Activation of human inspiratory muscles in an upside-down posture
Introduction
Respiratory muscle contraction to ventilate the lungs is a continual requirement. It is usually under automatic control by respiratory centres in the pontomedullary region of the brainstem and in mammals, inspiration is the crucial phase of the respiratory cycle (Feldman, 2011, Feldman and Del Negro, 2006). During quiet breathing in human subjects in the seated posture, there is coordinated contraction of the obligatory inspiratory muscles with differences in the timing and degree of activation of intramuscular EMG recordings (for review see Butler, 2007, Butler and Gandevia, 2008, Butler et al., 2014, Hudson et al., 2011).
The mechanical action of the respiratory muscles is determined, like all skeletal muscles, by the bony structure(s) to which they attach and the displacement of these structures when the muscles contract. In the upright posture, the mechanical actions of the inspiratory muscles expand the abdominal wall and expand and elevate the ribcage against gravity. Body posture changes often and in diverse ways that can acutely alter respiratory mechanics. As first described by Konno and Mead (1967), for the same change in lung volume there is greater ribcage expansion when upright, but abdominal expansion dominates in the supine posture. A change to the supine posture also affects pleural and abdominal pressures developed during tidal breathing and inspiratory muscle activity can, in some cases, be altered (e.g. Butler et al., 2001, Druz and Sharp, 1981, Segizbaeva et al., 2011, Steier et al., 2009). When present, changes in inspiratory muscle EMG are concerted, with a decrease in phasic EMG activity in all muscles (diaphragm, scalenes and parasternal intercostals) with a change from upright to supine posture (Druz and Sharp, 1981).
It is not known if the activation of inspiratory muscles can be differentially affected by the unusual posture of upside-down suspension. This is important as it establishes if the activation of respiratory muscles is adaptable to a wide range of postures that humans can adopt and if the coordinated (i.e. concerted) pattern of inspiratory muscle activation can be altered. If so, this suggests that neural drive from the pontomedullary respiratory centres to different inspiratory muscles can be differentially distributed in an adaptive manner. The rationale for an upside-down posture was that in addition to a decrease in functional residual capacity as known to occur in the supine posture compared to upright (e.g. Lim and Luft, 1959), an upside-down posture would elicit further mechanical changes and differentially affect inspiratory muscles. We hypothesised that in the upside-down posture, the reversed effects of gravity on chest wall, lung and respiratory muscle mechanics would alter thoracoabdominal movement and intrathoracic pressures generated by the muscles and that we would observe a concurrent differential adaptation of inspiratory muscle activity. Specifically, we expected: (i) an increase in inspiratory diaphragmatic activity to counteract the effect of gravity on the abdominal contents to generate comparable caudal movement of the muscle and transdiaphragmatic pressure and (ii) a decrease in scalene activity due a decreased requirement of the mechanical action of the scalenes to elevate the upper ribcage while upside down. We tested these hypotheses by determining the effect of the upside-down posture on ventilation, thoracoabdominal expansion, inspiratory pressures and inspiratory muscle activity during quiet breathing, compared to standing and lying supine.
Section snippets
Material and methods
The studies were carried out in 14 healthy subjects (5 females) aged 24–56 years. They gave informed written consent to the procedures, which conformed with the Declaration of Helsinki and were approved by the Human Research Ethics Committee of the University of New South Wales. Respiratory parameters and inspiratory scalene electromyographic activity (EMG) were recorded during quiet breathing in three postures: standing, lying supine and upside down (Fig. 1A). In a subset of six subjects,
Effect of body posture on lung volume and respiratory parameters during quiet breathing
Typical flow-volume loops during standing, lying supine and upside-down suspension are shown in Fig. 1B and group data are given in Table 1. Vital capacity (VC) was similar across all postures (F(2,24) = 2.5, p = 0.1), but inspiratory capacity (IC; F(2,24) = 11.4, p < 0.001) and expiratory reserve volume (ERV; F(2,24) = 16.5, p < 0.001) varied (see Table 1 for post hoc differences). While upside down, IC was greater, by ∼400 ml when compared to standing (p < 0.05). The increase in IC was associated with a
Discussion
We have shown that an upside-down posture modifies ribcage and diaphragm-abdominal compliance reflected by decreases in rib cage expansion and increases in inspiratory pressures compared to standing for the same change in tidal volume. We have also demonstrated that posture can differentially affect the activation of two inspiratory muscles as diaphragm EMG was unchanged when suspended upside down, but scalene activity decreased. Thus, the coordinated activity of the respiratory muscles that is
Acknowledgment
This work was supported by the National Health and Medical Research Council (Australia).
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