Fig. 1

Tidal volume loop during each dive condition in one diver. A positive transpulmonary pressure gradient (or positive static lung load: SLL+) is set when the rebreather is worn anteriorly (on the abdomen) by the diver in prone position (a). A positive pressure breathing (PPB) condition is created. Conversely, when the rebreather is worn posteriorly (b), the transpulmonary pressure gradient is negative in the prone position (negative static lung load, SLL−), and the diver is in condition of negative pressure breathing (NPB). In each condition, the diver completed two 30-min dives, one simply statically floating (static), and one with continuous fin swimming (exercise). Examples of tidal pressure-volume loops are sketched during both static and exercise in each PPB and NPB condition. The dashed lines indicate the SLL level in each condition. Peak insp. press., peak inspiratory pressure during; peak expir. press., peak expiratory pressure. Of note, in each PPB and NPB, Vt lengthening carried the main rest to exercise change, while pressure ranges were very similar during static and exercise dives