Introduction: In acute respiratory failure mechanical ventilation aims to unload respiratory muscles. However, some mechanically ventilated patients still recruit accessory respiratory muscles, probably indicating inadequate support by the ventilator. Monitoring accessory respiratory muscle activity might be helpful to optimize ventilator support.Aim: To evaluate the effect of ventilator settings during pressure support ventilation on accessory respiratory muscle recruitment in patients with acute respiratory failure.Methods: In seventeen mechanically ventilated patients, electrical activity of the parasternal intercostals, scalene, sternocleidomastoid, genioglossus and alae nasi was measured using surface electrodes. Esophageal electrodes were used to measure diaphragm activity. The pressure support level was reduced every five minutes with 3 cmH₂O, starting from 15 cmH₂O to CPAP. Muscle activity and recruitment times were assessed for every support level.Results: Significant differences in accessory respiratory muscle activity were found between combinations of high (15, 12 and 9 cmH₂O) versus low (6, 3 and 0 cmH₂O) support levels. The alae nasi recruited significantly earlier than the diaphragm (0.12±0.03 ms; p<0.05). The peak and termination of upper airway muscle activity occurred also earlier than for the other respiratory muscles.Conclusions: In response to lower support levels accessory respiratory muscle activity increases. Upper airway muscles recruit earlier than other respiratory muscles. Assessing accessory respiratory muscle recruitment by surface electromyography is a potentially novel and non-invasive tool to monitor inspiratory drive in ventilated ICU patients.