Muscles in microgravity: from fibres to human motion

Area of Science:

• Space medicine
• Human physiology
• Exercise science

Background:

• Microgravity leads to substantial human and animal postural muscle atrophy, with muscle mass stabilizing at approximately 70% of initial values after 270 days.
• Animal studies indicate preferential atrophy of slow-twitch fibers, while human studies show similar atrophy in both slow and fast fibers after 42 days of bed rest.
• Maximal muscle force decreases by 6-25% after microgravity exposure.

Discussion:

• Maximal power during short, explosive efforts (0.25-0.30s) falls significantly (to 45% of pre-flight values after 6 months), while longer-duration power (6-7s) is less affected (around 75% of pre-flight values).
• Lower limb muscle mass decline (9-13%) is less than the observed reduction in maximal power, suggesting motor coordination deterioration contributes to power loss.
• The absence of gravity significantly impacts motor control, leading to a substantial fraction of the maximal power decrease.

Key Insights:

• Explosive exercise should be incorporated into daily in-flight training to counteract the decay of maximal absolute power.
• A system using counter-rotating bicycles to simulate gravity via centrifugal acceleration is proposed for reducing cardiovascular deconditioning.
• Cycling on curved tracks on lunar bases can help maintain Earth-like gravitational effects for astronauts.

Outlook:

• Future space missions require targeted exercise protocols to preserve astronaut physical capabilities.
• Development of artificial gravity systems is crucial for long-duration spaceflight and lunar habitation.
• Understanding and mitigating the effects of altered gravity on neuromuscular function is essential for astronaut health and performance.