Cardiovascular effects of the respiratory muscle metaboreflexes in dogs: rest and exercise

Acute isometric and dynamic exercise do not alter cerebral sympathetic nerve activity in healthy humans

The impact of physiological stressors on cerebral sympathetic nervous activity (SNA) remains controversial. We hypothesized that cerebral noradrenaline (NA) spillover, an index of cerebral SNA, would not change during both submaximal isometric handgrip (HG) exercise followed by a post-exercise circulatory occlusion (PECO), and supine dynamic cycling exercise. Twelve healthy participants (5 females) underwent simultaneous blood sampling from the right radial artery and right internal jugular vein. Right internal jugular vein blood flow was measured using Duplex ultrasound, and tritiated NA was infused through the participants' right superficial forearm vein. Heart rate was recorded via electrocardiogram and blood pressure was monitored using the right radial artery. Total NA spillover increased during HG (P = 0.049), PECO (P = 0.006), and moderate cycling exercise (P = 0.03) compared to rest. Cerebral NA spillover remained unchanged during isometric HG exercise (P = 0.36), PECO after the isometric HG exercise (P = 0.45), and during moderate cycling exercise (P = 0.94) compared to rest. These results indicate that transient increases in blood pressure during acute exercise involving both small and large muscle mass do not engage cerebral SNA in healthy humans. Our findings suggest that cerebral SNA may be non-obligatory for exercise-related cerebrovascular adjustments.

Is the Lung Built for Exercise? Advances and Unresolved Questions

ABSTRACT

Nearly 40 yr ago, Professor Dempsey delivered the 1985 ACSM Joseph B. Wolffe Memorial Lecture titled: "Is the lung built for exercise?" Since then, much experimental work has been directed at enhancing our understanding of the functional capacity of the respiratory system by applying complex methodologies to the study of exercise. This review summarizes a symposium entitled: "Revisiting 'Is the lung built for exercise?'" presented at the 2022 American College of Sports Medicine annual meeting, highlighting the progress made in the last three-plus decades and acknowledging new research questions that have arisen. We have chosen to subdivide our topic into four areas of active study: (i) the adaptability of lung structure to exercise training, (ii) the utilization of airway imaging to better understand how airway anatomy relates to exercising lung mechanics, (iii) measurement techniques of pulmonary gas exchange and their importance, and (iv) the interactions of the respiratory and cardiovascular system during exercise. Each of the four sections highlights gaps in our knowledge of the exercising lung. Addressing these areas that would benefit from further study will help us comprehend the intricacies of the lung that allow it to meet and adapt to the acute and chronic demands of exercise in health, aging, and disease.

Consequences of group III/IV afferent feedback and respiratory muscle work on exercise tolerance in heart failure with reduced ejection fraction

Exercise intolerance and exertional dyspnoea are the cardinal symptoms of heart failure with reduced ejection fraction (HFrEF). In HFrEF, abnormal autonomic and cardiopulmonary responses arising from locomotor muscle group III/IV afferent feedback is one of the primary mechanisms contributing to exercise intolerance. HFrEF patients also have pulmonary system and respiratory muscle abnormalities that impair exercise tolerance. Thus, the primary impetus for this review was to describe the mechanistic consequences of locomotor muscle group III/IV afferent feedback and respiratory muscle work in HFrEF. To address this, we first discuss the abnormal autonomic and cardiopulmonary responses mediated by locomotor muscle afferent feedback in HFrEF. Next, we outline how respiratory muscle work impairs exercise tolerance in HFrEF through its effects on locomotor muscle O2 delivery. We then discuss the direct and indirect evidence supporting an interaction between locomotor muscle group III/IV afferent feedback and respiratory muscle work during exercise in HFrEF. Last, we outline future research directions related to locomotor and respiratory muscle abnormalities to progress the field forward in understanding the pathophysiology of exercise intolerance in HFrEF. NEW FINDINGS: What is the topic of this review? This review is focused on understanding the role that locomotor muscle group III/IV afferent feedback and respiratory muscle work play in the pathophysiology of exercise intolerance in patients with heart failure. What advances does it highlight? This review proposes that the concomitant effects of locomotor muscle afferent feedback and respiratory muscle work worsen exercise tolerance and exacerbate exertional dyspnoea in patients with heart failure.

Effects of sex and ageing on the human respiratory muscle metaboreflex

Intense inspiratory muscle work evokes a sympathetically mediated pressor reflex, termed the respiratory muscle metaboreflex, in which young females demonstrate an attenuated response relative to males. However, the effects of ageing and female sex hormones on the respiratory muscle metaboreflex are unclear. We tested the hypothesis that the pressor response to inspiratory work would be similar between older males and females, and higher relative to their younger counterparts. Healthy, normotensive young (26 ± 3 years) males (YM; n = 10) and females (YF; n = 10), as well as older (64 ± 5 years) males (OM; n = 10) and females (OF; n = 10), performed inspiratory pressure threshold loading (PTL) to task failure. Older adults had a greater mean arterial pressure (MAP) response to PTL than young (P < 0.001). YF had a lower MAP compared to YM (+10 ± 6 vs. +19 ± 15 mmHg, P = 0.026); however, there was no difference observed between OF and OM (+26 ± 11 vs. +27 ± 11 mmHg, P = 0.162). Older adults had a lower heart rate response to PTL than young (P = 0.002). There was no effect of sex between young females and males (+19 ± 9 and +27 ± 11 bpm, P = 0.186) or older females and males (+17 ± 7 and +20 ± 7 bpm, P = 0.753). We conclude the respiratory muscle metaboreflex response is heightened in older adults, and the sex effect between older males and post-menopause females is absent, suggesting an effect of circulating sex hormones. KEY POINTS: The arterial blood pressure response to the respiratory muscle metaboreflex is greater in older males and females. Compared to sex-matched young individuals, there is no sex differences in the blood pressure response between older males and post-menopause females. Our results suggest the differences between males and females in the cardiovascular response to high levels of inspiratory muscle work is abolished with reduced circulating female sex hormones.

The physiology and pathophysiology of exercise hyperpnea

In health, the near-eucapnic, highly efficient hyperpnea during mild-to-moderate intensity exercise is driven by three obligatory contributions, namely, feedforward central command from supra-medullary locomotor centers, feedback from limb muscle afferents, and respiratory CO₂ exchange (V̇CO₂). Inhibiting each of these stimuli during exercise elicits a reduction in hyperpnea even in the continuing presence of the other major stimuli. However, the relative contribution of each stimulus to the hyperpnea remains unknown as does the means by which V̇CO2 is sensed. Mediation of the hyperventilatory response to exercise in health is attributed to the multiple feedback and feedforward stimuli resulting from muscle fatigue. In patients with COPD, diaphragm EMG amplitude and its relation to ventilatory output are used to decipher mechanisms underlying the patients' abnormal ventilatory responses, dynamic lung hyperinflation and dyspnea during exercise. Key contributions to these exercise-limiting responses across the spectrum of COPD severity include high dead space ventilation, an excessive neural drive to breathe and highly fatigable limb muscles, together with mechanical constraints on ventilation. Major controversies concerning control of exercise hyperpnea are discussed along with the need for innovative research to uncover the link of metabolism to breathing in health and disease.

Changes in Body Surface Temperature Associated with High-Speed Treadmill Exercise in Beagle Dogs Measured by Infrared Thermography

Simple Summary

Heat exchange between the body surface and the external environment plays an important role in the regulation of body temperature in animals. Previous studies on the effect of exercise on the distribution of body surface temperature in dogs have been conducted on a variety of breeds and did not employ specific defined regions of interest. The aim of our research was to assess the influence of high-speed treadmill exercise on body surface temperature using infrared thermography in selected body regions of healthy Beagle dogs, taking into account gait and recovery time. The study was based exclusively on the Beagle breed, which presents short, uniform, and straight hair, conducting heat more readily from the skin. Statistical analysis indicated the highest temperature at the upper forearm and thigh, and the lowest on the croup, back, and neck. The peak surface temperature values in all examined areas were observed after the canter and the post-gallop walk, and the lowest were observed 2 h after exercise. Our study confirms that the body surface temperature of Beagle dogs is influenced by high-speed physical exercise on a treadmill as a result of muscle activity and changes in blood flow. The proximal forelimb and hindlimb were the most influenced by exercise.

Abstract

Evaluation of body surface temperature change in response to exercise is important for monitoring physiological status. The aim of the study was to assess the influence of high-speed treadmill exercise on body surface temperature using infrared thermography (IRT) in selected body regions of healthy Beagle dogs, taking into account gait and recovery time. Thermographic images of the dogs were taken before exercise (BE), after walk (AW), after trot (AT), after canter (AC), just after second walk (JAE), 5 min after exercise (5 AE), 15 min after exercise (15 AE), 30 min after exercise (30 AE), 45 min after exercise (45 AE), and 120 min after exercise (120 AE). Body surface temperature was measured at the neck, shoulder, upper forearm, back, chest, croup, and thigh. Statistical analysis indicated the highest temperature at the upper forearm, shoulder, and thigh, and the lowest on the croup, back, and neck. The peak values of surface temperature in all ROIs were at AC and JAE and the lowest at 120 AE. The study demonstrated that body surface temperature was influenced by high-speed physical exercise on a treadmill and IRT was a viable imaging modality that provided temperature data from specific body regions. The proximal forelimb and hindlimb were the most influenced by exercise.

Effects of physical exercise on baroreflex sensitivity and renal sympathetic nerve activity in chronic nicotine-treated rats

Chronic nicotine exposure may increase cardiovascular risk by impairing the cardiac autonomic function. Besides, physical exercise (PE) has shown to improve cardiovascular health. Thus, we aimed to investigate the effects of PE on baroreflex sensitivity (BRS), heart rate variability (HRV), and sympathetic nerve activity (SNA) in chronically nicotine-exposed rats. Male Wistar rats were assigned to four independent groups: Control (treated with saline solution), Control+Ex (treated with saline and submitted to treadmill training), Nicotine (treated with Nicotine), and Nicotine+Ex (treated with nicotine and submitted to treadmill training). Nicotine (1 mg·kg-1) was administered daily for 28 consecutive days. PE consisted of running exercise (60%-70% of maximal aerobic capacity) for 45 min, 5 days per week, for 4 weeks. At the end of the protocol, cardiac BRS, HRV, renal SNA (rSNA), and renal BRS were assessed. Nicotine treatment decreased absolute values of HRV indexes, increased low frequency/high frequency ratio of HRV, reduced the bradycardic and sympatho-inhibitory baroreceptor reflex responses, and reduced the rSNA. PE effectively restored time-domain HRV indexes, the bradycardic and sympatho-inhibitory reflex responses, and the rSNA in chronic nicotine-treated rats. PE was effective in preventing the deterioration of time-domain parameters of HRV, arterial baroreceptor dysfunction, and the rSNA after nicotine treatment.

The cardiovascular consequences of fatiguing expiratory muscle work in otherwise resting healthy humans

In 11 healthy adults (25 ± 4 yr; 2 female, 9 male subjects), we investigated the effect of expiratory resistive loaded breathing [65% maximal expiratory mouth pressure (MEP), 15 breaths·min^-1, duty cycle 0.5; ERL_Pm] on mean arterial pressure (MAP), leg vascular resistance (LVR), and leg blood flow ([Formula: see text]). On a separate day, a subset of five male subjects performed ERL targeting 65% of maximal expiratory gastric pressure (ERL_Pga). ERL-induced expiratory muscle fatigue was confirmed by a 17 ± 5% reduction in MEP (P < 0.05) and a 16 ± 12% reduction in the gastric twitch pressure response to magnetic nerve stimulation (P = 0.09) from before to after ERL_Pm and ERL_Pga, respectively. From rest to task failure in ERL_Pm and ERL_Pga, MAP increased (ERL_Pm = 31 ± 10 mmHg, ERL_Pga = 18 ± 9 mmHg, both P < 0.05), but group mean LVR and [Formula: see text] were unchanged (ERL_Pm: LVR = 0.78 ± 0.21 vs. 0.97 ± 0.36 mmHg·mL^-1·min, [Formula: see text] = 133 ± 34 vs. 152 ± 74 mL·min^-1; ERL_Pga: LVR = 0.70 ± 0.21 vs. 0.84 ± 0.33 mmHg·mL^-1·min, [Formula: see text] = 160 ± 48 vs. 179 ± 110 mL·min^-1) (all P ≥ 0.05). Interestingly, [Formula: see text] during ERL_Pga oscillated within each breath, increasing (∼66%) and decreasing (∼50%) relative to resting values during resisted expirations and unresisted inspirations, respectively. In conclusion, fatiguing expiratory muscle work did not affect group mean LVR or [Formula: see text] in otherwise resting humans. We speculate that any sympathetically mediated peripheral vasoconstriction was counteracted by transient mechanical effects of high intra-abdominal pressures during ERL.NEW & NOTEWORTHY Fatiguing expiratory muscle work in otherwise resting humans elicits an increase in sympathetic motor outflow; whether limb blood flow ([Formula: see text]) and leg vascular resistance (LVR) are affected remains unknown. We found that fatiguing expiratory resistive loaded breathing (ERL) did not affect group mean [Formula: see text] or LVR. However, within-breath oscillations in [Formula: see text] may reflect a sympathetically mediated vasoconstriction that was counteracted by transient increases in [Formula: see text] due to the mechanical effects of high intra-abdominal pressure during ERL.

Is the healthy respiratory system built just right, overbuilt, or underbuilt to meet the demands imposed by exercise?

In the healthy, untrained young adult, a case is made for a respiratory system (airways, pulmonary vasculature, lung parenchyma, respiratory muscles, and neural ventilatory control system) that is near ideally designed to ensure a highly efficient, homeostatic response to exercise of varying intensities and durations. Our aim was then to consider circumstances in which the intra/extrathoracic airways, pulmonary vasculature, respiratory muscles, and/or blood-gas distribution are underbuilt or inadequately regulated relative to the demands imposed by the cardiovascular system. In these instances, the respiratory system presents a significant limitation to O₂ transport and contributes to the occurrence of locomotor muscle fatigue, inhibition of central locomotor output, and exercise performance. Most prominent in these examples of an "underbuilt" respiratory system are highly trained endurance athletes, with additional influences of sex, aging, hypoxic environments, and the highly inbred equine. We summarize by evaluating the relative influences of these respiratory system limitations on exercise performance and their impact on pathophysiology and provide recommendations for future investigation.

Combined influence of inspiratory loading and locomotor subsystolic cuff inflation on cardiovascular responses during submaximal exercise

It is unknown if simultaneous stimulation of the respiratory and locomotor muscle afferents via inspiratory loading (IL) and locomotor subsystolic cuff inflation (CUFF) influences the cardiovascular responses during exercise. We hypothesized that combined IL and CUFF (IL + CUFF) will result in greater increases in blood pressure (MAP) and systemic vascular resistance (SVR) than IL and CUFF alone during exercise. Eight adults (6 males/2 females) were enrolled and performed four 10-min bouts of constant-load cycling eliciting 40% maximal oxygen uptake on a single day. For each exercise bout, the first 5 min consisted of spontaneous breathing. The second 5 min consisted of voluntary hyperventilation (i.e., breathing frequency of 40 breaths/min) with IL (30% maximum inspiratory pressure), CUFF (80 mmHg), IL + CUFF, or no intervention (CTL) in randomized order. During exercise, cardiac output and MAP were determined via open-circuit acetylene wash-in and manual sphygmomanometry, respectively, and SVR was calculated. Across CTL, IL, CUFF, and IL + CUFF, MAP was greater with each condition (CTL: 97 ± 14; IL: 106 ± 13; CUFF: 114 ± 14; IL + CUFF: 119 ± 15 mmHg, all P < 0.02). Furthermore, SVR was greater with IL + CUFF compared with IL, CUFF, and CTL (CTL: 6.6 ± 1.1; IL: 7.5 ± 1.4; CUFF: 7.5 ± 1.3; IL + CUFF: 8.2 ± 1.4 mmHg·L^-1·min^-1, all P < 0.02). Cardiac output was not different across conditions (CTL: 15.2 ± 3.8; IL: 14.8 ± 3.7; CUFF: 15.6 ± 3.5; IL + CUFF: 14.7 ± 4.3 L/min, all P > 0.05). These data demonstrate that simultaneous stimulation of respiratory and locomotor muscle afferent feedback results in additive MAP and SVR responses than IL and CUFF alone during submaximal exercise. These findings have important clinical implications for populations with exaggerated locomotor and respiratory muscle reflex feedbacks.NEW & NOTEWORTHY Reflexes arising from the respiratory and locomotor muscles influence cardiovascular regulation during exercise. However, it is unclear how the respiratory and locomotor muscle reflexes interact when simultaneously stimulated. Herein, we demonstrate that stimulation of the respiratory and locomotor muscle reflexes yielded additive cardiovascular responses during submaximal exercise.

Inspiratory muscle training in young, race-fit Thoroughbred racehorses during a period of detraining

Although inspiratory muscle training (IMT) is reported to improve inspiratory muscle strength in humans little has been reported for horses. We tested the hypothesis that IMT would maintain and/or improve inspiratory muscle strength variables measured in Thoroughbreds during detraining. Thoroughbreds from one training yard were placed into a control (Con, n = 3 males n = 7 females; median age 2.2±0.4 years) or treatment group (Tr, n = 5 males, n = 5 females; median age 2.1±0.3 years) as they entered a detraining period at the end of the racing/training season. The Tr group underwent eight weeks of IMT twice a day, five days per week using custom-made training masks with resistance valves and an incremental threshold of breath-loading protocol. An inspiratory muscle strength test to fatigue using an incremental threshold of breath-loading was performed in duplicate before (T₀) and after four (T₁) and eight weeks (T₂) of IMT/no IMT using a custom-made testing mask and a commercial testing device. Inspiratory measurements included the total number of breaths achieved during the test, average load, peak power, peak volume, peak flow, energy and the mean peak inspiratory muscle strength index (IMSi). Data were analysed using a linear mixed effects model, P≤0.05 significant. There were no differences for inspiratory measurements between groups at T₀. Compared to T₀, the total number of breaths achieved (P = 0.02), load (P = 0.003) and IMSi (P = 0.01) at T₂ had decreased for the Con group while the total number of breaths achieved (P<0.001), load (P = 0.03), volume (P = 0.004), flow (P = 0.006), energy (P = 0.01) and IMSi (P = 0.002) had increased for the Tr group. At T₂ the total number of breaths achieved (P<0.0001), load (P<0.0001), volume (P = 0.02), energy (P = 0.03) and IMSi (P<0.0001) were greater for the Tr than Con group. In conclusion, our results support that IMT can maintain and/or increase aspects of inspiratory muscle strength for horses in a detraining programme.

Inspiratory muscle training and testing: Rationale, development and feasibility

BACKGROUND

Inspiratory muscle training applies a training stimulus directly to the inspiratory muscles and is distinct from whole-body training. The potential benefits of inspiratory muscle training have yet to be explored in horses.

OBJECTIVES

The objectives were as follows: (a) to develop an equine-specific method of testing and training inspiratory muscles; (b) to assess tolerance and feasibility in a pilot study in a commercial Thoroughbred training establishment.

STUDY DESIGN

Field study.

METHODS

A mask was used to interface commercial human inspiratory muscle training equipment. Ten horses undertook inspiratory muscle training once daily while stood in the stable approximately 5 days/wk over a 9-week period. Inspiratory muscle strength testing employed a continuous incremental inspiratory loading protocol alternating two loaded and two minimally loaded breaths until failure to tolerate the load occurred or the maximum 60 breaths were completed. The inspiratory muscle strength testing was undertaken twice; firstly, in 10 horses with minimal acclimatisation and secondly, in eight horses experienced with the inspiratory muscle training programme.

RESULTS

The 10 horses undertook inspiratory muscle training for a median of 42 days, reaching a median peak training load of 32.5 cm H₂ O. One horse did not tolerate the mask with repeated snorting and was replaced. All horses completed the inspiratory muscle strength testing. The median peak value in inspiratory muscle strength testing protocol 1 was 27 cm H₂ O and in inspiratory muscle strength testing protocol 2 was 41 cm H₂ O. Two of 10 horses reached the maximum possible value in inspiratory muscle strength testing protocol 1; therefore, the test was adapted to permit a higher maximum value, despite this 3/8 horses reached the maximum possible value in inspiratory muscle strength testing protocol 2.

MAIN LIMITATIONS

A small number of horses were assessed. The inspiratory muscle strength testing protocol was refined during the study and requires additional refinement.

CONCLUSION

Inspiratory muscle testing and training were feasible and tolerated in horses. Further research is required to understand whether the inspiratory muscle strength testing values obtained correlate with other physiological/performance outcomes. The potential benefits and/or adverse effects of inspiratory muscle training warrant further investigation.

Respiratory Determinants of Exercise Limitation: Focus on Phrenic Afferents and the Lung Vasculature

We examine 2 means by which the healthy respiratory system contributes to exercise limitation. These include the activation of respiratory and locomotor muscle afferent reflexes, which constrain blood flow and hasten fatigue in both sets of muscles, and the excessive increases in pulmonary vascular pressures at high cardiac outputs, which constrain O₂ transport and precipitate maladaptive right ventricular remodeling in endurance-trained subjects.

Work of breathing influences muscle sympathetic nerve activity during semi-recumbent cycle exercise

Reducing the work of breathing during exercise improves locomotor muscle blood flow and reduces diaphragm and locomotor muscle fatigue and is thought to be the result of a sympathetically mediated reflex.

AIM

The aim of this study was to assess muscle sympathetic nerve activity (MSNA) when the work of breathing is experimentally lowered during dynamic exercise.

METHODS

Healthy subjects (n = 12; age = 29 ± 9 years) performed semi-recumbent cycling trials at 40%, 60%, and 80% of peak workload. Exercise trials consisted of spontaneous breathing, reduced work of breathing (proportional assist ventilator), followed by further spontaneous breathing (post-ventilator). MSNA was recorded from the median nerve.

RESULTS

There was no difference in work of breathing between PAV and post-PAV at 40% peak work. At 60% peak work, the ventilator significantly (P < 0.05) reduced work of breathing (103 ± 39 vs 144 ± 47 J min^-1 ), sympathetic nerve activity (35 ± 5 vs 42 ± 8 burst min^-1 ), and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:mover><mml:mi>V</mml:mi> <mml:mo>˙</mml:mo></mml:mover> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:mrow> </mml:math> (2.4 ± 0.5 vs 2.6 ± 0.5 L min^-1 ) without influencing ventilation (86 ± 9 vs 82 ± 10 L min^-1 ; P > 0.05), for PAV and post-PAV respectively. During 80% peak work (n = 8), the ventilator significantly (P < 0.05) reduced work of breathing (235 ± 110 vs. 361 ± 150 J min^-1 ), MSNA (48 ± 7 vs 54 ± 11 burst min^-1 ), and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:mover><mml:mi>V</mml:mi> <mml:mo>˙</mml:mo></mml:mover> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:mrow> </mml:math> (2.9 ± 0.6 vs 3.2 ± 0.7 L min^-1 ) but not ventilation (121 ± 20 vs 123 ± 20 L min^-1 ; P > 0.05), for PAV and post-PAV respectively. There was a significant relationship between MSNA and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow><mml:mover><mml:mi>V</mml:mi> <mml:mo>˙</mml:mo></mml:mover> <mml:msub><mml:mi>O</mml:mi> <mml:mn>2</mml:mn></mml:msub> </mml:mrow> </mml:math> (P < 0.0001) with a significant interaction due to the ventilator (P < 0.05).

CONCLUSION

Lowering the normally occurring work of breathing during exercise results in commensurate reductions in MSNA. Our findings provide evidence of a sympathetically mediated vasoconstrictor effect emanating from respiratory muscles during exercise.

The effect of inspiratory muscle fatigue on acid-base status and performance during race-paced middle-distance swimming

The aim of this study was to investigate the effect of pre-induced inspiratory muscle fatigue (IMF) on race-paced swimming and acid-base status. Twenty-one collegiate swimmers performed two discontinuous 400-m race-paced swims on separate days, with (IMF trial) and without (control trial) pre-induced IMF. Swimming characteristics, inspiratory and expiratory mouth pressures, and blood parameters were recorded. IMF and expiratory muscle fatigue (P < 0.05) were evident after both trials and swimming time was slower (P < 0.05) from 150-m following IMF inducement. Pre-induced IMF increased pH before the swim (P < 0.01) and reduced bicarbonate (P < 0.05) and the pressure of carbon dioxide (PCO₂) (P < 0.05). pH (P < 0.05), bicarbonate (P < 0.01) and PCO₂ (P < 0.05) were lower during swimming in the IMF trial. Blood lactate was similar before both trials (P > 0.05) but was higher (P < 0.01) in the IMF trial after swimming. Pre-induced IMF induced respiratory alkalosis, reduced bicarbonate buffering capacity and slowed swimming speed. Pre-induced and propulsion-induced IMF reflected metabolic acidosis arising from dual role breathing and propulsion muscle fatigue.

Sex differences in diaphragmatic fatigue: the cardiovascular response to inspiratory resistance

KEY POINTS

Diaphragmatic fatigue (DF) elicits a sympathetically mediated metaboreflex resulting in increased heart rate, blood pressure and limb vascular resistance. Women may be more resistant to DF compared to men, and therefore it was hypothesised that women would experience an attenuated inspiratory muscle metaboreflex during inspiratory pressure-threshold loading (PTL) performed to task failure. At the time of PTL task failure, the severity of DF was not different between sexes; however, inspiratory muscle endurance time was significantly longer in women than in men. For a given cumulative diaphragmatic force output, the severity of DF was less in women than in men. Women exhibited a blunted cardiovascular response to inspiratory resistance (i.e. metaboreflex) that may have implications for exercise tolerance.

ABSTRACT

Diaphragmatic fatigue (DF) elicits reflexive increases in sympathetic vasomotor outflow (i.e. metaboreflex). There is some evidence suggesting women may be more resistant to DF compared to men, and therefore may experience an attenuated inspiratory muscle metaboreflex. To this end, we sought to examine the cardiovascular response to inspiratory resistance in healthy young men (n = 9, age = 24 ± 3 years) and women (n = 9, age = 24 ± 3 years). Subjects performed isocapnic inspiratory pressure-threshold loading (PTL, 60% maximal inspiratory mouth pressure) to task failure. Diaphragmatic fatigue was assessed by measuring transdiaphragmatic twitch pressure (P_di,tw ) using cervical magnetic stimulation. Heart rate (HR) and mean arterial pressure (MAP) were measured beat-by-beat throughout PTL via photoplethysmography, and low-frequency systolic pressure (LF_SBP ; a surrogate for sympathetic vasomotor tone) calculated from arterial waveforms using power spectrum analysis. At PTL task failure, the degree of DF was similar between sexes (∼23% reduction in P_di,tw ; P = 0.33). However, time to task failure was significantly longer in women than in men (27 ± 11 vs. 16 ± 11 min, respectively; P = 0.02). Women exhibited less of an increase in HR (13 ± 8 vs. 19 ± 12 bpm; P = 0.02) and MAP (10 ± 8 vs. 14 ± 9 mmHg; P = 0.01), and significantly lower LF_SBP (23 ± 11 vs. 34 ± 8 mmHg² ; P = 0.04) during PTL compared to men. An attenuation of the inspiratory muscle metaboreflex may influence limb and respiratory muscle haemodynamics with implications for exercise performance.

Competition for blood flow distribution between respiratory and locomotor muscles: implications for muscle fatigue

Sympathetically induced vasoconstrictor modulation of local vasodilation occurs in contracting skeletal muscle during exercise to ensure appropriate perfusion of a large active muscle mass and to maintain also arterial blood pressure. In this synthesis, we discuss the contribution of group III-IV muscle afferents to the sympathetic modulation of blood flow distribution to locomotor and respiratory muscles during exercise. This is followed by an examination of the conditions under which diaphragm and locomotor muscle fatigue occur. Emphasis is given to those studies in humans and animal models that experimentally changed respiratory muscle work to evaluate blood flow redistribution and its effects on locomotor muscle fatigue, and conversely, those that evaluated the influence of coincident limb muscle contraction on respiratory muscle blood flow and fatigue. We propose the concept of a "two-way street of sympathetic vasoconstrictor activity" emanating from both limb and respiratory muscle metaboreceptors during exercise, which constrains blood flow and O₂ transport thereby promoting fatigue of both sets of muscles. We end with considerations of a hierarchy of blood flow distribution during exercise between respiratory versus locomotor musculatures and the clinical implications of muscle afferent feedback influences on muscle perfusion, fatigue, and exercise tolerance.

Respiratory Effects of Thoracic Load Carriage Exercise and Inspiratory Muscle Training as a Strategy to Optimize Respiratory Muscle Performance with Load Carriage

Many occupational and recreational settings require the use of protective and/or load-bearing apparatuses worn over the thoracic cavity, known as thoracic load carriage (LC). Compared to normal, unloaded exercise, thoracic LC exercise places an additional demand on the respiratory and limb locomotor systems by altering ventilatory mechanics as well as circulatory responses to exercise, thus accelerating the development of fatigue in the diaphragm and accessory respiratory muscles compared to unloaded exercise. This may be a consequence of the unique demands of thoracic LC, which places an additional mass load on the thoracic cavity and can restrict chest wall expansion. Therefore it is important to find effective strategies to ameliorate the detrimental effects of thoracic LC. Inspiratory muscle training is an intervention that aims to increase the strength and endurance of the diaphragm and accessory inspiratory muscle and may therefore be a useful strategy to optimize performance with thoracic LC.

Muscle metaboreflex-induced vasoconstriction in the ischemic active muscle is exaggerated in heart failure

When oxygen delivery to active muscle is insufficient to meet the metabolic demand during exercise, metabolites accumulate and stimulate skeletal muscle afferents, inducing a reflex increase in blood pressure, termed the muscle metaboreflex. In healthy individuals, muscle metaboreflex activation (MMA) during submaximal exercise increases arterial pressure primarily via an increase in cardiac output (CO), as little peripheral vasoconstriction occurs. This increase in CO partially restores blood flow to ischemic muscle. However, we recently demonstrated that MMA induces sympathetic vasoconstriction in ischemic active muscle, limiting the ability of the metaboreflex to restore blood flow. In heart failure (HF), increases in CO are limited, and metaboreflex-induced pressor responses occur predominantly via peripheral vasoconstriction. In the present study, we tested the hypothesis that vasoconstriction of ischemic active muscle is exaggerated in HF. Changes in hindlimb vascular resistance [femoral arterial pressure ÷ hindlimb blood flow (HLBF)] were observed during MMA (via graded reductions in HLBF) during mild exercise with and without α₁-adrenergic blockade (prazosin, 50 µg/kg) before and after induction of HF. In normal animals, initial HLBF reductions caused metabolic vasodilation, while reductions below the metaboreflex threshold elicited reflex vasoconstriction, in ischemic active skeletal muscle, which was abolished after α₁-adrenergic blockade. Metaboreflex-induced vasoconstriction of ischemic active muscle was exaggerated after induction of HF. This heightened vasoconstriction impairs the ability of the metaboreflex to restore blood flow to ischemic muscle in HF and may contribute to the exercise intolerance observed in these patients. We conclude that sympathetically mediated vasoconstriction of ischemic active muscle during MMA is exaggerated in HF. NEW & NOTEWORTHY We found that muscle metaboreflex-induced vasoconstriction of the ischemic active skeletal muscle from which the reflex originates is exaggerated in heart failure. This results in heightened metaboreflex activation, which further amplifies the reflex-induced vasoconstriction of the ischemic active skeletal muscle and contributes to exercise intolerance in patients.

Effect of cyclooxygenase inhibition on the inspiratory muscle metaboreflex-induced cardiovascular consequences in men

Inspiratory muscle metaboreflex activation increases mean arterial pressure (MAP) and limb vascular resistance (LVR) and decreases limb blood flow (Q̇_L). Cyclooxygenase (COX) inhibition has been found to attenuate limb skeletal muscle metaboreflex-induced increases in muscle sympathetic nerve activity. We hypothesized that compared with placebo (PLA), COX inhibition would attenuate inspiratory muscle metaboreflex-induced 1) increases in MAP and LVR and 2) decreases in Q̇_L Seven men (22 ± 1 yr) were recruited and orally consumed ibuprofen (IB; 10 mg/kg) or PLA 90 min before performing the cold pressor test (CPT) for 2 min and inspiratory resistive breathing task (IRBT) for 14.9 ± 2.0 min at 65% of maximal inspiratory pressure. Breathing frequency was 20 breaths/min with a 50% duty cycle during the IRBTs. MAP was measured via automated oscillometry, Q̇_L was determined via Doppler ultrasound, and LVR was calculated as MAP divided by Q̇_L Electromyography was recorded on the leg to ensure no muscle contraction occurred. The 65% IRBT led to greater increases (P = 0.02) in 6-keto-prostaglandin-F_1α with PLA compared with IB. IB, compared with PLA, led to greater (P < 0.01) increases in MAP (IB: 17 ± 7 mmHg vs. PLA: 8 ± 5 mmHg) and LVR (IB: 69 ± 28% vs. PLA: 52 ± 22%) at the final minute of the 65% IRBT. The decrease in Q̇_L was not different (P = 0.72) between IB (-28 ± 11%) and PLA (-27 ± 9%) at the final minute. The increase in MAP during the CPT was not different (P = 0.87) between IB (25 ± 11 mmHg) and PLA (24 ± 6 mmHg). Contrary to our hypotheses, COX inhibition led to greater inspiratory muscle metaboreflex-induced increases in MAP and LVR.NEW & NOTEWORTHY Cyclooxygenase (COX) products play a role in activating the muscle metaboreflex. It is not known whether COX products contribute to the inspiratory muscle metaboreflex. Herein, we demonstrate that COX inhibition led to greater increases in blood pressure and limb vascular resistance compared with placebo during inspiratory muscle metaboreflex activation.

Cardiovascular consequences of the inspiratory muscle metaboreflex: effects of age and sex

With inspiratory muscle metaboreflex activation, we hypothesized that, compared with their younger counterparts, older men and women would exhibit greater 1) increases in mean arterial pressure (MAP) and limb vascular resistance (LVR) and 2) decreases in limb blood flow (Q̇L) but 3) no sex differences would be present in older adults. Sixteen young adults [8 young men (YM) and 8 young women (YW), 18–24 yr] and 16 older adults [8 older men (OM) and 8 older women (OW), 60–73 yr] performed inspiratory resistive breathing tasks (IRBTs) at 2% and 65% of their maximal inspiratory pressure. During the IRBTs, breathing frequency was 20 breaths/min with a 50% duty cycle. At baseline and during the IRBTs, MAP was measured via automated oscillometry, Q̇L was determined via Doppler ultrasound, and LVR was calculated. The 65% IRBT led to significantly greater increases in MAP in OW (15.9 ± 8.1 mmHg) compared with YW (6.9 ± 1.4 mmHg) but not ( P > 0.05) between OM (12.3 ± 5.7 mmHg) and YM (10.8 ± 5.7 mmHg). OW (−20.2 ± 7.2%) had greater ( P < 0.05) decreases in Q̇L compared with YW (−9.4 ± 10.2%), but no significant differences were present between OM (−22.8 ± 9.7%) and YM (−22.7 ± 11.3%) during the 65% IRBT. The 65% IRBT led to greater ( P < 0.05) increases in LVR in OW (48.2 ± 25.5%) compared with YW (19.7 ± 15.0%), but no differences ( P > 0.05) existed among OM (54.4 ± 17.8%) and YM (47.1 ± 23.3%). No significant differences were present in MAP, Q̇L, or LVR between OM and OW. These data suggest that OW exhibit a greater inspiratory muscle metaboreflex compared with YW, whereas no differences between OM and YM existed. Finally, sex differences in the inspiratory muscle metaboreflex are not present in older adults.

NEW & NOTEWORTHY Premenopausal women exhibit an attenuated inspiratory muscle metaboreflex compared with young men; however, it is unknown whether these sex differences are present in older adults. Older women exhibited a greater inspiratory muscle metaboreflex compared with premenopausal women, whereas no differences were present between older and younger men.

Sex differences in the cardiovascular consequences of the inspiratory muscle metaboreflex

It is currently unknown whether sex differences exist in the cardiovascular consequences of the inspiratory muscle metaboreflex. We hypothesized that the activation of the inspiratory muscle metaboreflex will lead to less of an increase in mean arterial pressure (MAP) and limb vascular resistance (LVR) and less of a decrease in limb blood flow (Q̇L) in women compared with men. Twenty healthy men ( n = 10, 23 ± 2 yr) and women ( n = 10, 22 ± 3 yr) were recruited for this study. Subjects performed inspiratory resistive breathing tasks (IRBTs) at 2% or 65% of their maximal inspiratory mouth pressure (PIMAX). During the IRBTs, the breathing frequency was 20 breaths/min with a 50% duty cycle. At rest and during the IRBTs, MAP was measured via automated oscillometry, Q̇L was measured via Doppler ultrasound, and LVR was calculated. EMG was recorded on the leg to ensure no muscle contraction occurred. The 65% IRBT led to attenuated increases ( P < 0.01) from baseline in women compared with men for MAP (W: 7.3 ± 2.0 mmHg; M: 11.1 ± 5.0 mmHg) and LVR (W: 17.7% ± 14.0%; M: 47.9 ± 21.0%), as well as less of a decrease ( P < 0.01) in Q̇L (W: −7.5 ± 9.9%; M: −23.3 ± 10.2%). These sex differences in MAP, Q̇L, and LVR were still present in a subset of subjects matched for PIMAX. The 2% IRBT resulted in no significant changes in MAP, Q̇L, or LVR across time or between men and women. These data indicate premenopausal women exhibit an attenuated inspiratory muscle metaboreflex compared with age-matched men.

Neural control of circulation and exercise: a translational approach disclosing interactions between central command, arterial baroreflex, and muscle metaboreflex

The last 100 years witnessed a rapid and progressive development of the body of knowledge concerning the neural control of the cardiovascular system in health and disease. The understanding of the complexity and the relevance of the neuroregulatory system continues to evolve and as a result raises new questions. The purpose of this review is to articulate results from studies involving experimental models in animals as well as in humans concerning the interaction between the neural mechanisms mediating the hemodynamic responses during exercise. The review describes the arterial baroreflex, the pivotal mechanism controlling mean arterial blood pressure and its fluctuations along with the two main activation mechanisms to exercise: central command (parallel activation of central somatomotor and autonomic descending pathways) and the muscle metaboreflex, the metabolic component of exercise pressor reflex (feedback from ergoreceptors within contracting skeletal muscles). In addition, the role of the cardiopulmonary baroreceptors in modulating the resetting of arterial baroreflex is identified, and the mechanisms in the central nervous system involved with the resetting of baroreflex function during dynamic exercise are also described. Approaching a very relevant clinical condition, the review also presents the concept that the impaired arterial baroreflex function is an integral component of the metaboreflex-mediated exaggerated sympathetic tone in subjects with heart failure. This increased sympathetic activity has a major role in causing the depressed ventricular function observed during submaximal dynamic exercise in these patients. The potential contribution of a metaboreflex arising from respiratory muscles is also considered.

Sympathetic vasomotor outflow and blood pressure increase during exercise with expiratory resistance

The purpose of the present study was to elucidate the effect of increasing expiratory muscle work on sympathetic vasoconstrictor outflow and arterial blood pressure (BP) during dynamic exercise. We hypothesized that expiratory muscle fatigue would elicit increases in sympathetic vasomotor outflow and BP during submaximal exercise. The subjects performed four submaximal exercise tests; two were maximal expiratory pressure (PE_max) tests and two were muscle sympathetic nerve activity (MSNA) tests. In each test, the subjects performed two 10-min exercises at 40% peak oxygen uptake using a cycle ergometer in a semirecumbent position [spontaneous breathing for 5 min and voluntary hyperpnoea with and without expiratory resistive breathing for 5 min (breathing frequency: 60 breaths/min, inspiratory and expiratory times were set at 0.5 sec)]. PE_max was estimated before and immediately after exercises. MSNA was recorded via microneurography of the right median nerve at the elbow. PE_max decreased following exercise with expiratory resistive breathing, while no change was found without resistance. A progressive increase in MSNA burst frequency (BF) appeared during exercise with expiratory resistance (MSNA BF, without resistance: +22 ± 5%, with resistance: +44 ± 8%, P < 0.05), accompanied by an augmentation of BP (mean BP, without resistance: +5 ± 2%, with resistance: +29 ± 5%, P < 0.05). These results suggest that an enhancement of expiratory muscle activity leads to increases in sympathetic vasomotor outflow and BP during dynamic leg exercise.

Intercostal and forearm muscle deoxygenation during respiratory fatigue in patients with heart failure: potential role of a respiratory muscle metaboreflex

The purpose of this study was to determine the effect of respiratory muscle fatigue on intercostal and forearm muscle perfusion and oxygenation in patients with heart failure. Five clinically stable heart failure patients with respiratory muscle weakness (age, 66±12 years; left ventricle ejection fraction, 34±3%) and nine matched healthy controls underwent a respiratory muscle fatigue protocol, breathing against a fixed resistance at 60% of their maximal inspiratory pressure for as long as they could sustain the predetermined inspiratory pressure. Intercostal and forearm muscle blood volume and oxygenation were continuously monitored by near-infrared spectroscopy with transducers placed on the seventh left intercostal space and the left forearm. Data were compared by two-way ANOVA and Bonferroni correction. Respiratory fatigue occurred at 5.1±1.3 min in heart failure patients and at 9.3±1.4 min in controls (P<0.05), but perceived effort, changes in heart rate, and in systolic blood pressure were similar between groups (P>0.05). Respiratory fatigue in heart failure reduced intercostal and forearm muscle blood volume (P<0.05) along with decreased tissue oxygenation both in intercostal (heart failure, -2.6±1.6%; controls, +1.6±0.5%; P<0.05) and in forearm muscles (heart failure, -4.5±0.5%; controls, +0.5±0.8%; P<0.05). These results suggest that respiratory fatigue in patients with heart failure causes an oxygen demand/delivery mismatch in respiratory muscles, probably leading to a reflex reduction in peripheral limb muscle perfusion, featuring a respiratory metaboreflex.

Relation of respiratory muscle strength, cachexia and survival in severe chronic heart failure

BACKGROUND

Respiratory muscle (RM) function predicts prognosis in non-cachectic patients with chronic heart failure (CHF). We hypothesized that weakness of RM (maximum inspiratory mouth occlusion pressure, Pimax) is a function of body mass index, and that outcome is more a function of BMI than of Pimax or ventilatory drive (P0.1).

SUBJECTS AND METHODS

We enrolled 249 CHF patients (11.2 % female, median age 54.2 years) at the German Heart Institute Berlin. Patients were in NYHA classes I/II/III/IV by n = 16/90/108/35. All patients underwent tests of pulmonary function, RM (Pimax, P0.1), cardiopulmonary exercise testing (peakVO2, VE/VCO2-slope), and right heart catheterization.

RESULTS

Mean follow-up time was 18 (1-36) months, 47 patients (18.9 %) died or underwent cardiac assist implantation. Pimax correlated weakly with BMI (r = 0.19), peakVO2 (r = 0.15), and FEV1 (r = 0.34, all p < 0.02), and was lower in females compared to males (3.9 ± 1.7 vs. 6.6 ± 2.7 kPa; p < 0.001). P0.1 correlated with pulmonary pressure (rho = 0.2; p < 0.01) and peakVO2 (rho = -0.14; p < 0.02). Neither Pimax [hazard ratio (HR) 0.98; confidence interval (CI) 0.88-1.08] nor P0.1 (HR 0.52; 0.06-4.6) predicted survival. Multivariate regression analysis revealed gender, BMI, and FEV1 as cofactors of Pimax, with only BMI (HR 0.87; CI 0.80-0.95) predicting survival independently. The lowest quintile in BMI had the worst outcome (log-rank χ² = 13.5, p = 0.009). In CHF patients including cachexia and NYHA IV, Pimax does not predict survival. Pimax depends on gender, BMI, FEV1, and peakVO2, with only BMI and peakVO2 predicting survival. The impaired Pimax in CHF might be a result of catabolism and weight loss and is not a predictive factor in itself.

Ventilation and respiratory mechanics

During dynamic exercise, the healthy pulmonary system faces several major challenges, including decreases in mixed venous oxygen content and increases in mixed venous carbon dioxide. As such, the ventilatory demand is increased, while the rising cardiac output means that blood will have considerably less time in the pulmonary capillaries to accomplish gas exchange. Blood gas homeostasis must be accomplished by precise regulation of alveolar ventilation via medullary neural networks and sensory reflex mechanisms. It is equally important that cardiovascular and pulmonary system responses to exercise be precisely matched to the increase in metabolic requirements, and that the substantial gas transport needs of both respiratory and locomotor muscles be considered. Our article addresses each of these topics with emphasis on the healthy, young adult exercising in normoxia. We review recent evidence concerning how exercise hyperpnea influences sympathetic vasoconstrictor outflow and the effect this might have on the ability to perform muscular work. We also review sex-based differences in lung mechanics.

Control of breathing during exercise

During exercise by healthy mammals, alveolar ventilation and alveolar-capillary diffusion increase in proportion to the increase in metabolic rate to prevent PaCO2 from increasing and PaO2 from decreasing. There is no known mechanism capable of directly sensing the rate of gas exchange in the muscles or the lungs; thus, for over a century there has been intense interest in elucidating how respiratory neurons adjust their output to variables which can not be directly monitored. Several hypotheses have been tested and supportive data were obtained, but for each hypothesis, there are contradictory data or reasons to question the validity of each hypothesis. Herein, we report a critique of the major hypotheses which has led to the following conclusions. First, a single stimulus or combination of stimuli that convincingly and entirely explains the hyperpnea has not been identified. Second, the coupling of the hyperpnea to metabolic rate is not causal but is due to of these variables each resulting from a common factor which link the circulatory and ventilatory responses to exercise. Third, stimuli postulated to act at pulmonary or cardiac receptors or carotid and intracranial chemoreceptors are not primary mediators of the hyperpnea. Fourth, stimuli originating in exercising limbs and conveyed to the brain by spinal afferents contribute to the exercise hyperpnea. Fifth, the hyperventilation during heavy exercise is not primarily due to lactacidosis stimulation of carotid chemoreceptors. Finally, since volitional exercise requires activation of the CNS, neural feed-forward (central command) mediation of the exercise hyperpnea seems intuitive and is supported by data from several studies. However, there is no compelling evidence to accept this concept as an indisputable fact.

Hypoxic effects on sympathetic vasomotor outflow and blood pressure during exercise with inspiratory resistance

The purpose of the present study was to clarify the influence of inspiratory resistive breathing during exercise under hypoxic conditions on muscle sympathetic nerve activity (MSNA) and blood pressure (BP). Six healthy males completed this study. The subjects performed a submaximal exercise test using a cycle ergometer in a semirecumbent position under normoxic [inspired oxygen fraction (FiO2) = 0.21] and hypoxic (FiO2 = 0.12–0.13) conditions. The subjects carried out two 10-min exercises at 40% peak oxygen uptake [spontaneous breathing for 5 min and voluntary breathing with inspiratory resistance for 5 min (breathing frequency: 60 breaths/min, inspiratory and expiratory times were set at 0.5 s each)]. MSNA was recorded via microneurography of the right median nerve at the elbow. A progressive increase in MSNA burst frequency (BF) during leg-cycling exercise with inspiratory resistance in normoxia and hypoxia were accompanied by an augmentation of BP. The increased MSNA BF and mean arterial BP (MBP) during exercise with inspiratory resistive breathing in hypoxia (MSNA BF, 55.7 ± 1.4 bursts/min, MBP, 134.3 ± 6.6 mmHg) were higher than those in normoxia (MSNA BF, 39.2 ± 1.8 bursts/min, MBP, 123.6 ± 4.5 mmHg). These results suggest that an enhancement of inspiratory muscle activity under hypoxic condition leads to large increases in muscle sympathetic vasomotor outflow and BP during dynamic leg exercise.

New perspectives concerning feedback influences on cardiorespiratory control during rhythmic exercise and on exercise performance

The cardioaccelerator and ventilatory responses to rhythmic exercise in the human are commonly viewed as being mediated predominantly via feedforward 'central command' mechanisms, with contributions from locomotor muscle afferents to the sympathetically mediated pressor response. We have assessed the relative contributions of three types of feedback afferents on the cardiorespiratory response to voluntary, rhythmic exercise by inhibiting their normal 'tonic' activity in healthy animals and humans and in chronic heart failure. Transient inhibition of the carotid chemoreceptors during moderate intensity exercise reduced muscle sympathetic nerve activity (MSNA) and increased limb vascular conductance and blood flow; and reducing the normal level of respiratory muscle work during heavier intensity exercise increased limb vascular conductance and blood flow. These cardiorespiratory effects were prevented via ganglionic blockade and were enhanced in chronic heart failure and in hypoxia. Blockade of μ opioid sensitive locomotor muscle afferents, with preservation of central motor output via intrathecal fentanyl: (a) reduced the mean arterial blood pressure (MAP), heart rate and ventilatory responses to all steady state exercise intensities; and (b) during sustained high intensity exercise, reduced O(2) transport, increased central motor output and end-exercise muscle fatigue and reduced endurance performance. We propose that these three afferent reflexes - probably acting in concert with feedforward central command - contribute significantly to preserving O(2) transport to locomotor and to respiratory muscles during exercise. Locomotor muscle afferents also appear to provide feedback concerning the metabolic state of the muscle to influence central motor output, thereby limiting peripheral fatigue development.

Inspiratory muscle fatigue increases sympathetic vasomotor outflow and blood pressure during submaximal exercise

The purpose of this study was to elucidate the influence of inspiratory muscle fatigue on muscle sympathetic nerve activity (MSNA) and blood pressure (BP) response during submaximal exercise. We hypothesized that inspiratory muscle fatigue would elicit increases in sympathetic vasoconstrictor outflow and BP during dynamic leg exercise. The subjects carried out four submaximal exercise tests: two were maximal inspiratory pressure (PI(max)) tests and two were MSNA tests. In the PI(max) tests, the subjects performed two 10-min exercises at 40% peak oxygen uptake using a cycle ergometer in a semirecumbent position [spontaneous breathing for 5 min and with or without inspiratory resistive breathing for 5 min (breathing frequency: 60 breaths/min, inspiratory and expiratory times were each set at 0.5 s)]. Before and immediately after exercise, PI(max) was estimated. In MSNA tests, the subjects performed two 15-min exercises (spontaneous breathing for 5 min, with or without inspiratory resistive breathing for 5 min, and spontaneous breathing for 5 min). MSNA was recorded via microneurography of the right median nerve at the elbow. PI(max) decreased following exercise with resistive breathing, whereas no change was found without resistance. The time-dependent increase in MSNA burst frequency (BF) appeared during exercise with inspiratory resistive breathing, accompanied by an augmentation of diastolic BP (DBP) (with resistance: MSNA, BF +83.4%; DBP, +23.8%; without resistance: MSNA BF, +19.2%; DBP, -0.4%, from spontaneous breathing during exercise). These results suggest that inspiratory muscle fatigue induces increases in muscle sympathetic vasomotor outflow and BP during dynamic leg exercise at mild intensity.

Inspiratory muscle training enhances pulmonary O(2) uptake kinetics and high-intensity exercise tolerance in humans

Fatigue of the respiratory muscles during intense exercise might compromise leg blood flow, thereby constraining oxygen uptake (Vo(2)) and limiting exercise tolerance. We tested the hypothesis that inspiratory muscle training (IMT) would reduce inspiratory muscle fatigue, speed Vo(2) kinetics and enhance exercise tolerance. Sixteen recreationally active subjects (mean + or - SD, age 22 + or - 4 yr) were randomly assigned to receive 4 wk of either pressure threshold IMT [30 breaths twice daily at approximately 50% of maximum inspiratory pressure (MIP)] or sham treatment (60 breaths once daily at approximately 15% of MIP). The subjects completed moderate-, severe- and maximal-intensity "step" exercise transitions on a cycle ergometer before (Pre) and after (Post) the 4-wk intervention period for determination of Vo(2) kinetics and exercise tolerance. There were no significant changes in the physiological variables of interest after Sham. After IMT, baseline MIP was significantly increased (Pre vs. Post: 155 + or - 22 vs. 181 + or - 21 cmH(2)O; P < 0.001), and the degree of inspiratory muscle fatigue was reduced after severe- and maximal-intensity exercise. During severe exercise, the Vo(2) slow component was reduced (Pre vs. Post: 0.60 + or - 0.20 vs. 0.53 + or - 0.24 l/min; P < 0.05) and exercise tolerance was enhanced (Pre vs. Post: 765 + or - 249 vs. 1,061 + or - 304 s; P < 0.01). Similarly, during maximal exercise, the Vo(2) slow component was reduced (Pre vs. Post: 0.28 + or - 0.14 vs. 0.18 + or - 0.07 l/min; P < 0.05) and exercise tolerance was enhanced (Pre vs. Post: 177 + or - 24 vs. 208 + or - 37 s; P < 0.01). Four weeks of IMT, which reduced inspiratory muscle fatigue, resulted in a reduced Vo(2) slow-component amplitude and an improved exercise tolerance during severe- and maximal-intensity exercise. The results indicate that the enhanced exercise tolerance observed after IMT might be related, at least in part, to improved Vo(2) dynamics, presumably as a consequence of increased blood flow to the exercising limbs.

Intercostal muscle blood flow limitation in athletes during maximal exercise

We investigated whether, during maximal exercise, intercostal muscle blood flow is as high as during resting hyperpnoea at the same work of breathing. We hypothesized that during exercise, intercostal muscle blood flow would be limited by competition from the locomotor muscles. Intercostal (probe over the 7th intercostal space) and vastus lateralis muscle perfusion were measured simultaneously in ten trained cyclists by near-infrared spectroscopy using indocyanine green dye. Measurements were made at several exercise intensities up to maximal (WRmax) and subsequently during resting isocapnic hyperpnoea at minute ventilation levels up to those at WRmax. During resting hyperpnoea, intercostal muscle blood flow increased linearly with the work of breathing (R2 = 0.94) to 73.0 +/- 8.8 ml min-1 (100 g)-1 at the ventilation seen at WRmax (work of breathing approximately 550-600 J min-1), but during exercise it peaked at 80% WRmax (53.4 +/- 10.3 ml min-1 (100 g)-1), significantly falling to 24.7 +/- 5.3 ml min-1 (100 g)-1 at WRmax. At maximal ventilation intercostal muscle vascular conductance was significantly lower during exercise (0.22 +/- 0.05 ml min-1 (100 g)-1 mmHg-1) compared to isocapnic hyperpnoea (0.77 +/- 0.13 ml min-1 (100 g)-1 mmHg-1). During exercise, both cardiac output and vastus lateralis muscle blood flow also plateaued at about 80% WRmax (the latter at 95.4 +/- 11.8 ml min-1 (100 g)-1). In conclusion, during exercise above 80% WRmax in trained subjects, intercostal muscle blood flow and vascular conductance are less than during resting hyperpnoea at the same minute ventilation. This suggests that the circulatory system is unable to meet the demands of both locomotor and intercostal muscles during heavy exercise, requiring greater O2 extraction and likely contributing to respiratory muscle fatigue.

The effect of inspiratory muscle training on high-intensity, intermittent running performance to exhaustion

The effects of inspiratory muscle (IM) training on maximal 20 m shuttle run performance (Ex) during Yo-Yo intermittent recovery test and on the physiological and perceptual responses to the running test were examined. Thirty men were randomly allocated to 1 of 3 groups. The experimental group underwent a 6 week pressure threshold IM training program by performing 30 inspiratory efforts twice daily, 6 d/week, against a load equivalent to 50% maximal static inspiratory pressure. The placebo group performed the same training procedure but with a minimal inspiratory load. The control group received no training. In post-intervention assessments, IM function was enhanced by >30% in the experimental group. The Ex was improved by 16.3% ± 3.9%, while the rate of increase in intensity of breathlessness (RPB/4i) was reduced by 11.0% ± 6.2%. Further, the whole-body metabolic stress reflected by the accumulations of plasma ammonia, uric acid, and blood lactate during the Yo-Yo test at the same absolute intensity was attenuated. For the control and placebo groups, no significant change in these variables was observed. In comparison with previous observations that the reduced RPB/4i resulting from IM warm-up was the major reason for improved Ex, the reduced RPB/4i resulting from the IM training program was lower despite the greater enhancement of IM function, whereas improvement in Ex was similar. Such findings suggest that although both IM training and warm-up improve the tolerance of intense intermittent exercise, the underlying mechanisms may be different.

Effects of respiratory muscle unloading on leg muscle oxygenation and blood volume during high-intensity exercise in chronic heart failure

Blood flow requirements of the respiratory muscles (RM) increase markedly during exercise in chronic heart failure (CHF). We reasoned that if the RM could subtract a fraction of the limited cardiac output (QT) from the peripheral muscles, RM unloading would improve locomotor muscle perfusion. Nine patients with CHF (left ventricle ejection fraction = 26 ± 7%) undertook constant-work rate tests (70-80% peak) receiving proportional assisted ventilation (PAV) or sham ventilation. Relative changes (Δ%) in deoxy-hemoglobyn, oxi-Hb ([O2Hb]), tissue oxygenation index, and total Hb ([HbTOT], an index of local blood volume) in the vastus lateralis were measured by near infrared spectroscopy. In addition, QT was monitored by impedance cardiography and arterial O2 saturation by pulse oximetry (SpO2). There were significant improvements in exercise tolerance (Tlim) with PAV. Blood lactate, leg effort/Tlim and dyspnea/Tlim were lower with PAV compared with sham ventilation ( P < 0.05). There were no significant effects of RM unloading on systemic O2 delivery as QT and SpO2 at submaximal exercise and at Tlim did not differ between PAV and sham ventilation ( P > 0.05). Unloaded breathing, however, was related to enhanced leg muscle oxygenation and local blood volume compared with sham, i.e., higher Δ[O2Hb]% and Δ[HbTOT]%, respectively ( P < 0.05). We conclude that RM unloading had beneficial effects on the oxygenation status and blood volume of the exercising muscles at similar systemic O2 delivery in patients with advanced CHF. These data suggest that blood flow was redistributed from respiratory to locomotor muscles during unloaded breathing.

Effect of expiratory muscle fatigue on exercise tolerance and locomotor muscle fatigue in healthy humans

High-intensity exercise (≥90% of maximal O2 uptake) sustained to the limit of tolerance elicits expiratory muscle fatigue (EMF). We asked whether prior EMF affects subsequent exercise tolerance. Eight male subjects (means ± SD; maximal O2 uptake = 53.5 ± 5.2 ml·kg−1·min−1) cycled at 90% of peak power output to the limit of tolerance with (EMF-EX) and without (CON-EX) prior induction of EMF and for a time equal to that achieved in EMF-EX but without prior induction of EMF (ISO-EX). To induce EMF, subjects breathed against an expiratory flow resistor until task failure (15 breaths/min, 0.7 expiratory duty cycle, 40% of maximal expiratory gastric pressure). Fatigue of abdominal and quadriceps muscles was assessed by measuring the reduction relative to prior baseline values in magnetically evoked gastric twitch pressure (Pgatw) and quadriceps twitch force (Qtw), respectively. The reduction in Pgatw was not different after resistive breathing vs. after CON-EX (−27 ± 5 vs. −26 ± 6%; P = 0.127). Exercise time was reduced by 33 ± 10% in EMF-EX vs. CON-EX (6.85 ± 2.88 vs. 9.90 ± 2.94 min; P < 0.001). Exercise-induced abdominal and quadriceps muscle fatigue was greater after EMF-EX than after ISO-EX (−28 ± 9 vs. −12 ± 5% for Pgatw, P = 0.001; −28 ± 7 vs. −14 ± 6% for Qtw, P = 0.015). Perceptual ratings of dyspnea and leg discomfort (Borg CR10) were higher at 1 and 3 min and at end exercise during EMF-EX vs. during ISO-EX ( P < 0.05). Percent changes in limb fatigue and leg discomfort (EMF-EX vs. ISO-EX) correlated significantly with the change in exercise time. We propose that EMF impaired subsequent exercise tolerance primarily through an increased severity of limb locomotor muscle fatigue and a heightened perception of leg discomfort.

Exercise-induced respiratory muscle fatigue: implications for performance

It is commonly held that the respiratory system has ample capacity relative to the demand for maximal O2and CO2transport in healthy humans exercising near sea level. However, this situation may not apply during heavy-intensity, sustained exercise where exercise may encroach on the capacity of the respiratory system. Nerve stimulation techniques have provided objective evidence that the diaphragm and abdominal muscles are susceptible to fatigue with heavy, sustained exercise. The fatigue appears to be due to elevated levels of respiratory muscle work combined with an increased competition for blood flow with limb locomotor muscles. When respiratory muscles are prefatigued using voluntary respiratory maneuvers, time to exhaustion during subsequent exercise is decreased. Partially unloading the respiratory muscles during heavy exercise using low-density gas mixtures or mechanical ventilation can prevent exercise-induced diaphragm fatigue and increase exercise time to exhaustion. Collectively, these findings suggest that respiratory muscle fatigue may be involved in limiting exercise tolerance or that other factors, including alterations in the sensation of dyspnea or mechanical load, may be important. The major consequence of respiratory muscle fatigue is an increased sympathetic vasoconstrictor outflow to working skeletal muscle through a respiratory muscle metaboreflex, thereby reducing limb blood flow and increasing the severity of exercise-induced locomotor muscle fatigue. An increase in limb locomotor muscle fatigue may play a pivotal role in determining exercise tolerance through a direct effect on muscle force output and a feedback effect on effort perception, causing reduced motor output to the working limb muscles.