Interplay Between the Inspiratory and Postural Functions of the Human Parasternal Intercostal Muscles

Managing respiratory muscle weakness during weaning from invasive ventilation

Weaning is a critical stage of an intensive care unit (ICU) stay, in which the respiratory muscles play a major role. Weakness of the respiratory muscles, which is associated with significant morbidity in the ICU, is not limited to atrophy and subsequent dysfunction of the diaphragm; the extradiaphragmatic inspiratory and expiratory muscles also play important parts. In addition to the well-established deleterious effect of mechanical ventilation on the respiratory muscles, other risk factors such as sepsis may be involved. Weakness of the respiratory muscles can be suspected visually in a patient with paradoxical movement of the abdominal compartment. Measurement of maximal inspiratory pressure is the simplest way to assess respiratory muscle function, but it does not specifically take the diaphragm into account. A cut-off value of -30 cmH2O could identify patients at risk for prolonged ventilatory weaning; however, ultrasound may be better for assessing respiratory muscle function in the ICU. Although diaphragm dysfunction has been associated with weaning failure, this diagnosis should not discourage clinicians from performing spontaneous breathing trials and considering extubation. Recent therapeutic developments aimed at preserving or restoring respiratory muscle function are promising.

Effects of Inspiratory Muscle Warm-Up on Physical Exercise: A Systematic Review

This study aimed to systematically review the literature to examine the effects of inspiratory-muscle warm-up (IMW) on the inspiratory, metabolic, respiratory and performance parameters of a main exercise performed by athletes and healthy and active individuals. Methods: This systematic review included randomized studies in English based on the criteria of the PICOS model. The exclusion criteria adopted were studies that applied inspiratory exercise to: i. promote long-term adaptations through inspiratory training (chronic responses); ii. obtain acute responses to inspiratory load (overload) during and in breaks from physical effort and in an inspiratory-exercise session (acute training effect); iii. evaluate the effects of IMW on participants with cardiorespiratory and/or metabolic disease. Data Sources: PubMed, Embase, MedLine, Scopus, SPORTDiscus and Google Scholar (until 17 January 2023). Results: Thirty-one studies were selected. The performance and respiratory parameters were the most investigated (77% and 74%, respectively). Positive effects of IMW were reported by 88% of the studies that investigated inspiratory parameters and 45% of those that evaluated performance parameters. Conclusions: The analyzed protocols mainly had positive effects on the inspiratory and performance parameters of the physical exercises. These positive effects of IMW are possibly associated with the contractile and biochemical properties of inspiratory muscles.

Effects of specific inspiratory muscle training combined with whole-body endurance training program on balance in COPD patients: Randomized controlled trial

PURPOSE

This study aims to assess the effect of inspiratory muscle training (IMT) combined with endurance training (ET) on balance in patients with chronic obstructive pulmonary disease (COPD).

METHODS

We studied 32 male patients (62 ± 6 years) with moderate to very severe COPD. They were randomly assigned to an experimental group (IMT+ET) n = 16 or a control group (ET) n = 16 with similar characteristics. The evaluations were carried out at inclusion and after eight weeks of the training period. Functional balance was assessed by the Berg Balance Scale (BBS), the Timed-up and Go (TUG), the Single Leg Stance test (SLS), and the Activities-specific Balance Confidence (ABC) scale. The strength of the inspiratory muscles (PImax) was assessed by maximal inspiratory mouth pressure. Functional exercise performance was assessed by the 6 minutes walking test (6MWT). IMT program consists in performing two daily sets of 30 inspirations with 50% of PImax increased by 10% every two weeks. ET program consists in performing 30 min treadmill exercise at 60% to 80% of the average speed achieved during the 6MWT three days per week.

RESULTS

After the training period, the experimental group demonstrated greater improvements in BBS (IMT+ET vs. ET; p = 0.019), and in ABC (IMT+ET vs. ET; p = 0.014). However, no significant differences between groups were observed for TUG, SLS, and 6MWT. There was a significant difference between groups in PImax (IMT+ET vs. ET; p = 0.030). Significant moderate correlations were obtained between ΔPImax and ΔBBS for both groups (IMT+ET: r = 0.624, p = 0.010; ET r = 0.550, p = 0.027) as well as for ΔABC but only in the experimental group (IMT+ET: r = 0.550, p = 0.027).

CONCLUSION

Compared to ET alone, the results suggest that IMT combined with ET enhances inspiratory muscle function and functional balance according to BBS and ABC in patients with COPD. We suggest that inspiratory muscle training might be introduced as additional training to pulmonary rehabilitation programs aimed at improving balance in COPD patients.

TRIAL REGISTRATION

The trial registry name: Clinical Trials; Registration number: NCT04084405; URL: https://clinicaltrials.gov/ct2/show/NCT04084405.

Complex network model indicates a positive effect of inspiratory muscles pre-activation on performance parameters in a judo match

This study investigated the effects of inspiratory muscle pre-activation (IM_PA) on the interactions among the technical-tactical, physical, physiological, and psychophysiological parameters in a simulated judo match, based on the centrality metrics by complex network model. Ten male athletes performed 4 experimental sessions. Firstly, anthropometric measurements, maximal inspiratory pressure (MIP) and global strenght of the inspiratory muscles were determined. In the following days, all athletes performed four-minute video-recorded judo matches, under three conditions: without IM_PA (CON), after IM_PA at 15% (IM_PA15), and at 40% (IM_PA40) of MIP using an exerciser device. Blood lactate, heart rate and rating of perceived exertion were monitored, and the technical-tactical parameters during the match were related to offensive actions and the time-motion. Based on the complex network, graphs were constructed for each scenario (CON, IM_PA15, and IM_PA40) to investigate the Degree and Pagerank centrality metrics. IM_PA40 increased the connectivity of the physical and technical-tactical parameters in complex network and highlighted the combat frequency and average combat time in top-five ranked nodes. IM_PA15 also favoured the interactions among the psychophysiological, physical, and physiological parameters. Our results suggest the positive effects of the IM_PA, indicating this strategy to prepare the organism (IM_PA15) and to improve performance (IM_PA40) in judo match.

Contribution of Trunk Muscles to Upright Sitting with Segmental Support in Children with Spinal Cord Injury

To investigate and compare trunk control and muscle activation during uncompensated sitting in children with and without spinal cord injury (SCI). Static sitting trunk control in ten typically developing (TD) children (5 females, 5 males, mean (SD) age of 6 (2)y) and 26 children with SCI (9 females, 17 males, 5(2)y) was assessed and compared using the Segmental Assessment of Trunk Control (SATCo) test while recording surface electromyography (EMG) from trunk muscles. The SCI group scored significantly lower on the SATCo compared to the TD group. The SCI group produced significantly higher thoracic-paraspinal activation at the lower-ribs, and, below-ribs support levels, and rectus-abdominus activation at below-ribs, pelvis, and no-support levels than the TD group. The SCI group produced significantly higher lumbar-paraspinal activation at inferior-scapula and no-support levels. Children with SCI demonstrated impaired trunk control with the ability to activate trunk muscles above and below the injury level.

Ultrasound variations of diaphragm activity between prone position versus supine position in ventilated patients: a cross-sectional comparative study

Purpose

To evaluate the effect of the positioning from the supine position (SP) to the prone position (PP) on the diaphragm activity in ventilated patients; using the ultrasound (US) imaging.

Methods

A cross-sectional comparative study before/after PP was conducted on 40 ICU patients over 18 years who received invasive ventilation (IV) for at least 48 h. The considered ventilator modes were: assisted control volume with a low trigger flow (between − 2 and 2 L/mn) and pressure support mode. US diaphragmatic assessments were performed at SP and at 60 min of PP. Both End-inspiratory and End-expiratory diameters (EID/EED) were taken at 3 levels of axillary lines and determined by the average values of multiple measures. Diaphragmatic thickening fraction (DTF) was calculated as: DTF = (EID − EED/EED) × 100. Pairing and ANOVA tests were used for comparisons.

Results

Forty ventilated patients (42 years of median age) at 4 days [2–7] of median duration of ventilation were examined during the two positions: SP versus PP. EID decreased from the SP to the PP (2.8 mm in SP vs. 2.4 mm in PP, p = 0.001). No difference was showed regarding the expiratory thickness. Overall, DTF didn’t change in PP (37.4 vs. 42.05%, p = 0.36). When the patient was placed in PP, the best DTF value was showed at the posterior part of diaphragm (posterior: 45%, median: 31% and anterior: 38%, p = 0.049).

Conclusion

The ventral placement in ventilated patients reduced end-inspiratory diameter and tended to decrease DTF. In PP, the best contractile activity was detected at the posterior region of diaphragm.

Recruitment pattern of the diaphragm and extradiaphragmatic inspiratory muscles in response to different levels of pressure support

Background

Inappropriate ventilator assist plays an important role in the development of diaphragm dysfunction. Ventilator under-assist may lead to muscle injury, while over-assist may result in muscle atrophy. This provides a good rationale to monitor respiratory drive in ventilated patients. Respiratory drive can be monitored by a nasogastric catheter, either with esophageal balloon to determine muscular pressure (gold standard) or with electrodes to measure electrical activity of the diaphragm. A disadvantage is that both techniques are invasive. Therefore, it is interesting to investigate the role of surrogate markers for respiratory dive, such as extradiaphragmatic inspiratory muscle activity. The aim of the current study was to investigate the effect of different inspiratory support levels on the recruitment pattern of extradiaphragmatic inspiratory muscles with respect to the diaphragm and to evaluate agreement between activity of extradiaphragmatic inspiratory muscles and the diaphragm.

Methods

Activity from the alae nasi, genioglossus, scalene, sternocleidomastoid and parasternal intercostals was recorded using surface electrodes. Electrical activity of the diaphragm was measured using a multi-electrode nasogastric catheter. Pressure support (PS) levels were reduced from 15 to 3 cmH₂O every 5 min with steps of 3 cmH₂O. The magnitude and timing of respiratory muscle activity were assessed.

Results

We included 17 ventilated patients. Diaphragm and extradiaphragmatic inspiratory muscle activity increased in response to lower PS levels (36 ± 6% increase for the diaphragm, 30 ± 6% parasternal intercostals, 41 ± 6% scalene, 40 ± 8% sternocleidomastoid, 43 ± 6% alae nasi and 30 ± 6% genioglossus). Changes in diaphragm activity correlated best with changes in alae nasi activity (r² = 0.49; P < 0.001), while there was no correlation between diaphragm and sternocleidomastoid activity. The agreement between diaphragm and extradiaphragmatic inspiratory muscle activity was low due to a high individual variability. Onset of alae nasi activity preceded the onset of all other muscles.

Conclusions

Extradiaphragmatic inspiratory muscle activity increases in response to lower inspiratory support levels. However, there is a poor correlation and agreement with the change in diaphragm activity, limiting the use of surface electromyography (EMG) recordings of extradiaphragmatic inspiratory muscles as a surrogate for electrical activity of the diaphragm.

Increased diaphragm motor unit discharge frequencies during quiet breathing in people with chronic tetraplegia

KEY POINTS

Respiratory muscle strength is compromised in people with tetraplegia, which may be compensated for by an increase in neural drive to the diaphragm. We found that the discharge frequencies of diaphragm motor units are higher in people with chronic tetraplegia compared with able-bodied people during quiet breathing. Furthermore, we found that the area of single motor unit potentials was increased in people with tetraplegia. These results suggest an increased motoneurone output to the diaphragm and remodelling of diaphragm motor units to maintain ventilation in tetraplegia.

ABSTRACT

People with tetraplegia have reduced inspiratory muscle strength, ∼40% of able-bodied individuals. Paralysed or partially paralysed respiratory muscles as a result of tetraplegia compromise lung function, increase the incidence of respiratory infections and can cause dyspnoea. We hypothesised that reduced inspiratory muscle strength in tetraplegia may increase neural drive to the inspiratory muscles to maintain ventilation. We recorded the discharge properties of single motor units from the diaphragm in participants with chronic tetraplegia (8 males, 42-78 years, C3-C6 injury, AIS A-C) and able-bodied control participants (6 males matched for age and body mass index). In each group, 117 and 166 single motor units, respectively, were discriminated from recordings in the costal diaphragm using a monopolar electrode. A linear mixed-effects model analysis showed higher peak discharge frequencies of motor units during quiet breathing in tetraplegia (17.8 ± 4.9 Hz; mean ± SD) compared with controls (12.4 ± 2.2 Hz) (P < 0.001). There were no differences in tidal volume, inspiratory time or mean air flow between groups. Motor unit potentials in tetraplegia, compared with controls, were larger in amplitude (1.1 ± 0.7 mV and 0.5 ± 0.3 mV, respectively, P = 0.007) and area (1.83 ± 1.49 µV ms and 0.69 ± 0.52 µV ms, respectively, P = 0.003). The findings indicate that diaphragm motor unit remodelling is likely to have occurred in people with chronic tetraplegia and that there is an increase in diaphragm motor unit discharge rates during quiet breathing. These neural changes ensure that ventilation is maintained in people with chronic tetraplegia.

Respiratory muscle activity in voluntary breathing tracking tasks: Implications for the assessment of respiratory motor control

How the involuntary (bulbospinal) and voluntary (corticospinal) pathways interact in respiratory muscle control is not established. To determine the role of excitatory corticobulbar pathways in humans, studies typically compare electromyographic activity (EMG) or evoked responses in respiratory muscles during hypercapnic and voluntary tasks. Although ventilation is matched between tasks by having participants track signals of ventilation, these tasks may not result in matched respiratory muscle activity. The aim of this study was to describe respiratory muscle activity and ribcage and abdominal excursions during two different voluntary conditions, compared to hypercapnic hyperventilation. Ventilation was matched in the voluntary conditions via (i) a simple target of lung volume ('volume tracking') or (ii) targets of both ribcage and abdominal excursions, adjusted to end-expiratory lung volume in hypercapnic hyperventilation ('bands tracking'). Compared to hypercapnic hyperventilation, respiratory parameters such as tidal volume were similar, but the ratio of ribcage to abdominal excursion was higher for both voluntary tasks. Inspiratory scalene and parasternal intercostal muscle activity was higher in volume tracking, but diaphragm and abdominal muscle activity showed little to no change. There were no differences in muscle activity in bands tracking for any muscle, compared to hypercapnic hyperventilation. An elevated ratio of ribcage to abdominal excursion in the bands tracking task indicates that participants could not accurately match the targets in this condition. Inspiratory muscle activity is altered in some muscles in some voluntary tasks, compared to hypercapnia. Therefore, differences in muscle activity should be considered in interpretation of studies that use these protocols to investigate respiratory muscle control.

Assessment of Work of Breathing in Patients with Acute Exacerbations of Chronic Obstructive Pulmonary Disease

The assessment of the work of breathing (WOB) of patients with acute exacerbations of chronic obstructive pulmonary disease (COPD) is difficult, particularly when the patient first presents with acute hypercapnia and respiratory acidosis. Acute exacerbations of COPD patients are in significant respiratory distress and noninvasive measurements of WOB are easier for the patient to tolerate. Given the interest in using alternative therapies to noninvasive ventilation, such as high flow nasal oxygen therapy or extracorporeal carbon dioxide removal, understanding the physiological changes are key and this includes assessment of WOB. This narrative review considers the role of three different methods of assessing WOB in patients with acute exacerbations of COPD. Esophageal pressure is a very well validated measure of WOB, however the ability of patients with acute exacerbations of COPD to tolerate esophageal tubes is poor. Noninvasive alternative measurements include parasternal electromyography (EMG) and electrical impedance tomography (EIT). EMG is easily applied and is a well validated measure of neural drive but is more likely to be degraded by the electrical environment in intensive care or high dependency. EIT is less well validated as a tool for WOB in COPD but extremely well tolerated by patients. Each of the different methods assess WOB in a different way and have different advantages and disadvantages. For research into therapies treating acute exacerbations of COPD, combinations of EIT, EMG and esophageal pressure are likely to be better than only one of these.

Muscles of Breathing: Development, Function, and Patterns of Activation

This review is a comprehensive description of all muscles that assist lung inflation or deflation in any way. The developmental origin, anatomical orientation, mechanical action, innervation, and pattern of activation are described for each respiratory muscle fulfilling this broad definition. In addition, the circumstances in which each muscle is called upon to assist ventilation are discussed. The number of "respiratory" muscles is large, and the coordination of respiratory muscles with "nonrespiratory" muscles and in nonrespiratory activities is complex-commensurate with the diversity of activities that humans pursue, including sleep (8.27). The capacity for speech and adoption of the bipedal posture in human evolution has resulted in patterns of respiratory muscle activation that differ significantly from most other animals. A disproportionate number of respiratory muscles affect the nose, mouth, pharynx, and larynx, reflecting the vital importance of coordinated muscle activity to control upper airway patency during both wakefulness and sleep. The upright posture has freed the hands from locomotor functions, but the evolutionary history and ontogeny of forelimb muscles pervades the patterns of activation and the forces generated by these muscles during breathing. The distinction between respiratory and nonrespiratory muscles is artificial, as many "nonrespiratory" muscles can augment breathing under conditions of high ventilator demand. Understanding the ontogeny, innervation, activation patterns, and functions of respiratory muscles is clinically useful, particularly in sleep medicine. Detailed explorations of how the nervous system controls the multiple muscles required for successful completion of respiratory behaviors will continue to be a fruitful area of investigation. © 2019 American Physiological Society. Compr Physiol 9:1025-1080, 2019.

Aminophylline increases parasternal muscle action in awake canines

The traditional theophylline bronchodilator, aminophylline, is still widely used, especially in the treatment of COPD. The effects of aminophylline on ventilation and action of the costal diaphragm have been previously defined, but other respiratory muscles - notably the chest wall, are not well determined. Therefore, we investigated the effects of aminophylline on the Parasternal intercostal, a key obligatory inspiratory muscle, examining muscle length, shortening and EMG. We studied 11 awake canines, chronically implanted with sonomicrometer crystals and fine-wire EMG electrodes in the parasternal muscle. Ventilatory parameters, muscle length (shortening), and moving average muscle EMG activity, were measured at baseline and with aminophylline, during resting and hypercapnic stimulated breathing. Experiments were carried out prior to administration of aminophylline (baseline), and 1.5 h after loading and ongoing infusion. Minute ventilation, tidal volume and respiratory frequency all increased significantly with aminophylline, both during resting breathing and at equivalent levels of hypercapnic stimulated breathing. Parasternal baseline muscle length was entirely unchanged with aminophylline. Parasternal shortening increased significantly with aminophylline while corresponding parasternal EMG activity remained constant, consistent with increased contractility. Thus, in awake, intact mammals, aminophylline, in the usual therapeutic range, elicits increased ventilation and increased contractility of all primary inspiratory respiratory muscles, including both chest wall and diaphragm.

Reflex response to airway occlusion in human inspiratory muscles when recruited for breathing and posture

Briefly occluding the airway during inspiration produces a short-latency reflex inhibition in human inspiratory muscles. This occlusion reflex seems specific to respiratory muscles; however, it is not known whether the reflex inhibition has a uniform effect across a motoneuron pool when a muscle is recruited concurrently for breathing and posture. In this study, participants were seated and breathed through a mouthpiece that occluded inspiratory airflow for 250 ms at a volume threshold of 0.2 liters. The reflex response was measured in the scalene and sternocleidomastoid muscles during 1) a control condition with the head supported in space and the muscles recruited for breathing only, 2) a postural condition with the head unsupported and the neck flexors recruited for both breathing and to maintain head posture, and 3) a large-breath condition with the head supported and the volume threshold raised to between 0.8 and 1.0 liters to increase inspiratory muscle activity. When normalized to its preocclusion mean, the reflex response in the scalene muscles was not significantly different between the large-breath and control conditions, whereas concomitant recruitment of these muscles for posture control reduced the reflex response by half compared with the control condition. A reflex response occurred in sternocleidomastoid when it contracted phasically as an accessory muscle for inspiration during the large-breath condition. These results indicate that the occlusion reflex does not produce a uniform effect across the motoneuron pool and that afferent inputs for this reflex most likely act via intersegmental networks of premotoneurons rather than at a motoneuronal level.

NEW & NOTEWORTHY In this study, we investigated the effect of nonrespiratory activity on the reflex response to brief sudden airway occlusions in human inspiratory muscles. We show that the reflex inhibition in the scalene muscles was not uniform across the motoneuron pool when the muscle was recruited concurrently for breathing and postural control. The reflex had a larger effect on respiratory-driven motoneurons than those recruited to maintain head posture.

Breathing with neuromuscular disease: Does compensatory plasticity in the motor drive to breathe offer a potential therapeutic target in muscular dystrophy?

Duchenne muscular dystrophy is a fatal neuromuscular disease associated with respiratory-related morbidity and mortality. Herein, we review recent work by our group exploring deficits and compensation in the respiratory control network governing respiratory homeostasis in a pre-clinical model of DMD, the mdx mouse. Deficits at multiple sites of the network provide considerable challenges to respiratory control. However, our work has also revealed evidence of compensatory neuroplasticity in the motor drive to breathe enhancing diaphragm muscle activity during increased chemical drive. The finding may explain the preserved capacity for mdx mice to increase ventilation in response to chemoactivation. Given the profound dysfunction in the primary pump muscle of breathing, we argue that activation of accessory muscles of breathing may be especially important in mdx (and perhaps DMD). Notwithstanding the limitations resulting from respiratory muscle dysfunction, it may be possible to further leverage intrinsic physiological mechanisms serving to compensate for weak muscles in attempts to preserve or restore ventilatory capacity. We discuss current knowledge gaps and the need to better appreciate fundamental aspects of respiratory control in pre-clinical models so as to better inform intervention strategies in human DMD.

Task-dependent output of human parasternal intercostal motor units across spinal levels

KEY POINTS

During breathing, there is differential activity in the human parasternal intercostal muscles and the activity is tightly coupled to the known mechanical advantages for inspiration of the same regions of muscles. It is not known whether differential activity is preserved for the non-respiratory task of ipsilateral trunk rotation. In the present study, we compared single motor units during resting breathing and axial rotation of the trunk during apnoea. We not only confirmed non-uniform recruitment of motor units across parasternal intercostal muscles in breathing, but also demonstrated that the same motor units show an altered pattern of recruitment in the non-respiratory task of trunk rotation. The output of parasternal intercostal motoneurones is modulated differently across spinal levels depending on the task and these results help us understand the mechanisms that may govern task-dependent differences in motoneurone output.

ABSTRACT

During inspiration, there is differential activity in the human parasternal intercostal muscles across interspaces. We investigated whether the earlier recruitment of motor units in the rostral interspaces compared to more caudal spaces during inspiration is preserved for the non-respiratory task of ipsilateral trunk rotation. Single motor unit activity (SMU) was recorded from the first, second and fourth parasternal interspaces on the right side in five participants in two tasks: resting breathing and 'isometric' axial rotation of the trunk during apnoea. Recruitment of the same SMUs was compared between tasks (n = 123). During resting breathing, differential activity was indicated by earlier recruitment of SMUs in the first and second interspaces compared to the fourth space in inspiration (P < 0.01). By contrast, during trunk rotation, the same motor units showed an altered pattern of recruitment because SMUs in the first interspace were recruited later and at a higher rotation torque than those in the second and fourth interspaces (P < 0.05). Tested for a subset of SMUs, the reliability of the breathing and rotation tasks, as well as the SMU recruitment measures, was good-excellent [intraclass correlation (2,1): 0.69-0.91]. Thus, the output of parasternal intercostal motoneurones is modulated differently across spinal levels depending on the task. Given that the differential inspiratory output of parasternal intercostal muscles is linked to their relative mechanical effectiveness for inspiration and also that this output is altered in trunk rotation, we speculate that a mechanism matching neural drive to muscle mechanics underlies the task-dependent differences in output of axial motoneurone pools.

Parasternal intercostal muscle activity during methacholine-induced bronchoconstriction

NEW FINDINGS

What is the central question of this study? The parasternal intercostal electromyogram (EMGpara) is known to provide an accurate, non-invasive index of respiratory load-capacity balance. Although relationships between EMGpara and both airflow obstruction and hyperinflation have been shown, the independent contribution of each factor has not been examined. What is the main finding and its importance? Reductions in airway calibre and inspiratory capacity along with increases in EMGpara were induced via methacholine challenge. A strong inverse relationship was observed between EMGpara and airway obstruction, with no influence of inspiratory capacity. These data suggest that EMGpara is more strongly influenced by airway calibre than by changes in end-expiratory lung volume during airway challenge testing. Neural respiratory drive, measured via the parasternal intercostal electromyogram (EMGpara), provides a non-invasive index of the load-capacity balance of the respiratory muscle pump. Previous studies in patients with obstructive lung disease have shown strong relationships between EMGpara and the extent of both airflow obstruction and hyperinflation. The relative influence of the two factors has not, however, been described. Airflow obstruction was induced via methacholine challenge testing in 25 adult humans. Forced expiratory volume in 1 s (FEV₁ ) and surface EMGpara during tidal breathing were measured after each dose, with 20 of the participants also undergoing measurements of inspiratory capacity (IC) at each stage. Linear mixed model analysis was used to assess dose-wise changes in FEV₁ and EMGpara, and thereafter to determine the influence of changes in FEV₁ and IC on change in EMGpara. Median (interquartile range) FEV₁ decreased significantly [from 96.00 (80.00-122.30) to 67.80 (37.98-92.27)% predicted, P < 0.0001] and EMGpara increased significantly [from 5.37 (2.25-8.92) to 6.27 (3.37-19.60) μV, P < 0.0001] from baseline to end of test. Linear mixed model analysis showed a significant interaction between methacholine dose and induced change in EMGpara, with an increase in EMGpara of 0.24 (95% confidence interval 0.11-0.37) μV per methacholine dose² . Change in FEV₁ further influenced this relationship [increase in slope of 0.002 (0.004-0.001) μV dose^-2 per % predicted fall in FEV₁ , P = 0.011], but not with change in IC. These data suggest that bronchoconstriction exerts a more potent influence on levels of EMGpara than changes in end-expiratory lung volume during methacholine challenge.

Effect of cadence on locomotor-respiratory coupling during upper-body exercise

Introduction

Asynchronous arm-cranking performed at high cadences elicits greater cardiorespiratory responses compared to low cadences. This has been attributed to increased postural demand and locomotor–respiratory coupling (LRC), and yet, this has not been empirically tested. This study aimed to assess the effects of cadence on cardiorespiratory responses and LRC during upper-body exercise.

Methods

Eight recreationally-active men performed arm-cranking exercise at moderate and severe intensities that were separated by 10 min of rest. At each intensity, participants exercised for 4 min at each of three cadences (50, 70, and 90 rev min⁻¹) in a random order, with 4 min rest-periods applied in-between cadences. Exercise measures included LRC via whole- and half-integer ratios, cardiorespiratory function, perceptions of effort (RPE and dyspnoea), and diaphragm EMG using an oesophageal catheter.

Results

The prevalence of LRC during moderate exercise was highest at 70 vs. 50 rev min⁻¹ (27 ± 10 vs. 13 ± 9%, p = 0.000) and during severe exercise at 90 vs. 50 rev min⁻¹ (24 ± 7 vs. 18 ± 5%, p = 0.034), with a shorter inspiratory time and higher mean inspiratory flow (p < 0.05) at higher cadences. During moderate exercise, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \dot{V}{\text{O}}_{ 2} $$\end{document}V˙O2 and f_C were higher at 90 rev min⁻¹ (p < 0.05) relative to 70 and 50 rev min⁻¹ (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$ \dot{V}{\text{O}}_{ 2} $$\end{document}V˙O2 1.19 ± 0.25 vs. 1.05 ± 0.21 vs. 0.97 ± 0.24 L min⁻¹; f_C 116 ± 11 vs. 101 ± 13 vs. 101 ± 12 b min⁻¹), with concomitantly elevated dyspnoea. There were no discernible cadence-mediated effects on diaphragm EMG.

Conclusion

Participants engage in LRC to a greater extent at moderate-high cadences which, in turn, increase respiratory airflow. Cadence rate should be carefully considered when designing aerobic training programmes involving the upper-limbs.

Postural visual dependence in asymptomatic type 2 diabetic patients without peripheral neuropathy during a postural challenging task

Increasing evidence indicates that diabetes may negatively affect vestibular function, and postural control more so under postural challenging conditions. Healthy and diabetic subjects were compared on visual control of posture during a postural challenging task.

METHODS

Forty-eight asymptomatic patients with type 2 diabetes free of peripheral neuropathy and 29 age-matched normal subjects were compared on postural stability on a high-density foam block with computerized posturography. Sway parameters were measured and Romberg ratios calculated and compared between the two groups.

RESULTS

For subjects who succeeded in maintaining balance, all sway parameters were larger in the diabetics with vision available. With eye closure, only the sway area was almost significantly larger and of greater variability in diabetics, but with a smaller and less variable Romberg ratio. Among the two groups and visual conditions, the tightest anterioposterior-mediolateral (AP-ML) coupling was found in diabetics during eye closure. Differences in anthropometric factors did not influence postural sway.

CONCLUSIONS

Despite the smaller Romberg ratios in diabetics than controls, findings still suggest greater but masked postural visual dependence in diabetics faced with postural challenging situations due to subclinical vestibular deficits. They also indicate that diabetics may be vulnerable before any clinical signs of peripheral neuropathy arise to falls on unstable surfaces especially in poorly lit areas, and may require to employ other complex postural tactics such as stiffening to maintain their balance.

Does the supplementary motor area keep patients with Ondine's curse syndrome breathing while awake?

BACKGROUND

Congenital central hypoventilation syndrome (CCHS) is a rare neuro-respiratory disorder associated with mutations of the PHOX2B gene. Patients with this disease experience severe hypoventilation during sleep and are consequently ventilator-dependent. However, they breathe almost normally while awake, indicating the existence of cortical mechanisms compensating for the deficient brainstem generation of automatic breathing. Current evidence indicates that the supplementary motor area plays an important role in modulating ventilation in awake normal humans. We hypothesized that the wake-related maintenance of spontaneous breathing in patients with CCHS could involve supplementary motor area.

METHODS

We studied 7 CCHS patients (5 women; age: 20-30; BMI: 22.1 ± 4 kg.m(-2)) during resting breathing and during exposure to carbon dioxide and inspiratory mechanical constraints. They were compared with 8 healthy individuals. Segments of electroencephalographic tracings were selected according to ventilatory flow signal, from 2.5 seconds to 1.5 seconds after the onset of inspiration. After artefact rejection, 80 or more such segments were ensemble averaged. A slow upward shift of the EEG signal starting between 2 and 0.5 s before inspiration (pre-inspiratory potential) was considered suggestive of supplementary motor area activation.

RESULTS

In the control group, pre-inspiratory potentials were generally absent during resting breathing and carbon dioxide stimulation, and consistently identified in the presence of inspiratory constraints (expected). In CCHS patients, pre-inspiratory potentials were systematically identified in all study conditions, including resting breathing. They were therefore significantly more frequent than in controls.

CONCLUSIONS

This study provides a neurophysiological substrate to the wakefulness drive to breathe that is characteristic of CCHS and suggests that the supplementary motor area contributes to this phenomenon. Whether or not this "cortical breathing" can be taken advantage of therapeutically, or has clinical consequences (like competition with attentional resources) remains to be determined.

Coordination of breathing with nonrespiratory activities

Many articles in this section of Comprehensive Physiology are concerned with the development and function of a central pattern generator (CPG) for the control of breathing in vertebrate animals. The action of the respiratory CPG is extensively modified by cortical and other descending influences as well as by feedback from peripheral sensory systems. The central nervous system also incorporates other CPGs, which orchestrate a wide variety of discrete and repetitive, voluntary and involuntary movements. The coordination of breathing with these other activities requires interaction and coordination between the respiratory CPG and those governing the nonrespiratory activities. Most of these interactions are complex and poorly understood. They seem to involve both conventional synaptic crosstalk between groups of neurons and fluid identity of neurons as belonging to one CPG or another: neurons that normally participate in breathing may be temporarily borrowed or hijacked by a competing or interrupting activity. This review explores the control of breathing as it is influenced by many activities that are generally considered to be nonrespiratory. The mechanistic detail varies greatly among topics, reflecting the wide variety of pertinent experiments.

Different respiratory behaviors disclosed by induced bronchoconstriction in mild asthma patients

Respiratory responses to bronchoconstriction in asthma have been partially assessed and their significance is unclear. In 44 mild asthma patients we investigated respiratory responses during increasing levels of methacholine-induced bronchoconstriction. Inspiratory muscle activity, tidal volume, inspiratory and expiratory times were continuously monitored; breathing discomfort was rated. Mean inspiratory flow, respiratory frequency and ventilation were calculated. Lung function was assessed prior to and at maximum bronchoconstriction. Bronchoconstriction "dose-dependently" increased inspiratory muscle activity and breathing discomfort (P<0.01). In 37 patients (84.1%), the increase in inspiratory muscle activity was associated with increases in mean inspiratory flow and ventilation (P<0.01) because of selective rises in breathing depth (volume responders), or rate (frequency responders) or both (dual responders). In seven patients (15.9%) ventilation was unchanged. Individual respiratory responses were reproducible. With bronchoconstriction, frequency responders displayed greater hyperinflation and stronger breathing discomfort than volume responders (P<0.01). Analysis of the responses to induced bronchoconstriction disclosed distinctive and reproducible respiratory adjustments that may identify functionally different asthma subpopulations.

Effect of airway control by glottal structures on postural stability

Maintenance of upright posture involves complex neuromotor processes that include control of thoracic and abdominal pressures. Control of airflow by glottal structures is a primary determinant of thoracic pressure and may have a role in control of postural stability. This study aimed to investigate the effect of modulation of airway control on upright postural stability during postural perturbations. Standing balance was gently perturbed in the sagittal plane during 7 breathing/voicing tasks that ranged from completely closed (breath-hold), to partially opened (voicing) or completely open (sigh) glottal conditions in 11 healthy adults. Dependent measures were peak amplitude of displacement of the thorax and center of pressure (CoP). When the glottis was completely open during sigh, thoracic displacement in response to the perturbation was greater than in all other conditions, regardless of direction of perturbation (post hoc, all P < 0.002). The absolute amplitude of CoP displacement was greater with backward perturbation (main effect, Direction P = 0.001) and was greater at both extremes of glottal modulation (glottis closed and completely open) than when the glottis was partially opened during counting out loud (post hoc, all P < 0.04). These results show that airway modulation affects postural control during upright perturbations. The thorax was more stable when the glottis was engaged than when it was required to remain open, whereas control of CoP displacement appeared more optimal during the natural dynamic mid-range airway modulation of voicing. These data suggest that glottal control influences balance, and that glottal control strategies may be an important consideration for patients with breathing and/or balance disorders.

Neural respiratory drive measured during inspiratory threshold loading and acute hypercapnia in healthy individuals

Understanding the effects of respiratory load on neural respiratory drive and respiratory pattern are key to understanding the regulation of load compensation in respiratory disease. The aim of the study was to examine and compare the recruitment pattern of the diaphragm and parasternal intercostal muscles when the respiratory system was loaded using two methods. Twelve subjects performed incremental inspiratory threshold loading up to 50% of their maximal inspiratory pressure, and 10 subjects underwent incremental, steady-state hypercapnia to a maximal inspired CO2 of 5%. The diaphragmatic electromyogram (EMGdi) was measured using a multipair oesophageal catheter, and the parasternal intercostal muscle EMG (sEMGpara) was recorded from bipolar surface electrodes positioned in the second intercostal space. The EMGdi and sEMGpara were analysed over the last minute of each increment of both protocols, normalized using the peak EMG recorded during maximal respiratory manoeuvres and expressed as EMG%max. The EMGdi%max and sEMGpara%max increased in parallel during the two loading methods, although EMGdi%max was consistently greater than sEMGpara%max in both conditions, inspiratory threshold loading [bias (SD) 9 (3)%, 95% limits of agreement 4-15%] and hypercapnia [bias (SD) 6 (3)%, 95% limits of agreement -0.05 to 12%]. Inspiratory threshold loading resulted in more pronounced increases in mean (SD) EMGdi%max [10 (7)-45 (28)%] and sEMGpara%max [5.3 (3.1)-40 (28)%] from baseline compared with EMGdi%max [7 (4)-21 (8)%] and sEMGpara%max [4.7 (2.3)-10 (4)%] during hypercapnia, despite comparable levels of ventilation. These data support the use of sEMGpara%max, as a non-invasive alternative to EMGdi%max recorded with an invasive oesophageal electrode catheter, for the quantification of neural respiratory drive. This technique should make evaluation of respiratory muscle function easier to undertake and therefore more readily acceptable in patients with respiratory disease, in whom transduction of neural respiratory drive to pressure generation can be compromised.

Effects of body positions on respiratory muscle activation during maximal inspiratory maneuvers

We evaluated the maximal mouth inspiratory pressure and the EMG patterns of major respiratory and accessory muscles used in the generation of voluntary inspiratory maneuvers during different body positions. Ten healthy subjects (F/M-4/6), the mean age 22.000B10.6 years, participated in the study. The maximal inspiratory mouth pressure (MIP) during Müller's maneuver was measured from residual volume in the standing, sitting, right-sided (RSL) and left-sided lying (LSL), supine, and head-down-tilt (HDT) (3000B0; relatively horizon) positions. EMG of the diaphragmatic (D), parasternal (PS), sternocleidomastoid (SM), and genioglossus (GG) muscles were assessed in each body position. The baseline MIP was 105.3 00B1; 12.0 in men and 59.9 00B110.1 cmH(2)O in women in the standing position and did not appreciable differ in the other positions, except the HDT where it was lower by 23 and 27% in men and women, respectively (P003C0.05). During Müllers maneuver, diaphragmatic EMG activity also was similar in all the body positions, but it was significantly enhanced in the HDT. In contrast, PS EMG showed the highest level of activation in the standing position, taken as the control, reference level, and was lower in the HDT. Activation of SM during the maneuver was near the control in the sitting position, lower in the supine (79%), RSL (85%), LSL (80%), and HDT (72%) positions (P 003C0.05). GG EMG was significantly greater during maximal inspiratory effort in the supine and HDT positions (125and 130%, respectively), while it was lower in the sitting, LRS, and LLS positions (76, 57, and 43%) compared with standing (P 003C; 0.05). We conclude that the inspiratory pressure generated during Muller maneuver is a reflection of complex interactions between several muscle groups during changes in body positions.

Effects of breathing on multijoint control of center of mass position during upright stance

Breathing exerts destabilizing effects on postural stability during quiet stance. The authors conducted an experiment to evaluate the role of motor abundance in the organization of the postural synergy compensating for the effects of breathing. They measured the kinematic pattern of covariation among the major joints of the postural chain (ankle, knee, hip, trunk, and neck) as a function of different breathing modes (spontaneous, paced, and increased volume) and different stance instructions (still vs. relaxed) using the uncontrolled manifold approach. Joint variability was structured to preserve the stable position of the center of mass. This result supports the hypothesis that motor abundance of the postural chain plays an important role in the flexible compensation for breathing during quiet stance.

Respiratory muscle plasticity

Muscle plasticity is defined as the ability of a given muscle to alter its structural and functional properties in accordance with the environmental conditions imposed on it. As such, respiratory muscle is in a constant state of remodeling, and the basis of muscle's plasticity is its ability to change protein expression and resultant protein balance in response to varying environmental conditions. Here, we will describe the changes of respiratory muscle imposed by extrinsic changes in mechanical load, activity, and innervation. Although there is a large body of literature on the structural and functional plasticity of respiratory muscles, we are only beginning to understand the molecular-scale protein changes that contribute to protein balance. We will give an overview of key mechanisms regulating protein synthesis and protein degradation, as well as the complex interactions between them. We suggest future application of a systems biology approach that would develop a mathematical model of protein balance and greatly improve treatments in a variety of clinical settings related to maintaining both muscle mass and optimal contractile function of respiratory muscles.

Effect of precision aiming on respiration and the postural-respiratory synergy

Precision manual tasks require a stable postural background which might be facilitated by respiratory modulations. We investigated the influence of performing a manual precision aiming task on respiratory rate and dynamics, and the coherence between respiration and center of pressure (COP) fluctuations (i.e., the postural-respiratory synergy). Participants aimed a pointer at targets of different sizes while seated or standing. Respiratory rate increased during the aiming period compared to a pre-task phase, but did not vary as a function of aiming difficulty. Recurrence quantification analysis revealed an increased incidence of slowly changing periods of chest movements during the most difficult aiming condition, which required the highest level of manual precision. Aiming, irrespective of difficulty, led to increases in the regularity of the respiratory pattern. Increases in respiratory rate during aiming were accompanied by an increased level of coherence for the seated but not the standing posture, suggesting that task demands affect the organization of coordination across the postural-respiratory synergy. Functional demands of the task likely shape the effectiveness of compensation for respiration during precision aiming.

Role of the diaphragm in trunk rotation in humans

The objectives of the present study were to test the hypothesis that the costal diaphragm contracts during ipsilateral rotation of the trunk and that such trunk rotation increases the motor output of the muscle during inspiration. Monopolar electrodes were inserted in the right costal hemidiaphragm in six subjects, and electromyographic (EMG) recordings were made during isometric rotation efforts of the trunk to the right ("ipsilateral rotation") and to the left ("contralateral rotation"). EMG activity was simultaneously recorded from the parasternal intercostal muscles on the right side. The parasternal intercostals were consistently active during ipsilateral rotation but silent during contralateral rotation. In contrast, the diaphragm was silent in the majority of rotations in either direction, and whenever diaphragm activity was recorded, it involved very few motor units. In addition, whereas parasternal inspiratory activity substantially increased during ipsilateral rotation and decreased during contralateral rotation, inspiratory activity in the diaphragm was essentially unaltered and the discharge frequency of single motor units in the muscle remained at 13-14 Hz in the different postures. It is concluded that 1) the diaphragm makes no significant contribution to trunk rotation and 2) even though the diaphragm and parasternal intercostals contract in a coordinated manner during resting breathing, the inspiratory output of the two muscles is affected differently by voluntary drive during trunk rotation.