Respiratory muscle fatigue during simulated air combat maneuvering (SACM)

This study tested the hypothesis that respiratory muscle fatigue occurs during a simulated air combat maneuver (SACM) centrifuge profile.

Six subjects (four males, two females) were exposed to a SACM consisting of alternating +4 Gz to +7 Gz plateaus until volitional fatigue. Electrical activities of the diaphragm, scalene, intercostal and external oblique muscles were monitored along with esophageal pressure and ventilation.

SACM times averaged 358 +/- 115 s. The mean frequency of the electromyographic signal of the diaphragm and scalenes during inspiration, and the intercostals during the static expiratory portion of the anti-G straining maneuver (AGSM), decreased significantly (p < 0.05) over the course of the SACM by about 9%, 10% and 4%, respectively, indicating muscle fatigue. Esophageal pressure (PES) during the straining portion of the AGSM decreased significantly from 72 +/- 20 to 66 +/- 21 mmHg (mean +/- SD). Ventilation increased significantly both at +4 Gz and +7 Gz (32 +/- 16 L.min-1 to 56 +/- 19 L.min-1 at +4 Gz and 27 +/- 5 to 35 +/- 9 L.min-1 at +7 Gz) over time. PES during a maximal AGSM at +1.4 Gz (baseline) was decreased post- vs. pre-SACM (p = 0.0575). PES exerted during a maximal static inspiratory maneuver at residual lung volume was not changed after the SACM. Inspiratory work increased significantly during the SACM. The elastance of the respiratory system did not change during the SACM, but it was significantly increased when tested after the SACM.

Increased inspiratory work, decreased pressure generation during a maximal AGSM and EMG frequency shifts suggest respiratory muscle fatigue coincides with the termination of SACM.