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Evolution of abdominal contribution to tidal volume over time, for all classes. All preflight values were averaged. *0.05 < P < 0.1, **P < 0.05 compared with preflight. The gray area indicates the intersubject variability (SE) for the awake class. Intersubject variability for the other sleep classes considered was similar or lower. μG, microgravity; REM, rapid eye movement.
