A significant increase in inspiratory occlusion pressure, which equals alveolar pressure, was observed immediately after the head-down-tilt (Fig. 1). All subjects demonstrated an approximately two-fold increase in P_occl and R_aw (Table 1). These changes were reversed on return to the standing position. The MIP ranged from 52 to 113 cm H₂O in the standing position. These values are approximately in the normal range as found by others (Sachs et al. 2009; Hautmann et al. 2000). The HDT lowered the MIP by 17.4 % (P < 0.05). After 30 min of head-down-tilting, PIF and PEF decreased significantly by 16.3 % and 20.0 %, respectively, compared with the standing position.

During quiet breathing while standing, phasic inspiratory activity was observed in the D and PS muscles of the subjects. In HDT, there are marked differences in the patterns of respiratory muscle activity during quiet breathing, compared with baseline, which may be related to CHV. Phasic PS activity increased more than twice (P < 0.01), which may contribute to the inspiratory rib cage expansion in this condition (Fig. 2A). Peak amplitude of the integrated EMG of the diaphragm decreased immediately the onset of HDT; the decrease was maintained (~40 %) throughout the 30-min tilt (P < 0.01). In contrast, after shifting to supine and upright position, a reverse pattern of EMG-responses was observed; peak amplitude of D and PS EMG returned to the control levels (Fig. 2A). Furthermore, minimal electrical activity was present in the SC and GG muscles during quiet breathing in the upright position. This activity was phasic, occurring during inspiration (Fig. 3). The transient EMG responses to shifting from the upright to HDT position consisted of a rapid increase in peak amplitude of GG EMG (~65 %), whereas SC activity decreased during spontaneous breathing (~25 %) (Figs. 2B and 3).