组合的相关穴位进行针刺,探索不同的穴位配伍刺激方案对提高心肌及肝脏组织抗氧化能力的效果,筛选消除运动性疲劳的最有效的穴位组合。 方法：选取雄性健康成年大鼠70只随机分为对照组(n=10)、模型组(n=15)、三阴交+后三里组(n=15)、肾俞+后三里组(n=15)和大椎+后三里组(n=15)。后4组大鼠通过为期10 d的游泳训练(在4 d的适应性游泳训练后)建立力竭模型。三阴交+后三里组、肾俞+后三里组和大椎+后三里组大鼠,每次训练结束休息15 min后,分别对其三阴交+后三里、肾俞+后三里和大椎+后三里进行穴位针刺。 结果与结论：与对照组相比,模型组大鼠心肌和肝组织中超氧化物歧化酶活性、谷胱甘肽过氧化物酶活性显著降低,心肌丙二醛含量明显增加;与模型组相比,大椎+后三里组大鼠心肌组织超氧化物歧化酶活性、谷胱甘肽过氧化物酶活性显著升高,肾俞+后三里组大鼠肝组织中超氧化物歧化酶活性显著升高,三阴交+后三里组和大椎+后三里组大鼠心肌丙二醛含量明显降低,三阴交+后三里组大鼠肝组织丙二醛含量明显降低。提示不同穴位组合针刺对大鼠心肌、肝脏组织超氧化物歧化酶、谷胱甘肽过氧化物酶和丙二醛有不同的影响,穴位针刺对提高大鼠心肌抗氧化能力具有一定的效果,其中针刺大椎加后三里效果最明显,而对于肝脏组织抗氧化能力,针刺肾俞加后三里和三阴交加后三里有一定的特异性效果。 中国组织工程研究杂志出版内容重点：肾移植;肝移植;移植;心脏移植;组织移植;皮肤移植;皮瓣移植;血管移植;器官移植;组织工程     

过度疲劳状态下运动性猝死模型大鼠甲状腺及心脏功能变化

BACKGROUND: Exercise fatigue can lead to lactic acid accumulation, metabolism decrease and energy intake disturbance following by overtraining. Further, constant development of negative superposition after overtraining will contribute to karoshi and exercise-induced sudden death. OBJECTIVE:To investigate the changes of rat thyroid function under excessive fatigue state and to further explore the relationship between the exercise-induced sudden cardiac death and thyroid function. METHODS: Seven rats were randomly selected from 30 male Sprague-Dawley rats as blank control group. The remaining rats were trained continuously with exhaustive loaded-swimming exercises to achieve the excessive fatigue state, and seven as fatigue group, seven as sudden death group. Rat samples were collected immediately after exercise to observe the morphology, structure and intercellular substance changes of thyroid follicular epithelial cells and cardiac muscle fibers by hematoxylin-eosin staining;   levels of serum triiodothyronine (T3), tetraiodothyronine (T4), thyroid stimulating hormone (TSH) and lactic dehydrogenase 2 (LDH-2) were detected by ELISA. RESULTS AND CONCLUSION:(1) In the sudden death group, intermyocardiac vessels expanded extremely, some thin myocardial fibers ruptured, and severe congestion and hemorrhage occurred; thyroid follicles were in a hypochromatism with loose intercellular substance, and extensive visible vacuolization in the follicular colloid. (2) The levels of serum LDH-2 and T3 in the sudden death group were significantly higher than those in the blank control group (P < 0.05). (3) The levels of serum T4 and TSH were the highest in the fatigue group, followed by sudden death group, and lowest in the blank control group (P < 0.05). (4) These results suggest that excessive fatigue caused by long-term high-intensity training will lead to thyroid morphological abnormality and myocardial impairment. Increasing serum LDH-2 following excessive fatigue aggravates myocardial damages. Fatigue accumulation causing thyroid dysfunction, abnormal secretion of TSH, T3 and T4, may be one of the causes of exercise-induced sudden death. Exercise-induced sudden death belongs to a multiple organ dysfunction syndrome, and turbulence of thyroid causing cardiac structural and functional alteration may induce sudden cardiac death. 中国组织工程研究杂志出版内容重点：肾移植;肝移植;移植;心脏移植;组织移植;皮肤移植;皮瓣移植;血管移植;器官移植;组织工程     

Effects of exercise-induced fatigue on postural balance: a comparison of treadmill versus cycle fatiguing protocols

The authors of this study examined the effects of muscle fatigue on balance indices and recovery time in recreationally trained individuals after incremental tests on a treadmill and a cycle ergometer. Sixteen participants (male N = 11, female N = 5) (mean age = 21.2 ± 2 years) completed this study. Balance measures were performed on a Biodex Balance System via the Dynamic Balance Test. Balance was measured pre-exercise, immediately post-exercise, and at 3-, 6-, 9-, 12-, 15-, 18-, and 21-min post-exercise. Immediately following the fatiguing treadmill test, balance increased significantly in the overall stability index (SI) (from 4.38 ± 2.48 to 6.09 ± 1.80) and the anterior/posterior index (API) (from 3.49 ± 2.18 to 5.28 ± 1.81) (p < 0.01). Immediately following the fatiguing cycle test, balance was not altered significantly in SI or API. Balance was not altered significantly for the medial/lateral index for either exercise test at any time point. Additionally, there were no significant differences in time to recovery. At 12-min post-exercise, all indices were below pre-exercise values, indicating that fatiguing exercise has a positive effect on balance over time. These results are consistent with previous research, suggesting that any effects of fatigue on balance are seen immediately and are diminished as time after exercise increases.     

Independence of synaptic specificity from neuritic guidance

Neuronal circuits are interconnected with a high degree of specificity. While axonal guidance has been demonstrated to be crucial for the choice of the correct target region, its role in specificity at the level of individual cells remains unclear. Specificity of synapse formation may either result from precise guidance of axonal outgrowth onto the target or depend on a molecular "match" between pre- and postsynapse. To distinguish between these possibilities, an in vitro system was used in which neuritic outgrowth of rat cortical neurons is accurately guided along the narrow pathways of a surface micropattern. The micropattern consisted of a blend of extracellular matrix molecules applied to a cell repellent background of polystyrene by microcontact printing. The system reproduces guidance by attractant and repellent surface cues while no other signals that may influence synapse formation, like gradients of trophic factors or accumulations of signaling molecules, are provided. While the number of contact points between neighboring cells was strongly reduced on patterned substrates due to the geometrical restrictions, frequency of synapse formation was not different from homogeneous cultures. Thus it was unaffected by stringent guidance onto the target cell or by the number of cell-cell contacts. Moreover, a statistically significant enrichment of reciprocal contacts between mixed pairs of excitatory and inhibitory neurons over probabilistic predictions was found, which has similarly been shown by others in dissociated neuronal cultures. Our results indicate that precise axonal guidance is insufficient for target-specific synapse formation and suggest that instead recognition between individual cells is required.