Expiratory and inspiratory action of transversus abdominis during eupnea and hypercapnic ventilation

•The transversus abdominis is an obligatory expiratory muscle and expiration is not a passive process during resting breathing.•The transversus abdominis is progressively recruited during hypercapnic ventilation.•In early inspiration, transversus abdominis contributes to minute ventilation, during hypercapnia, and respiratory failure.