Activity Of Accessory Respiratory Muscles Research Articles

Accessory Muscle Support of Breathing in Young Dystrophin‐deficient mdx Mice

Duchenne muscular dystrophy (DMD) is a genetic neuromuscular disorder that occurs due to the absence of the structural protein dystrophin and secondary consequences. DMD is characterized by severe and progressive skeletal muscle weakness, which extends to the respiratory muscles. In young dystrophin‐deficient mdx mice, peak inspiratory pressure‐generating capacity is preserved despite diaphragm muscle weakness and reduced electromyogram (EMG) activitya. Our overarching hypothesis is that accessory muscle compensation limits ventilatory deficit in early dystrophic disease. Four‐month‐old male wild‐type (n=39) and mdx (n=41) mice were studied. Breathing was assessed in conscious mice using whole body plethysmography during normoxia and chemochallenge (10% O2/6% CO2). Thoracic oesophageal pressure and obligatory (diaphragm and external intercostal) respiratory muscle EMG activities were recorded in urethane (1.7g/kg i.p.) anaesthetised spontaneously breathing mice during baseline and sustained tracheal occlusion (maximal muscle activation). Diaphragm and scalene (accessory respiratory muscle) force output were examined ex vivo. Data were statistically compared using unpaired Student’s t test or two‐way ANOVA with Bonferroni post hoc test. Peak diaphragm force ex vivo (P < 0.0001) and baseline diaphragm EMG activity (P = 0.01) were significantly lower in mdx compared with wild‐type. Minute ventilation was equivalent in age‐matched wild‐type and mdx mice during baseline and chemochallenge (P = 0.1). Peak diaphragm (P < 0.0001) and external intercostal (P = 0.01) EMG activities during obstruction were significantly lower in mdx mice compared to wild‐type, while peak inspiratory pressure was equivalent (P = 0.4). Interestingly, peak scalene muscle force ex vivo was preserved in 4‐month‐old mdx mice (P = 0.8). Despite a decline in diaphragm muscle performance (decreased EMG activity and mechanical weakness) at baseline, ventilatory capacity is preserved in mdx mice. Furthermore, despite lower peak electrical activation of the obligatory muscles of breathing and diaphragm muscle functional deficit, peak inspiratory pressure is preserved in 4‐month‐old mdx mice, confirming our previous findings in 2‐month‐old micea. Our finding of preserved scalene muscle force suggests that compensation may be provided by accessory muscles to support ventilatory and non‐ventilatory behaviours in early dystrophic disease. Current studies are examining the EMG activity of accessory respiratory muscles (e.g. scalene, sternomastoid, cleidomastoid and trapezius) in early and advanced dystrophic disease in mdx mice.References a Burns DP, Murphy KH, Lucking EF, O'Halloran KD. Inspiratory pressure‐generating capacity is preserved during ventilatory and non‐ventilatory behaviours in young dystrophic mdx mice despite profound diaphragm muscle weakness. J. Physiol. 2019;597(3):831‐848.

Does diaphragm ultrasound improve the rapid shallow breathing index accuracy for predicting the success of weaning from mechanical ventilation?

BackgroundThis prospective study investigated whether taking into account diaphragmatic excursion (DE) measured by ultrasonography would improve the performance of the rapid shallow breathing index (RSBI) to predict extubation success or failure. ObjectivesThe aim of the study was to compare the new composite index named the rapid shallow diaphragmatic index (RSDI), and the RSBI measured during a spontaneous breathing trial regarding their ability to predict the need for re-intubation at 72 h. MethodsOne hundred mechanically ventilated patients underwent daily 30-min spontaneous breathing trials (SBTs) under pressure support ventilation of 6 cm H2O and end-expiratory pressure of 0 cm H2O until the SBT was considered successful and followed by extubation. The performances of RSBI (respiratory rate/tidal volume) and of the ratio RSBI/DE measured at 5 and 25 min of the successful SBT were compared in terms of area under the receiver operating characteristics curve (AUC), for predicting extubation success at 72 h. As secondary analysis, extubation and weaning success at 7 d were also considered. As exploratory analyses, predictive indices incorporating both clinical characteristics, the DE, and ultrasound diaphragm thickening fraction (DTF) were investigated. ResultsRSBI and RSBI/DE showed AUCs with 95% confidence intervals consistently extending below 0.50, either at the 5th (0.55 [0.36–0.74] and 0.55 [0.34–0.75], respectively) or at the 25th minute of SBT (0.49 [0.27–0.71] and 0.50 [0.29–0.68], respectively) for predicting weaning success at 72 h or at 7 d (5th min: 0.53 [0.37–0.70] and 0.54 [0.37–0.70], respectively; 25th min: 0.54 [0.37–0.71] and 0.55 [0.39–0.71], respectively). An exploratory index incorporating the accessory respiratory muscle activity, DE, DTF, and respiratory rate at 5th min of SBT showed AUCs for predicting extubation success at 7 d in the 78 patients with DTF measurement (0.77 [0.64–0.90]) that were significantly higher than that of the RSBI (P = 0.017) and RSBI/DE (P < 0.001) in the same respective populations. ConclusionsThe RSBI and the ratio RSBI/DE failed to predict weaning success when measured during an SBT performed under minimal pressure support. Predictive indices incorporating ultrasound DE and DTF may merit further investigation.

Comparison of Physiotherapeutic Intervention Programs on Handgrip Force and Myoelectric Activity in Individuals with COPD

Background: Chronic obstructive pulmonary disease (COPD) modifies the electrical activity of accessory respiratory muscles, also leading to a marked peripheral muscular dysfunction, measured by the handgrip strength taken as an indicator of total body strength. Objective: To compare the effects of two different physiotherapeutic programs on handgrip strength and myoelectric activity in COPD patients. Methods: Participated in the study 17 individuals, randomly allocated into 2 groups: 1) Maneuvers: 62.7±15.4 years, 1.65±0.12 m, 81.4±18.2 kg, BMI: 29.9±5.0 kg/m2 ; 2) Threshold: 64.4±11.2 years, 1.58±0.08 m, 70.7±9.4 kg, BMI: 28.6±3.7 kg/m2 , diagnosed with COPD, submitted to surface electromyographic (sEMG) evaluation of sternocleidomatoid (SCM) and anterior scalene (AS), and handgrip strength, before and after the interventions. The data were analyzed with SPSS (20.0). Pared t-student and Wilcoxon tests were used to compare mean values of the handgrip strength and the Root Mean Square (RMS) of the sEMG signal (SCM and AS muscles), pre- and post-interventions. In addition, independent t-Student and Mann-Whitney tests were used to compare the RMS average of the sEMG signal between the right and left sides. Significant level was set at 5%. Results: When comparing pre and post-intervention values, there was a significant increase for the Threshold group handgrip strength (P=0.037). It was observed a sEMG activity of SCM and AS during dynamometry, vital capacity, forced vital capacity on inspiratory and expiratory pressure (P&lt;0.05) for both analyzed groups. In the comparison of the myoelectric activity between the right and left sides for the SCM and AS, there was a significant difference in handgrip strength (P=0.009) and forced vital capacity (P=0.001) of the SCM muscle, post intervention for the Threshold group. Conclusion: The proposed physiotherapeutic programs improved the overall muscle strength for the Threshold group and the electromyographic activity of SCM and AS in the 2 groups.This study was registered in ReBEC: RBR-4VGP58

Severe diaphragmatic dysfunction with preserved activity of accessory respiratory muscles in a critically ill child: a case report of failure of neurally adjusted ventilatory assist (NAVA) and successful support with pressure support ventilation (PSV)

BackgroundNeurally adjusted ventilatory assist (NAVA) is an alternative to pressure support ventilation (PSV) potentially improving patient-ventilator interaction. During NAVA, diaphragmatic electrical activity (EAdi) is used to trigger the ventilator and perform a proportional respiratory assistance. We present a case in which the presence of severe bilateral diaphragmatic dysfunction led to a failure of NAVA. On the contrary, the preserved activity of the accessory inspiratory muscles allowed a successful respiratory assistance using PSV.Case presentationA 10-year-old girl developed quadriplegia after neurological surgery. Initially, no spontaneous breathing activity was present and volume controlled ventilation was necessary. Two months later spontaneous inspiratory efforts were observed and a maximal negative inspiratory force of − 20 cmH2O was recorded. In addition, a NAVA nasogastric tube was placed. The recorded EAdi signal, despite showing a phasic activity, had a very low amplitude (1–2 μV). Two brief (15 min) breathing trials to compare PSV (pressure support = 8 cmH2O) with NAVA (Gain = 5 cmH2O/μV, inspiratory trigger = 0.3 μV) were performed. On PSV, the patient was well adapted with stable tidal volumes, respiratory rates, minute ventilation, end-tidal and venous carbon dioxide levels. When switched to NAVA, her breathing pattern became irregular and she showed clear sign of increased work of breathing and distress: tidal volume dropped and respiratory rate rose, leading to an increase in total minute ventilation. Nevertheless, end-tidal and venous carbon dioxide rapidly increased (from 49 to 55 mmHg and from 52 to 57 mmHg, respectively). An electromyographic study documented an impairment of the diaphragm with preserved activity of the accessory inspiratory muscles.ConclusionsWe document the failure of mechanical assistance performed with NAVA due to bilateral diaphragmatic dysfunction in a critically ill child. The preserved activity of some accessory respiratory muscles allowed to support the patient effectively with pressure support ventilation, i.e. by applying a pneumatic trigger. The present case underlines (i) the importance of the integrity of the respiratory centers, phrenic nerves and diaphragm in order to perform NAVA and (ii) the possible diagnostic role of EAdi monitoring in complex cases of weaning failure.

Forward trunk lean with arm support affects the activity of accessory respiratory muscles and thoracoabdominal movement in healthy individuals

Despite the reported benefits of postures involving leaning the trunk forward with arm support for relieving dyspnea, how those postures influence the mechanics of breathing remains unclear. In response, the aim of the study reported here was to evaluate how posture (i.e., standing and sitting) and leaning the trunk forward with arm support affect the activity of accessory respiratory muscles and thoracoabdominal movement in healthy individuals. Thirty-five volunteers (15 males and 20 females) aged 18–29 years breathed with the same rhythm in standing and sitting positions while upright and while leaning the trunk forward with arm support. Surface electromyography was performed to assess the activity of accessory inspiratory (i.e., during inspiration) and abdominal (i.e., during inspiration and expiration) muscles, and a motion capture system was used to assess thoracoabdominal movement. Results revealed that upper trapezius activity was significantly lower in forward-leaning postures than in upright ones (P = 005; ηp2 = 0.311), although the activity of the sternocleidomastoideus and scalenus (P < 0.001; ηp2 = 0.427–0.529), along with the anterior-to-posterior movement of the upper ribcage (P < 0.001; ηp2 = 0.546), were significantly greater in forward-leaning postures than in upright ones. The activity of the external oblique and transversus abdominis/internal oblique was significantly lower in sitting than in standing postures (P < 0.050; ηp2 = 0.206–0.641), and though the activity of the transversus abdominis/internal oblique was significantly lower in forward-leaning than in upright postures (P ≤ 0.001; ηp2 = 0.330–0.541), a significantly greater anterior-to-posterior movement of the abdomen was observed (P < 0.001; ηp2 = 0.662). However, the magnitude of the lower ribcage’s medial-to-lateral movement was significantly lower in forward-leaning than in upright postures (P = 0.039; ηp2 = 0.149). Leaning the trunk forward with arm support not only increased the use of accessory inspiratory muscles but also decreased the use of the transversus abdominis/internal oblique, which improved thoracoabdominal movement.

Repeated Measurement of Respiratory Muscle Activity and Ventilation in Mouse Models of Neuromuscular Disease.

Accessory respiratory muscles help to maintain ventilation when diaphragm function is impaired. The following protocol describes a method for repeated measurements over weeks or months of accessory respiratory muscle activity while simultaneously measuring ventilation in a non-anesthetized, freely behaving mouse. The technique includes the surgical implantation of a radio transmitter and the insertion of electrode leads into the scalene and trapezius muscles to measure the electromyogram activity of these inspiratory muscles. Ventilation is measured by whole-body plethysmography, and animal movement is assessed by video and is synchronized with electromyogram activity. Measurements of muscle activity and ventilation in a mouse model of amyotrophic lateral sclerosis are presented to show how this tool can be used to investigate how respiratory muscle activity changes over time and to assess the impact of muscle activity on ventilation. The described methods can easily be adapted to measure the activity of other muscles or to assess accessory respiratory muscle activity in additional mouse models of disease or injury.

Effects of continuous positive airway pressure on diaphragmatic kinetics and breathing pattern in healthy individuals.

In this study, we investigate the changes in diaphragmatic kinetics, breathing pattern and work of breathing induced by 10 cmH2 O of continuous positive airway pressure (CPAP). We used sonography to study diaphragmatic kinetics and measured energy expenditure using indirect calorimetry in 50 healthy volunteers at 0 cmH2 O positive end expiratory pressure (ZEEP) and after application of 10 cmH2 O CPAP. In a subgroup of 14 subjects, the changes in thoracic and abdominal volumes and thoraco-abdominal asynchrony were recorded with inductive plethysmography, while accessory respiratory muscle activity was recorded with electromyography. Continuous positive airway pressure breathing induced acute lung hyperinflation of 600 mL above passive functional residual capacity. This hyperinflation induced changes in diaphragmatic kinetics and breathing pattern; diaphragmatic excursion, thickness and thickness ratio, tidal volume (Vt) and oxygen consumption (VO2) increased while respiratory rate decreased. The increase in Vt with CPAP was mainly due to rib cage contribution. Activation of the accessory inspiratory (scalene) and expiratory (transversus abdominis) muscles was recorded. The raised respiratory muscles workload induced an increase in VO2. In healthy volunteers, CPAP therapy leads to lung overdistention and recruitment of respiratory muscles. These mechanisms operate at a high energy cost.

Effects of forward head posture on forced vital capacity and respiratory muscles activity.

[Purpose] This study investigated the effects of forward head posture on forced vital capacity and deep breathing. [Subjects] Twenty-six subjects, divided into the two groups (normal and forward head posture groups), participated in this study. [Methods] Forced vital capacity and forced expiratory volume in 1 second were measured using respiratory function instrumentation that met the American Thoracic Society’s recommendation for diagnostic spirometry. Accessory respiratory muscle activity during deep breathing was measured by electromyography. A Mann-Whitney test was used to compare the measure variables between the normal and forward head posture group. [Results] Forced vital capacity and forced expiratory volume in 1 second were significantly lower in the forward head posture group than in the normal group. Accessory respiratory muscle activity was also lower in the forward head posture group than in the normal group. In particular, the sternocleidomastoid and pectoralis major activity of the forward head posture group was significantly lower than that of normal group. Activities of the other muscles were generally decreased with forward head posture, but were not significantly different between the two groups. [Conclusion] These results indicate that forward head posture could reduce vital capacity, possibly because of weakness or disharmony of the accessory respiratory muscles.

0449. Surface EMG of extra-diaphragmatic muscles can assess muscle unloading during neurally adjusted ventilatory support

Neurally adjusted ventilatory assist (NAVA) is used to adapt mechanical ventilation to patient demand while unloading respiratory muscles. Titration methods are focused on sustained unloading of the diaphragm while maintaining a stable tidal volume [1]. It is hypothesized that the activity of accessory respiratory muscles can supply extra information about muscle unloading and patient comfort during NAVA ventilation.

Correlation between pulmonary function, posture, and body composition in patients with asthma

AimAsthma may result in postural disorders due to increased activity of accessory respiratory muscles and hyperinflation. Our primary objective was to assess the correlation between pulmonary function and posture in adult patients with asthma. Secondarily, we aimed to study the correlation between body composition and body posture in this group of patients. MethodThis was a cross-sectional study including 34 patients with asthma who were subjected to postural assessment (photogrammetry), pulmonary function testing (spirometry, whole-body plethysmography, diffusing capacity for carbon monoxide, and respiratory muscle strength), and body composition estimation by means of bioelectrical impedance. ResultsMost patients were female (70.6%) with a median age of 32.5 years (range: 23–42 years old). We found a significant correlation between horizontal alignment of head (anterior view) and the ratio of forced expiratory volume in 1s to forced vital capacity (FEV1/FVC; ρ=−0.37; P=0.03), total lung capacity (TLC; ρ=0.42; P=0.01), and residual volume (RV; ρ=0.45; P<0.001). Bronchial obstruction and respiratory muscle strength variables also correlated with postural assessment measures on the right and left lateral views. Both body mass index and the percentage of fat mass were correlated with horizontal alignment of head, horizontal alignment of the pelvis, and the frontal angle of the lower limbs. ConclusionAdult patients with asthma exhibit specific postural disorders that correlate with pulmonary function and body composition. The assessment of postural variables may provide a better pulmonary rehabilitation approach for these patients.

Correlação entre função pulmonar, postura e composição corporal em pacientes com asma

AimAsthma may result in postural disorders due to increased activity of accessory respiratory muscles and hyperinflation. Our primary objective was to assess the correlation between pulmonary function and posture in adult patients with asthma. Secondarily, we aimed to study the correlation between body composition and body posture in this group of patients. MethodThis was a cross-sectional study including 34 patients with asthma who were subjected to postural assessment (photogrammetry), pulmonary function testing (spirometry, whole-body plethysmography, diffusing capacity for carbon monoxide, and respiratory muscle strength), and body composition estimation by means of bioelectrical impedance. ResultsMost patients were female (70.6%) with a median age of 32.5 years (range: 23-42 years old). We found a significant correlation between horizontal alignment of head (anterior view) and the ratio of forced expiratory volume in 1 second to forced vital capacity (FEV1/FVC; ρ=−0,37; P=.03), total lung capacity (TLC; ρ=0,42; P=.01), and residual volume (RV; ρ=0,45; P<.001). Bronchial obstruction and respiratory muscle strength variables also correlated with postural assessment measures on the right and left lateral views. Both body mass index and the percentage of fat mass correlated with horizontal alignment of head, horizontal alignment of the pelvis, and the frontal angle of the lower limbs. ConclusionAdult patients with asthma exhibit specific postural disorders that correlate with pulmonary function and body composition. The assessment of postural variables may provide a better pulmonary rehabilitation approach for these patients.

Bronchoscopy During Non-Invasive Mechanical Ventilation: A Review of Techniques and Procedures

Non-invasive mechanical ventilation (NIMV) is used to treat acute respiratory failure by improving gas exchange abnormalities and reducing the signs of respiratory effort, dyspnea and the activity of accessory respiratory muscles. Bronchoscopy is a key technique in the study of respiratory diseases that is necessary to perform in acute and critical patients, most of the times only after orotracheal intubation (OTI) due to possible complications of the technique. In this review, we evaluate the evidence of NIMV use during bronchoscopy, concluding that its use should be considered in severe patients as an alternative that is capable of preventing the complications related with OTI and mechanical ventilation, especially in patients with chronic obstructive pulmonary disease and with a tendency toward developing hypercapnia.

Broncoscopia durante la ventilación mecánica no invasiva: revisión de técnicas y procedimientos

Non-invasive mechanical ventilation (NIMV) is used to treat acute respiratory failure by improving gas exchange abnormalities and reducing the signs of respiratory effort, dyspnea and the activity of accessory respiratory muscles. Bronchoscopy is a key technique in the study of respiratory diseases that is necessary to perform in acute and critical patients, often times only after orotracheal intubation (OTI) due to possible complications of the technique. In this review, we evaluate the evidence of NIMV use during bronchoscopy, concluding that its use should be considered in severe patients as an alternative that is capable of preventing the complications related with OTI and mechanical ventilation, especially in patients with chronic obstructive pulmonary disease and with a tendency towards developing hypercapnia.

Diaphragm shortening and intrathoracic pressure during hypercapnia in rats

The purpose of this study was to determine the relationship between intrathoracic pressure (ΔITP) and diaphragm shortening (DS) during the development of diaphragm fatigue. Fatigue of the diaphragm was produced by having rats breath 15% CO 2 in O 2. Diaphragm shortening increased significantly to 178% of control during the first 5 min of hypercapnia and then decreased to 86% of control at approximately 80 min. Twenty minutes after terminating hypercapnia, DS increased to 115% of the prehypercapnic value. ΔITP increased to 199% of control following 5 min of hypercapnia and continued to increase, reaching 267% of control at the end of the hypercapnic period. Twenty minutes later, ΔITP was 147% of control. These results illustrate that during increased respiratory work, DS can decrease while intrathoracic pressure remains increased. These findings suggest that intrathoracic pressure may not always reflect the contractile status of the diaphragm. These findings are consistent with other studies indicating that as the diaphragm fatigues, accessory respiratory muscle activity increases to maintain ΔITP.

Respiratory failure and sleep in neuromuscular disease.

Sleep hypoxaemia in non-rapid eye movement (non-REM) and rapid eye movement (REM) sleep was examined in 20 patients with various neuromuscular disorders with reference to the relation between oxygen desaturation during sleep and daytime lung and respiratory muscle function. All the patients had all night sleep studies performed and maximum inspiratory and expiratory mouth pressures (PI and Pemax), lung volumes, single breath transfer coefficient for carbon monoxide (KCO), and daytime arterial oxygen (PaO2) and carbon dioxide tensions (PaCO2) determined. Vital capacity in the erect and supine posture was measured in 14 patients. Mean (SD) PI max at RV was low at 33 (19) cm H2O (32% predicted). Mean PE max at TLC was also low at 53 (24) cm H2O (28% predicted). Mean daytime PaO2 was 67 (16) mm Hg and PaCO2 52 (13) mm Hg (8.9 (2.1) and 6.9 (1.7) kPa). The mean lowest arterial oxygen saturation (SaO2) was 83% (12%) during non-REM and 60% (23%) during REM sleep. Detailed electromyographic evidence in one patient with poliomyelitis showed that SaO2% during non-REM sleep was maintained by accessory respiratory muscle activity. There was a direct relation between the lowest SaO2 value during REM sleep and vital capacity, daytime PaO2, PaCO2, and percentage fall in vital capacity from the erect to the supine position (an index of diaphragm weakness). The simple measurement of vital capacity in the erect and supine positions and arterial blood gas tensions when the patient is awake provide a useful initial guide to the degree of respiratory failure occurring during sleep in patients with neuromuscular disorders. A sleep study is required to assess the extent of sleep induced respiratory failure accurately.

Effect of dorsolateral pontine lesions on diaphragmatic activity during REMS

Muscle atonia is a feature of normal rapid-eye-movement sleep (REMS). The suppression of accessory respiratory muscle activity has been investigated and a role for sleep-disordered breathing hypothesized, but the suppression of diaphragmatic activity has rarely been considered. We hypothesized that the activity of the diaphragm was suppressed by an area of the dorsolateral pons during REMS. Lesions in this region have previously been shown to abolish the atonia of REMS. The diaphragmatic electromyogram (EMG) activity was analyzed in five naturally sleeping cats before and after pontine lesions leading to REMS without atonia. Although respiratory timing parameters were not altered by the lesion, the inspiratory rate of rise was significantly increased in all cats, and the brief pauses (40-100 ms) in the diaphragmatic EMG normally seen in REMS were virtually abolished. We conclude that the dorsolateral pons has a role in suppressing diaphragmatic activation during REMS. This suppression affects the average rate of rise of diaphragmatic activity and also leads to brief intermittent complete cessation of ongoing muscle activity. These decrements in diaphragm activity could jeopardize ventilation during REMS.

Ventilatory Muscle Support in Respiratory Failure with Nasal Positive Pressure Ventilation

Long-term intermittent mechanical ventilation results in improvements in ventilatory performance and clinical status between ventilation sessions in patients with chronic respiratory failure. The application of intermittent positive pressure ventilation through a nasal mask (NPPV) is a simple, noninvasive method for the provision of chronic intermittent ventilatory support. We investigated the effects of NPPV on inspiratory muscle activity in three normal subjects and nine patients with acute or chronic ventilatory failure due to restrictive (four subjects) or obstructive (five subjects) respiratory disorders. NPPV resulted in reductions of phasic diaphragm electromyogram amplitude to 6.7 +/- 0.7 percent (mean +/- SEM) of values obtained during spontaneous breathing in the normal subjects, 6.4 +/- 3.2 percent in the restrictive group, and 8.3 +/- 5.1 percent in the obstructive group. Simultaneous decreases in activity of accessory respiratory muscles were observed. The reductions in inspiratory muscle activity were confirmed by the finding of positive intrathoracic pressure swings on inspiration in all subjects. With NPPV, oxygen saturation and PCO2 remained stable or improved as compared with values obtained during spontaneous breathing. These results indicate that NPPV can noninvasively provide ventilatory support while reducing inspiratory muscle energy expenditure in acute and chronic respiratory failure of diverse etiology. Long-term assisted ventilation with NPPV may be useful in improving ventilatory performance by resting the inspiratory muscles.

Role of Selected Muscles of Respiration as Influenced by Posture and Tidal Volume

Studies of the activity of accessory respiratory muscles were conducted in 30 normal subjects (15 male and 15 female subjects, age 5 to 62 years). Electromyograms were recorded over the right serratus anterior muscle and the adjacent sixthe intercostal space, and the results were correlated with tidal volume. Phasic respiratory activity in the serratus anterior muscle was detected in every subject but was markedly affected by the prevailing posture and level of ventilation (appearing at mean tidal volumes equivalent to 60.5 percent of vital capacity when subjects were standing relaxed, and equivalent to 35.2 percent of vital capacity when subjects were seated, leaning forward with elbows supported). The activity was predominantly inspiratory in timing but in some subjects extended into early expiration. No significant differences were noted in subjects of different ages, sex, height, or weight. Similar results were recorded from the inspiratory intercostal muscle. On the basis of these findings, we conclude that, contrary to reports, the serratus anterior muscle should be classified as an accessory muscle of respiration, at least at increased levels of ventilation, and that its activity is most pronounced in postures that place the muscles' origin in an advantageous position for moving the ribs.