Helmet CPAP (H-CPAP) and prone position proved to be lifesaving as non-invasive respiratory support during COVID-19. Although helmets (INTERSURGICAL) are equipped with an inflatable neck cushion, air leaks still remain an issue for the measurements of flow/volume with a flowmeter. We used opto-electronic plethysmography, which does not require instrumentation at the opening of the airways, to quantify the ventilatory pattern and the end-expiratory chest wall volume variation (?EEcw) on 10 healthy volunteers (24.4 yrs; 173 cm; 70 kg) at rest and during H-CPAP (PEEP=10 cmH2O and Flow= 50 L/min) in semi-recumbent and prone position. PEEP increases EEcw (p=0.010), but it does not affect the ventilatory pattern (p>0.05). Prone position reduces minute ventilation because both tidal volume and frequency diminish, without affecting EEcw (Table). In the prone position, lung compression is less, blood and air flow are better redistributed, and heart function may improve. In addition to these already-known benefits, we have shown that prone positioning reduces the ventilatory requirements of the subject and that PEEP-induced volume recruitment is not affected (compared to semi-recumbent position). We may speculate that thanks to the prone position less support from the ventilator may be needed to achieve adequate oxygen levels, therefore reducing the risk of ventilator-induced lung injury, while maintaining the same level of PEEP.