Effect of Simulated Microgravity and Lunar Gravity on Human Inspiratory Muscle Function: 'Selena-T' 2015 Study

Area of Science:

• Space Physiology
• Exercise Physiology
• Respiratory Muscle Function

Background:

• Prolonged bed rest simulates microgravity and lunar gravity conditions.
• Understanding the impact of these conditions on respiratory muscles is vital for astronaut health.
• Previous research has not fully elucidated the specific effects of different bed rest angles on inspiratory muscle fatigue.

Purpose of the Study:

• To investigate the effects of head-down bed rest (HDBR) and head-up bed rest (HUBR) on inspiratory muscle fatigue during incremental exercise.
• To compare the fatigue patterns of respiratory muscles after 21 days of HDBR (simulating microgravity) versus a mixed HDBR/HUBR protocol (simulating lunar gravity).

Main Methods:

• 22 healthy males underwent incremental exercise tests to exhaustion before and after 21 days of bed rest.
• Two bed rest protocols were used: HDBR (-6°) for 10 subjects and HDBR followed by HUBR (+9.6°) for 12 subjects.
• Inspiratory muscle function was assessed using maximal inspiratory pressure (MIP) and electromyograms (EMG) of respiratory muscles.

Main Results:

• Exercise induced inspiratory muscle fatigue (decreased MIP and EMG centroid frequency) in all subjects pre-bed rest.
• After 3 weeks of HDBR, inspiratory muscles showed significantly greater fatigue compared to pre-bed rest.
• HUBR did not lead to increased inspiratory muscle fatigue, and HDBR's effects were primarily attributed to postural changes rather than just hypokinesia.

Conclusions:

• Head-down bed rest (HDBR) exacerbates inspiratory muscle fatigue during high-intensity exercise, potentially limiting maximal ventilation and contributing to exercise intolerance in simulated microgravity.
• Head-up bed rest (HUBR) does not induce similar fatigue, suggesting that the postural effects of HDBR are key drivers of respiratory muscle dysfunction.
• These findings highlight the importance of considering postural influences in spaceflight and developing countermeasures for respiratory health.