The effect of posture on respiratory activity of the abdominal muscles

The purpose of this study was to determine the influence of posture on the expiratory activity of the abdominal muscles. Fifteen young adult men participated in the study. Activities of the external oblique abdominis, internal oblique abdominis, and rectus abdominis muscles were measured electromyographically in various postures. We used a pressure threshold in order to activate the abdominal muscles as these muscles are silent at rest. A spirometer was used to measure the lung volume in various postures. Subjects were placed in the supine, standing, sitting, and sitting-with-elbow-on-the-knee (SEK) positions. Electromyographic activity and mouth pressure were measured during spontaneous breathing and maximal voluntary ventilation under the respiratory load. We observed that the lung volume changed with posture; however, the breathing pattern under respiratory load did not change. During maximal voluntary ventilation, internal oblique abdominis muscle expiratory activity was lower in the SEK position than in any other position, external oblique abdominis muscle inspiratory activity was lower in the supine position than in any other position, and internal oblique abdominis muscle activity was higher in the standing position than in any other position. During spontaneous breathing, external oblique abdominis muscle activity was higher during expiration and inspiration in the SEK position than in any other position. The internal oblique abdominis muscle activity was higher during both inspiration and expiration in the standing position than in any other position. The rectus abdominis muscle activity did not change with changes in posture during both inspiration and expiration. Increase in the external oblique abdominis activity in the SEK position was due to anatomical muscle arrangement that was consistent with the direction of lower rib movement. On the other hand, increase in the internal oblique abdominis activity in the standing position was due to stretching of the abdominal wall by the viscera. We concluded that differences in activity were due to differences in the anatomy of the abdominal muscles and the influence of gravity.     

Action of abdominal muscles on rib cage in humans

To assess the actions of the rectus abdominis and external oblique muscles on the rib cage in humans, these two muscles were stimulated with surface electrodes in four normal supine subjects at functional residual capacity. Changes in anteroposterior and transverse rib cage diameters and changes in xiphipubic distance were measured with pairs of magnetometers. Stimulation of rectus abdominis produced a marked decrease in the xiphipubic distance and in the anteroposterior diameter, thus making the rib cage more elliptic. In contrast, stimulation of the external oblique caused a decrease in the transverse diameter, making the rib cage more cylindrical. When both muscles were stimulated simultaneously, the resultant rib cage distortion depended on the relative voltage at which each muscle was stimulated. Electromyogram recordings showed that there was no cross contamination or activity of the diaphragm during the muscle stimulations. Transdiaphragmatic pressure increased with the voltage of stimulation, suggesting passive lengthening of the diaphragm. X-ray studies were performed in two subjects and confirmed the main magnetometer findings. These studies thus confirm that the rib cage in humans is more easily distortable than conventionally thought. The abdominal muscles can distort it in either direction depending on which muscles are contracting.     

Activity of respiratory muscles in upright and recumbent humans

It is established that during tidal breathing the rib cage expands more than the abdomen in the upright posture, whereas the reverse is usually true in the supine posture. To explore the reasons for this, we studied nine normal subjects in the supine, standing, and sitting postures, measuring thoracoabdominal movement with magnetometers and respiratory muscle activity via integrated electromyograms. In eight of the subjects, gastric and esophageal pressures and diaphragmatic electromyograms via esophageal electrodes were also measured. In the upright postures, there was generally more phasic and tonic activity in the scalene, sternocleidomastoid, and parasternal intercostal muscles. The diaphragm showed more phasic (but not more tonic) activity in the upright postures, and the abdominal oblique muscle showed more tonic (but not phasic) activity in the standing posture. Relative to the esophageal pressure change with inspiration, the inspiratory gastric pressure change was greater in the upright than in the supine posture. We conclude that the increased rib cage motion characteristic of the upright posture owes to a combination of increased activation of rib cage inspiratory muscles plus greater activation of the diaphragm that, together with a stiffened abdomen, acts to move the rib cage more effectively.     

Aging effect on oxygen consumption of respiratory muscles in humans

The first purpose of the present study was to develop a new method to examine oxygen consumption of respiratory muscles (VO2resp) in human subjects. The apparatus consists of an expandable dead space and a respirometer. When the dead space was increased at a constant rate (approximately 100 ml/min), minute ventilation (VE) and VO2resp increased gradually. Because the logarithm of VO2 was found to be approximately linearly related to VE, we characterized this relationship by the slope (logVO2/VE) and the intercept at VE = 0 (VO2met) of the semilog regression line. The second purpose of this study was to examine the relationship between VO2resp and aging. Six anthropometric and spirometric factors (age, height, weight, vital capacity, forced expiratory volume in 1 s, and body surface area) were analyzed in 37 normal subjects by simple and stepwise multiple regression analyses. We found a significant increase in logVO2/VE and a significant decrease in VO2met with age. In conclusion, 1) the present method is convenient to use, and we are able to study VO2resp over a wide range of ventilation without voluntary effort, and 2) age per se is one of the factors accounting for the observed increase in VO2resp with age.     

Differential response of respiratory muscles to airway occlusion in infants

The effect of end-expiratory occlusion on respiratory muscle activity was studied in 10 unsedated preterm infants during sleep. Electromyograms (EMG) of the upper airway were recorded from surface electrodes placed over the submental (SM) area; diaphragm (DIA) EMGs were obtained with identical electrodes over the right subcostal margin. Phasic SM EMG accompanied 56 +/- 36% of breaths during spontaneous breathing and increased to 80 +/- 26% (P less than 0.05) on the first inspiratory effort after occlusion. Occlusion increased peak amplitude (P less than 0.001) and total duration (P less than 0.005) of the SM EMG without significant changes in its initial rate of rise. In contrast, only the total duration of the DIA EMG increased (P less than 0.005) during occlusion. Inspiratory time increased from 470 +/- 120 to 720 +/- 210 ms (P less than 0.001) during the first occluded effort, but expiratory time did not change. With sustained occlusion, peak amplitude of the SM EMG progressively increased, but DIA EMG only significantly increased by the third occluded effort. Pharyngeal patency was invariably maintained throughout the induced airway occlusions. Sharp bursts of SM EMG activity coincided with resolution of spontaneous obstructive apneic episodes in four infants. The immediate increase in SM EMG associated with airway occlusion may be a mechanism that prevents the development of obstructive apnea.     

Differential expression of sarcoplasmic proteins in four heterogeneous ovine skeletal muscles

Fiber-type distribution is known to vary widely within and between muscles according to differences in muscle functions. 2-DE and MALDI-MS were used to investigate the molecular basis of muscle fiber type-related variability. We compared four lamb skeletal muscles with heterogeneous fiber-type composition that are relatively rich in fast-twitch fiber types, i.e., the semimembranosus, vastus medialis, longissimus dorsi, and tensor fasciae latae (TL). Our results clearly showed that none of the glycolytic metabolism enzymes detected, including TL which was most strongly glycolytic, made intermuscular differentiation possible. Muscle differentiation was based on the differential expression of proteins involved in oxidative metabolism, including not only citric acid cycle enzymes but also other classes of proteins with functions related to oxidative metabolism, oxidative stress, and probably to higher protein turnover. Detected proteins were involved in transport (carbonate dehydratase, myoglobin, fatty acid-binding protein), repair of misfolding damage (heat shock protein (HSP) 60 kDa, HSP-27 kDa, alpha-crystallin beta subunit, DJ1, stress-induced phosphoprotein), detoxification or degradation of impaired proteins (GST-Pi, aldehyde dehydrogenase, peroxiredoxin, ubiquitin), and protein synthesis (tRNA-synthetase). The fractionating method led to the detection of proteins involved in different functions related to oxidative metabolism that have not previously been shown concomitancy.     

Long-term activity in upper- and lower-limb muscles of humans.

Despite limited data on humans, previous studies suggest that there is an association between the duration of daily muscle activity and the proportion of type I muscle fibers. We quantified the activity of limb muscles in healthy men and women during normal use and compared these measurements with published reports on fiber-type proportions. Seven men (age range = 21-28 yr) and seven women (age range = 18-26 yr) participated in two 10-h recording sessions. Electromyogram (EMG) activity of four muscles in nondominant upper (first dorsal interosseus and biceps brachii) and lower limbs (vastus medialis and vastus lateralis) was recorded with surface electrodes. Hand and arm muscles were active for 18% of the recording time, whereas leg muscles were active for only 10% of the recording time. On average, upper-limb muscles were activated 67% more often than lower-limb muscles. When lower-limb muscles were activated, however, the mean amplitude of each burst was greater in leg muscles [18 and 17% maximum voluntary contraction (MVC)] compared with hand (8% MVC) and arm (6% MVC) muscles. Temporal association in activity between pairs of muscles was high for the two lower-limb muscles (r2 = 0.7) and relatively weak for the two upper-limb muscles (r2 = 0.09). Long-term muscle activity was only different between men and women for the biceps brachii muscle. We found no relation between duration of muscle activity in 10-h recordings and the reported values of type I fibers in men and women.     

Contractions of specific abdominal muscles in postural tasks are affected by respiratory maneuvers

Hodges, Paul W., Simon C. Gandevia, and Carolyn A. Richardson. Contractions of specific abdominalmuscles in postural tasks are affected by respiratory maneuvers.J. Appl. Physiol. 83(3): 753-760, 1997.The influence of respiratory activity of the abdominal muscleson their reaction time in a postural task was evaluated. Theelectromyographic (EMG) onsets of the abdominal muscles and deltoidwere evaluated in response to shoulder flexion initiated by a visualstimulus occurring at random throughout the respiratory cycle.Increased activity of the abdominal muscles was produced by inspiratoryloading, forced expiration below functional residual capacity, and astatic glottis-closed expulsive maneuver. During quiet breathing, thelatency between activation of the abdominal muscles and deltoid was notinfluenced by the respiratory cycle. When respiratory activity of theabdominal muscles increased, the EMG onset of transversus abdominis andinternal oblique, relative to deltoid, was significantly earlier formovements beginning in expiration, compared with inspiration [by97-107 ms (P < 0.01) and64-90 ms (P < 0.01),respectively]. However, the onset of transversus abdominis EMGwas delayed by 31-54 ms (P < 0.01) when movement was performed during a static expulsive effort,compared with quiet respiration. Thus changes occur in earlyanticipatory contraction of transversus abdominis during respiratorytasks but they cannot be explained simply by existing activation of themotoneuron pool.

     

Differential ultrastructure of synaptic terminals on ventral longitudinal abdominal muscles in Drosophila larvae

The innervation of ventral longitudinal abdominal muscles (muscles 6, 7, 12, and 13) of third-instar Drosophila larvae was investigated with Nomarski, confocal, and electron microscopy to define the ultrastructural features of synapse-bearing terminals. As shown by previous workers, muscles 6 and 7 receive in most abdominal segments “Type I” endings, which are restricted in distribution and possess relatively prominent periodic terminal enlargements (“boutons”); whereas muscles 12 and 13 have in addition “Type II” terminals, which are more widely distributed and have smaller “boutons.” Serial sectioning of the Type I innervation of muscles 6 and 7 showed that two axons with distinctive endings contribute to it. One axon (termed Axon 1) has somewhat larger boutons, containing numerous synapses and presynaptic dense bodies (putative active zones for transmitter release). This axon also has more numerous intraterminal mitochondria, and a profuse subsynaptic reticulum around or under the synaptic boutons. The second axon (Axon 2) provides somewhat smaller boutons, with fewer synapses and dense bodies per bouton, fewer intraterminal mitochondria, and less-developed subsynaptic reticulum. Both axons contain clear synaptic vesicles, with occasional large dense vesicles. Approximately 800 synapses are provided by Axon 1 to muscles 6 and 7, and approximately 250 synapses are provided by Axon 2. In muscles 12 and 13, endings with predominantly clear synaptic vesicles, generally similar to the Type I endings of muscles 6 and 7, were found, along with another type of ending containing predominantly dense-cored vesicles, with small clusters of clear synaptic vesicles. This second type of ending was found most frequently in muscle 12, and probably corresponds to a subset of the “Type II” endings seen in the light microscope. Type I endings are thought to generate the ?fast’? and ?slow’? junctional potentials seen in electrophysiological recordings, whereas the physiological actions of Type II endings are presently not known. © 1993 John Wiley & Sons, Inc.     

Differential timing of respiratory muscles in response to chemical stimuli in awake dogs

We assessed changes in respiratory muscle timing in response to hyperpnea and shortened inspiratory and expiratory times caused by chemoreceptor stimuli in six awake dogs. Durations of postinspiratory inspiratory activity of costal and crural diaphragm (PIIA), the delay in diaphragm electromyogram (EMG) after the initiation of inspiratory airflow, postexpiratory expiratory activity of the transversus abdominis (PEEA), and the delay of abdominal expiratory muscle activity after the initiation of expiratory airflow were measured. In control, four out of six dogs showed PIIA [8-10% of expiratory time (TE)]; all showed delay of diaphragm [19% of inspiratory time (TI)], delay of abdominal muscle activation (21% of TE), and PEEA (24% of TI). Hypercapnia decreased PIIA (4-9% of TE), maintained diaphragm delay at near control values (23% of TI), increased PEEA (36% of TI), eliminated delay of abdominal muscle activation (4% of TE), and decreased end-expiratory lung volume (EELV). Hypocapnic hypoxia increased PIIA (24-25% of TE), eliminated diaphragm delay (3% of TI), eliminated PEEA (3% of TI), reduced delay of abdominal muscle activation (14% of TE), and increased EELV. Most of these effects of hypoxic hypocapnia vs. hypercapnia on the within-breath EMG timing parameters corresponded to differences in the magnitude of expiratory muscle activation. These changes exerted significant influences on flow rates and EELV.     

Non-invasive detection of respiratory muscles activity during assisted ventilation

The instantaneous pressure applied by the respiratory muscles [Pmus(t)] of a patient under ventilatory support may be continuously assessed with the help of a model of the passive respiratory system updated cycle by cycle. Inspiratory activity (IA) is considered present when Pmus goes below a given threshold. In six patients, we compared IA with (i) inspiratory activity (IAref) obtained from esophageal pressure and diaphragmatic EMG and (ii) that (IAvent) detected by the ventilator. In any case, a ventilator support onset coincides with an IA onset but the opposite is not true. IA onset is always later than IAref beginning ((0.21 +/- 0.10 s) and IA end always precedes IAref end (0.46 +/- 0.16 s). These results clearly deteriorate when the model is not updated.     

Electrical and mechanical activity of respiratory muscles during hypercapnia

In nine anesthetized supine spontaneously breathing dogs, we compared moving average electromyograms (EMGs) of the costal diaphragm and the third parasternal intercostal muscles with their respective respiratory changes in length (measured by sonomicrometry). During resting O2 breathing the pattern of diaphragm and intercostal muscle inspiratory shortening paralleled the gradually incrementing pattern of their moving average EMGs. Progressive hypercapnia caused progressive increases in the amount and velocity of respiratory muscle inspiratory shortening. For both muscles there were linear relationships during the course of CO2 rebreathing between their peak moving average EMGs and total inspiratory shortening and between tidal volume and total inspiratory shortening. During single-breath airway occlusions, the electrical activity of both the diaphragm and intercostal muscles increased, but there were decreases in their tidal shortening. The extent of muscle shortening during occluded breaths was increased by hypercapnia, so that both muscles shortened more during occluded breaths under hypercapnic conditions (PCO2 up to 90 Torr) than during unoccluded breaths under normocapnic conditions. These results suggest that for the costal diaphragm and parasternal intercostal muscles there is a close relationship between their electrical and mechanical behavior during CO2 rebreathing, this relationship is substantially altered by occluding the airway for a single breath, and thoracic respiratory muscles do not contract quasi-isometrically during occluded breaths.     

Changes in respiratory activity induced by mastication in humans.

We studied the influence of mastication on respiratory activity in nine healthy volunteers who were requested to masticate a 5-g chewing gum bolus at a spontaneous rate (SR) for 5 min and "at the maximum possible rate" (MPR) for 1 min. Significant increases in respiratory frequency were induced by SR mastication due to a decrease in both the inspiratory and expiratory time. Tidal volume displayed slight nonsignificant decreases, but minute ventilation and mean inspiratory flow significantly increased. The duty cycle (TI/TT) did not change significantly. Total airway resistance significantly increased. Both peak and rate of rise of the integrated electromyographic activity of inspiratory muscles presented marked increases, accompanied by the appearance of a low level of tonic muscular activity. Similar but more intense effects on respiratory activity were induced by MPR mastication; in addition, a significant decrease in tidal volume and a significant increase in TI/TT were observed. Rhythmic handgrip exercise performed at metabolic rates comparable to those attained during SR or MPR mastication induced similar changes in the drive and time components of the breathing pattern, although accompanied respectively by nonsignificant or significant increases in tidal volume. Furthermore, the frequency of SR mastication significantly entrained the respiratory rhythm. The results suggest that mastication-induced hyperpnea does not merely represent a ventilatory response to exercise but also reflects complex interactions between respiratory and nonrespiratory functions of the upper airway and chest wall muscles.