The influence of posture on parasternal intercostal muscle activity in healthy young adults.

Parasternal intercostal muscle electromyography (EMGpara) has been used as an index of respiratory load in health and disease. While reference values are available, such data have been obtained with subjects in the seated position only. The objective of the current study was to determine the influence of posture on measurements of EMGpara. Fifty-one healthy adult participants underwent measurement of EMGpara, respiratory flow and volume in the seated, reclined at 45°, and supine positions. Resting peak EMGpara activity per breath was determined and expressed both as the raw signal and normalised to that obtained during a maximum inspiratory effort (EMGpara%max). Neural respiratory drive index (NRDI, the product of EMGpara%max and respiratory rate) and neuroventilatory efficiency (NVE, tidal volume divided by EMGpara) were also calculated. No significant differences were observed in raw EMGpara, EMGpara%max, NRDI, NVE or tidal volume from the seated to reclined or supine positions. Respiratory rate and minute ventilation were significantly lower in the supine position compared to seated (p   =  0.0043 and 0.0266 respectively). Poor agreement was observed between raw EMGpara and EMGpara%max, likely due to submaximal efforts or cross-talk from adjacent musculature during the maximal manoeuvres. Agreement was notably poorer in the supine posture. Posture does not have a significant effect on EMGpara activity, suggesting that measurements can be made in the reclined or supine position if required or requested by the participant. Normalising the EMGpara signal to a maximal respiratory effort may give unreliable estimates of respiratory load.