Activation of human inspiratory muscles in an upside-down posture

• The upside-down posture reduces end-expiratory lung volume compared to standing.
• Tidal volume is maintained, but thoracoabdominal movement and intrathoracic pressures vary.
• Crural diaphragm EMG is unchanged, but scalene muscle EMG is reduced by ∼half.
• The mechanical effect of an upside-down posture differentially modifies inspiratory muscle activation.

During quiet breathing, activation of obligatory inspiratory muscles differs in timing and magnitude. To test the hypothesis that this coordinated activation can be modified, we determined the effect of the upside-down posture compared with standing and lying supine. Subjects (n = 14) breathed through a pneumotachometer with calibrated inductance bands around the chest wall and abdomen. Surface electromyographic activity (EMG) was recorded from the scalene muscles. Crural diaphragmatic EMG and oesophageal and gastric pressures were measured in a subset of six subjects. Quiet breathing and standard lung function manoeuvres were performed. The upside-down posture reduced end-expiratory lung volume. During quiet breathing, for the same inspiratory airflow and tidal volume, ribcage contribution decreased, abdominal contribution increased and transdiaphragmatic pressure swing doubled in the upside-down posture compared to standing (p < 0.05). Despite this, crural diaphragm EMG was unchanged, whereas scalene muscle EMG was reduced by ∼half (p < 0.05). Thus, the mechanical effect of an upside-down posture differentially affects inspiratory muscle activation.