Expulsive Maneuvers Research Articles

Impact of sarcopenia on diaphragm muscle fatigue.

What is the central question of this study? Is the residual force generated by the diaphragm muscle after repeated activation reduced with sarcopenia, and is the residual force generated after fatiguing activation sufficient to sustain ventilatory behaviours of diaphragm muscle in young and old rats? What is the main finding and its importance? After diaphragm muscle fatigue, the residual specific force after 120s of repeated stimulation was unaffected by ageing and was sufficient to accomplish ventilatory behaviours, but not expulsive manoeuvres (e.g. coughing). The inability to perform expulsive behaviours might underlie the increased susceptibility of older individuals to respiratory tract infections. Type IIx and/or IIb diaphragm muscle (DIAm) fibres make up more fatigable motor units that are more vulnerable to sarcopenia, i.e. age-associated reductions of specific force and cross-sectional area. In contrast, type I and IIa DIAm fibres form fatigue-resistant motor units that are relatively unchanged with age. The fatigue resistance of the DIAm is assessed by normalizing the residual force generated after a period of repeated supramaximal stimulation (e.g. 120s) to the initial maximal force. Given that sarcopenia primarily affects more fatigable DIAm motor units, apparent fatigue resistance improves with ageing. However, the central question is whether there is an ageing-related difference in the residual force generated by the DIAm after repeated stimulation and whether this force is sufficient to sustain ventilatory behaviours of DIAm. In 6- and 24-month-old Fischer 344 rats, we assessed the loss of ex vivo DIAm force throughout 120s of repeated supramaximal stimulation at 10, 40 and 75Hz. We found that relative fatigue resistance improved in older rats at 40 and 75Hz stimulation. Across all stimulation frequencies, DIAm residual force was unchanged with age (∼5Ncm-2 ). We conclude that ageing increases the relative contribution of type I and IIa fibres to DIAm force, with decreased contributions of type IIx and/or IIb fibres. The residual force generated by the DIAm after repeated stimulation is sufficient to accomplish ventilatory behaviours, regardless of age.

Age does not increase muscle fatigue resistance of the diaphragm

The diaphragm muscle (DIAm) comprises a mixed population of motor units, with FInt and FF units (type IIx and/or IIb fibers) vulnerable to sarcopenia ‐ age‐associated reductions of specific force and cross‐sectional area (CSA). Fatigue‐resistant S and FR units (type I and IIa fibers) are relatively spared. There is controversy as to whether aging confers an enhanced resistance to DIAm fatigue, with interpretations relying on relative measurements of force. We assessed the loss of DIAm force in 6‐ and 24‐month old Fischer 344 rats at 10, 40 and 75 Hz stimulation in 330 ms duration trains repeated each s (33% duty cycle) across a 2 min period. Although the initial specific force of the DIAm was lower in older rats across all stimulation frequencies, the final residual DIAm specific force after 2 min of stimulation was the same (~5 N/cm2) at all ages. This gave the misconception of an improved fatigue resistance, especially at 40 and 75 Hz. We also found that aging is associated with an increase in the relative contributions of type I and IIa fiber CSA to DIAm mass with decreased contributions of type IIx and/or IIb fibers. When these changing contributions are factored into a fiber type‐specific model of DIAm force generation, we found that ventilatory behaviors require activation of only type I and IIa fibers, regardless of age or fatigue. By contrast, the model predicts that aging impairs the ability of the DIAm to effectively perform expulsive manoeuvres, even in the non‐fatigued condition. In summary, relative fatigue measurements are not indicative of improved fatigue resistance in aging and are an artifact of reduced initial specific force contributions from FInt and FF units. These results are consistent with previous studies showing the relative vulnerability of FInt and FF units and conserved ventilatory behaviors in aging DIAm.Support or Funding InformationR01‐AG044615This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Sex differences in diaphragmatic fatigue and the metaboreflex following inspiratory pressure-threshold loading.

Sex differences in diaphragmatic fatigue and the metaboreflex following inspiratory pressure-threshold loading.

Blood Shift During Cough: Negligible or Significant?

Rationale: It was reported how forceful rhythmic coughing can provide effective blood flow during ventricular fibrillation without direct chest compression. This mechanism of cough-assisted cardiopulmonary resuscitation constitutes a form of “cardiac massage” secondary to the intrathoracic and intra-abdominal pressure changes during cough. We have previously shown that significant blood shifts (BSs) occurs from the thorax to the extremities during expulsive maneuvers and that abdominal pressure controls the outflow of blood from the splanchnic vasculature. This mechanism was called abdominal circulatory pump. BS was quantified by using double body plethysmography (DBP), which combines total body plethysmography and opto-electronic plethysmography.Aim: We hypothesized that coughing activates also the abdominal circulatory pump, being an additional mechanism that displaces a circulatory output sufficient to maintain consciousness in a patient with a non-beating heart.Methods and Results: We studied seven healthy subjects (age: 28.6 ± 2.5 years) during series of voluntary coughs at three different operating volumes: after a spontaneous tidal volume, at total lung capacity (TLC) and at an intermediate volume. BS from the thorax to the extremities were measured by DBP during quiet breathing and during cough at each operating lung volume. BS during cough resulted significantly higher than during quiet breathing (p < 0.05). During the compressive phase, the blood outflow is around 200 ml, whereas during the expulsive phase BS increased (p < 0.05) with increasing operating volume, being almost 700 ml at TLC. At lower operating volume it is almost 400 ml.Conclusion: Deep, vigorous coughing and the consequent fluctuations in intra-thoracic and intra-abdominal pressure activate both the thoracic and the abdominal pump mechanism. The former leads the low-resistance pulmonary veins to empty into the left heart. The latter can generate a circulatory output from the splanchnic region, which acts as a blood reservoir, to other body tissues. These findings might help to better understand the cardiopulmonary interactions during cough, particularly in patients with unstable cardiac function, and the mechanism by which coughing during unstable cardiac rhythms can maintain consciousness in human subjects.

Concomitant ventilatory and circulatory functions of the diaphragm and abdominal muscles

Expulsive maneuvers (EMs) caused by simultaneous contraction of diaphragm and abdominal muscles shift substantial quantities of blood from the splanchnic circulation to the extremities. This suggests that the diaphragm assisted by abdominal muscles might accomplish ventilation and circulation simultaneously by repeated EMs. We tested this hypothesis in normal subjects by measuring changes (Δ) in body volume (Vb) by whole body plethysmography simultaneously with changes in trunk volume (Vtr) by optoelectronic plethysmography, which measures the same parameters as whole body plethysmography plus the volume of blood shifts (Vbs) between trunk and extremities: Vbs = ΔVtr-ΔVb. We also measured abdominal pressure, pleural pressure, the arterial pressure wave, and cardiac output (Qc). EMs with abdominal pressure ~100 cmH(2)O for 1 s, followed by 2-s relaxations, repeated over 90 s, produced a "stroke volume" from the splanchnic bed of 0.35 ± 0.07 (SD) liter, an output of 6.84 ± 0.75 l/min compared with a resting Qc of 5.59 ± 1.14 l/min. Refilling during relaxation was complete, and the splanchnic bed did not progressively empty. Diastolic pressure increased by 25 mmHg during each EM. Between EMs, Qc increased to 7.09 ± 1.14 l/min due to increased stroke volume and heart rate. The circulatory function of the diaphragm assisted by simultaneous contractions of abdominal muscles with appropriate pressure and duration at 20 min(-1) can produce a circulatory output as great as resting Qc, as well as ventilation. These combined functions of the diaphragm have potential for cardiopulmonary resuscitation. The abdominal circulatory pump can act as an auxiliary heart.

The The abdominal muscles – a functional review

The role of the abdominal muscles in human function as wellas their physiological mechanisms for these has been the subject of an increasing volume of reported research for more than 20 years. The abdominal muscles participate in a wide range of diverse functional tasks.These include breathing, postural stability, expulsive maneuvers and speech control. To perform these functions effectively and often concurrently, based on demand, the muscles are anatomically, physiologically and biochemical suited. The idea that the abdominal muscles function as a unit has been dispelled by many scientific reports. It is now known that they function differentially as individual muscles and also differentially at the motor unit level. Regional differences in the activation patterns of these muscles are also known. In addition the motor units within muscles are functionally organized to either subserve a specific function or more than one function. Knowledge of the functional organization of the abdominal muscles is critical to any rehabilitation process especially since rehabilitation is holistic and strives to achieve functional goals. This functional review is based on current scientific evidence for the multi functionality of all abdominal muscles. Literature based on animal studies has been excluded as far as is possible.

Effect of increased diaphragm activation on diaphragm power spectrum center frequency

Increased transdiaphragmatic pressure, reduced muscle blood flow, and increased duty cycle have all been associated with a reduction in the center frequency (CFdi) of the diaphragm's electrical activity (EAdi). However, the specific influence of diaphragm activation on CFdi is unknown. We evaluated whether increased diaphragm activation would result in a greater decline in the CFdi when pressure-time product (PTPdi) was kept constant. Five healthy subjects performed periods of intermittent quasi-static diaphragmatic contractions with a fixed duty cycle. In separate runs, subjects targeted transdiaphragmatic pressures (Pdi) by performing end-inspiratory holds with the glottis open and expulsive maneuvers at end-expiratory lung volume (EELV). Diaphragm activation and pressures were measured with an electrode array and balloons mounted on an esophago-gastric catheter, respectively. The EAdi, which was 25+/-8%(S.D.) of maximum at EELV, increased to 61+/-8% (P<0.001) when an identical Pdi (averaging 31+/-13 cmH2O) was generated at a higher lung volume (77% of inspiratory capacity). The latter was associated with a 17% greater decline in CFdi (P=0.012). In order to reproduce at EELV, the decrease in CFdi observed at the increased lung volume, a two-fold increase in PTPdi was required. We conclude that CFdi responds specifically to increased diaphragm activation when pressure-time product remains constant.

Assessment of abdominal muscle contractility, strength, and fatigue.

We evaluated abdominal muscle contractility and fatigue by measuring twitch gastric pressure (Pgat) after percutaneous supramaximal electrical stimulation of the abdominal wall before and after sit-ups to task failure. Mouth pressures during maximal voluntary expulsive maneuvers (PEmax) at TLC and FRC with superimposed twitches, and maximum voluntary ventilation (MVV) were also assessed. Mean fresh Pgat was 36.1 +/- 3.0 cm H2O with a coefficient of variation that ranged between 3.0 to 4.8%. Pgat decreased by 25% (p < 0.001) and 37% (p < 0.001) at 1 and 30 min after sit-ups. During maximal voluntary contraction twitch occlusion never occurred. PEmax at TLC and FRC decreased by 15% (p < 0.001) and 11% (p < 0.017) at 1 min, and 8% (p < 0.036) and 9% (p < 0.030) at 30 min after sit-ups, respectively. Despite the abdominal muscle fatigue, MVV values at 1 and 30 min after sit-ups were not significantly different from the value obtained before the sit-ups. We conclude that (1) Pgat is a useful objective indicator of abdominal muscle contractility and fatigue; (2) during maximal voluntary expulsive maneuvers the abdominal muscles are never fully activated; (3) sit-ups lead to substantial low-frequency fatigue but little high-frequency fatigue of the abdominal muscles, which has little effect on maximal breathing capacity.

Regulation of ventilatory muscle blood flow.

The ventilatory muscles perform various functions such as ventilation of the lungs, postural stabilization, and expulsive maneuvers (e.g., coughing). They are classified in functional terms as inspiratory muscles, which include the diaphragm, parasternal intercostal, external intercostal, scalene, and sternocleidomastoid muscles; and expiratory muscles, which include the abdominal muscles, internal intercostal, and triangularis sterni. The ventilatory muscles require high-energy phosphate compounds such as ATP to fuel the biochemical and physical processes of contraction and relaxation. Maintaining adequate intracellular concentrations of these compounds depends on adequate intracellular substrate levels and delivery of these substrates by arterial blood flow. In addition to the delivery of substrates, blood flow influences muscle function through the removal of metabolic by-products, which, if accumulated, could exert negative effects on several excitatory and contractile processes. Skeletal muscle substrate utilization is also dependent on the ability to extract substrates from arterial blood, which, in turn, is accomplished by increasing the total number of perfused capillaries. It follows that matching perfusion to metabolic demands is critical for the maintenance of normal muscle contractile function. In this article, I review the factors that influence ventilatory muscle blood flow. Major emphasis is placed on the diaphragm because a large number of published reports deal with diaphragmatic blood flow. The second reason for focusing on the diaphragm is because it is the largest and most important inspiratory muscle.

Transmission of pressure within the abdomen.

The extent to which transmission of pressure within the abdomen is accomplished in accordance with the laws of fluid mechanics, i.e., homogeneous transmission to all portions of the abdomen, is controversial. To examine the cranial-to-caudal as well as side-to-side transmission of pressure within the abdomen in humans, we measured intra-abdominal pressure at four sites in five subjects undergoing colonoscopy. Liquid-filled catheters were inserted into the colon, and intracolonic pressure was measured in the rectum and in transverse, descending, and sigmoid colon. Differences in intracolonic pressure were recorded during breaths to total lung capacity and brief expulsive maneuvers. Measurements were taken in the supine, right lateral, and seated position. Comparison of pressure swings at all sites showed that the pressure changes were nearly equal during both inspiratory and expulsive maneuvers. The changes in pressure were uniform in the cephalocaudal axis as from side to side. We conclude that transmission of abdominal pressure in humans is nearly homogeneous. Our findings provide support for the hydraulic model of abdominal mechanics.

Dynamic changes in the zone of apposition and diaphragm length during maximal respiratory efforts.

Clinical tests of diaphragmatic strength are limited by the wide normal variation in maximal pressure which result, in part, from changes in diaphragmatic length. During relaxation at different lung volumes diaphragmatic length (LDI) can be estimated from the length of the zone of apposition (LZapp) and the transverse diameter of the rib cage (DRC). A study was carried out in two subjects using sequential digital radiography at six frames/second to determine whether these relations apply during maximal respiratory efforts which distort the rib cage and diaphragm. The length of the anteroposterior contour of the diaphragm and DRC were determined by curve fitting. LZapp was measured with a millimetre rule. A significant correlation was found between LDI and LZapp during both maximal inspiratory and expulsive manoeuvres (R2 = 0.88 and 0.52). LDI was estimated from the measurements of LZapp and DRC using a multiple regression equation derived from measurements during static relaxation. Despite the complex dynamic events at the onset of these "static" manoeuvres, actual LDI correlated strongly with derived LDI using all data for the two manoeuvres in each subject (R2 = 0.95 and 0.84). Measurements with ultrasonography (12 cm linear probe) and magnetometers confirmed the changes in LZapp and DRC during inspiratory and expulsive efforts. Non-invasive measurements of LZapp and DRC can be used to derive an accurate estimate of diaphragmatic length under dynamic conditions.

Evaluation of isotope proctography in constipated subjects

Twelve patients with longstanding constipation were examined by isotope proctography. Radio-labelled potato mash was inserted rectally to provoke the urge to defaecate and expulsive manoeuvres were recorded using a Gamma-camera. The method illustrated dynamic alterations in the anorectal angles (ARA) which became more acute on 'squeezing', less so on coughing and more obtuse on straining, and maximally so on evacuation. The pelvic floor (PF) movements were consistently upwards on squeezing, less so on coughing, downwards on straining, and maximally so on evacuation. A useful addition was the measurement of the completeness of evacuation and the time involved. The results were comparable to radiological videoproctography (P < 0.01), but the isotope method gave greater discrimination for both ARA and PF movement changes. It also allowed correlations to be made between the pelvic floor descent (P < 0.05) and anorectal angle changes (P < 0.01) with rectal evacuation.

Effect of fatigue on maximal inspiratory pressure-flow capacity.

The inspiratory muscles can be fatigued by repetitive contractions characterized by high force (inspiratory resistive loads) or high velocities of shortening (hyperpnea). The effects of fatigue induced by inspiratory resistive loaded breathing (pressure tasks) or by eucapnic hyperpnea (flow tasks) on maximal inspiratory pressure-flow capacity and rib cage and diaphragm strength were examined in five healthy adult subjects. Tasks consisted of sustaining an assigned breathing frequency, duty cycle, and either a "pressure-time product" of esophageal pressure (for the pressure tasks) or peak inspiratory flow rate (for the flow tasks). Esophageal pressure was measured during maximal inspiratory efforts against a closed glottis (Pesmax), maximal transdiaphragmatic pressure was measured during open-glottis expulsive maneuvers (Pdimax), and maximal inspiratory flow (VImax) was measured during maximal inspiratory efforts with no added external resistance before and after fatiguing pressure and flow tasks. The reduction in Pesmax) with pressure fatigue (-25 +/- 7%) was significantly greater than the change in Pesmax with flow fatigue (-8 +/- 8%, P less than 0.01). In contrast, the reductions in Pdimax (-11 +/- 8%) and VImax (-16 +/- 3%) with flow fatigue were greater than the changes in Pdimax (-0.6 +/- 4%, P less than 0.05) or VImax (-3 +/- 4%, P less than 0.05) with pressure fatigue. We conclude that respiratory muscle performance is dependent not only on the presence of fatigue but whether fatigue was induced by pressure tasks or flow tasks. The specific impairment of Pesmax and not of Pdimax or flow with pressure fatigue may reflect selective fatigue of the rib cage muscles.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of human respiratory muscles during different voluntary manoeuvres.

1. This study used three techniques (bilateral phrenic nerve stimulation, motor cortex stimulation and quantitative electromyography) to assess the degree of activation of the diaphragm, intercostal-accessory muscles and abdominal muscles during postural tasks and respiratory manoeuvres. They included maximal inspiratory and expulsive efforts. 2. Bilateral phrenic nerve stimuli at supramaximal levels produced an average change in transdiaphragmatic pressure (Pdi) of 28 cmH2O during relaxation. During maximal inspiratory or expulsive efforts, all subjects were able to activate the diaphragm fully at functional residual capacity as judged by the failure of stimuli delivered during the voluntary efforts to increase Pdi. Peak voluntary Pdi was about 30% less for inspiratory than expulsive manoeuvres. 3. By contrast, transcranial activation of motor cortical output to the diaphragm and abdominal muscles produced an increment in abdominal pressure of 25 +/- 7 cmH2O during maximal voluntary expulsive efforts. Given the lack of response to phrenic nerve stimulation at similar voluntary pressures, this suggests that abdominal muscles, and not the diaphragm, fail to generate their full contractile force during maximal voluntary expulsive manoeuvres. 4. Motor cortical stimulation during weak inspiratory efforts produced a small reduction in oesophageal pressure (i.e. increase in net inspiratory force) of 7-14 cmH2O. This response could not be extinguished during maximal voluntary inspiratory efforts in two of three subjects. This occurred despite the cortical co-activation of 'antagonist' muscles in the chest wall and abdomen, and passive transmission of pressure from the abdominal to thoracic compartments. 5. Integrated electromyographic activity (EMG) recorded from abdominal muscles (rectus abdominis, external oblique) was greater during trunk flexion than during maximal expulsive efforts. Similarly, integrated EMG of the intercostal-accessory muscles (sternomastoid, scalenes, parasternal intercostals) was greater during tasks requiring head and/or neck flexion than during the maximal inspiratory efforts. 6. These data show that the diaphragm can be fully activated by the central nervous system during voluntary respiratory tasks but that other agonist 'respiratory' muscles need not be activated fully. Given the complex actions of 'inseries' respiratory muscles revealed here, it is argued that differences in the transdiaphragmatic pressure during various postural and respiratory tasks do not necessarily imply variation in the level of diaphragmatic neural drive.

Abdominal pressure transmission in humans during slow breathing maneuvers

Pressure transmission within the abdomen was studied in four subjects standing and supine, breathing slowly and performing slow breathing maneuvers. Pressures were measured in the stomach and rectum with air-containing balloon-catheter systems Pga(air) and Prec and in the stomach with a water-filled catheter system Pga(liq). Changes in Pga(air), Pga(liq), and Prec were nearly in phase and linearly related. The changes in Pga(liq) and Prec were nearly equal in all maneuvers, whereas the changes in Pga(air) were systematically greater than those of Pga(liq) and Prec during quiet breathing and relaxation maneuvers in the upright position. During expulsive maneuvers in either posture the abdominal shape and stomach position were relatively constant, and the changes in Pga(air) were nearly equal to those of Pga(liq) and Prec. Discrepancies between changes in Pga(air) and the other pressures were consistent with presumed changes in the height of the gastric air bubble. We conclude that abdominal pressure in humans is essentially hydrostatic during quiet breathing and slow breathing maneuvers.

Transversus abdominis muscle function in humans

We used a high-resolution ultrasound to make electrical recordings from the transversus abdominis muscle in humans. The behavior of this muscle was then compared with that of the external oblique and rectus abdominis in six normal subjects in the seated posture. During voluntary efforts such as expiration from functional residual capacity, speaking, expulsive maneuvers, and isovolume "belly-in" maneuvers, the transversus in general contracted together with the external oblique and the rectus abdominis. In contrast, during hyperoxic hypercapnia, all subjects had phasic expiratory activity in the transversus at ventilations between 10 and 18 l/min, well before activity could be recorded from either the external oblique or the rectus abdominis. Similarly, inspiratory elastic loading evoked transversus expiratory activity in all subjects but external oblique activity in only one subject and rectus abdominis activity in only two subjects. We thus conclude that in humans 1) the transversus abdominis is recruited preferentially to the superficial muscle layer of the abdominal wall during breathing and 2) the threshold for abdominal muscle recruitment during expiration is substantially lower than conventionally thought.

Comparison of human motor cortical projections to abdominal muscles and intrinsic muscles of the hand

Percutaneous electrical stimulation of the motor cortex was used to activate rapidly conducting corticofugal pathways to human abdominal muscles. Following cortical stimulation the response latencies for the abdominal muscles were similar to those for limb muscles which are a similar distance from the motor cortex. Cortically evoked responses recorded from the abdominal muscles had the same latency and similar amplitude during several voluntary tasks including expiration, expulsive manoeuvres and trunk flexion. Responses could also be evoked when the chemical drive to breathe was increased by rebreathing. In addition, the properties of the cortical projection to muscles of the abdominal wall were directly compared with those of the projection to the intrinsic muscles of the hand. The latencies of responses in abdominal muscles and intrinsic muscles of the hand were measured during static contractions over a range of strengths in the same subjects (0-100% maximal voluntary contraction, MVC). For both muscle groups, cortically evoked muscle responses of minimal latency occurred when background contractions reached 10-20% MVC with responses of maximal amplitude at 60% MVC. The variability in latency of fifty consecutive responses were similar for the two muscle groups. Furthermore, post-stimulus time histograms for 4 rectus abdominis motoneurones revealed a brief initial excitatory peak of 1.15 ms duration (range 0.96-1.34 ms) following cortical stimulation. The characteristics of this peak are the same as reported for motoneurones of intrinsic hand muscles. These findings demonstrate a powerful rapidly conducting pathway from the motor cortex to the human abdominal muscles. This pathway has many of the same properties as the monosynaptic corticospinal projection to the distal muscles of the upper limb.

Preferential fatigue of the rib cage muscles during inspiratory resistive loaded ventilation.

Because the inspiratory rib cage muscles are recruited during inspiratory resistive loaded breathing, we hypothesized that such loading would preferentially fatigue the rib cage muscles. We measured the pressure developed by the inspiratory rib cage muscles during maximal static inspiratory maneuvers (Pinsp) and the pressure developed by the diaphragm during maximal static open-glottis expulsive maneuvers (Pdimax) in four human subjects, both before and after fatigue induced by an inspiratory resistive loaded breathing task. Tasks consisted of maintaining a target esophageal pressure, breathing frequency, and duty cycle for 3-5 min, after which the subjects maintained the highest esophageal pressure possible for an additional 5 min. After loading, Pinsp decreased in all subjects [control, -128 +/- 14 (SD) cmH2O; with fatigue, -102 +/- 18 cmH2O; P less than 0.001, paired t test]. Pdimax was unchanged (control, -192 +/- 23 cmH2O; fatigue, -195 +/- 27 cmH2O). These data suggest that 1) inability to sustain the target during loading resulted from fatigue of the inspiratory rib cage muscles, not diaphragm, and 2) simultaneous measurement of Pinsp and Pdimax may be useful in partitioning muscle fatigue into rib cage and diaphragmatic components.

Triangularis sterni muscle use in supine humans.

The electrical activity of the triangularis sterni (transversus thoracis) muscle was studied in supine humans during resting breathing and a variety of respiratory and nonrespiratory maneuvers known to bring the abdominal muscles into action. Twelve normal subjects, of whom seven were uninformed and untrained, were investigated. The electromyogram of the triangularis sterni was recorded using a concentric needle electrode, and it was compared with the electromyograms of the abdominal (external oblique and rectus abdominis) muscles. The triangularis sterni was usually silent during resting breathing. In contrast, the muscle was invariably activated during expiration from functional residual capacity, expulsive maneuvers, "belly-in" isovolume maneuvers, static head flexion and trunk rotation, and spontaneous events such as speech, coughing, and laughter. When three trained subjects expired voluntarily with considerable recruitment of the triangularis sterni and no abdominal muscle activity, rib cage volume decreased and abdominal volume increased. These results indicate that unlike in the dog, spontaneous quiet expiration in supine humans is essentially a passive process; the human triangularis sterni, however, is a primary muscle of expiration; and its neural activation is largely coupled with that of the abdominals. The triangularis sterni probably contributes to the deflation of the rib cage during active expiration.