Effects of 17-day spaceflight on knee extensor muscle function and size

Area of Science:

• Space physiology
• Human adaptation to microgravity
• Musculoskeletal system response to spaceflight

Background:

• Microgravity exposure is widely believed to cause muscle dysfunction and atrophy in space travelers.
• Existing observations regarding the specifics of these changes are limited and sometimes contradictory.
• This study addresses the need for more consistent data on spaceflight-induced muscle alterations.

Purpose of the Study:

• To quantify changes in knee extensor muscle function and size in astronauts after a short-duration spaceflight.
• To compare microgravity-induced muscle changes with those observed in ambulatory individuals over a similar time frame.
• To investigate potential discrepancies with previous findings on ankle plantar flexor force.

Main Methods:

• Four male astronauts underwent testing before and after a 17-day spaceflight (STS-78).
• Muscle function was assessed via maximal voluntary isometric, iso-inertial concentric, and eccentric knee extensor actions.
• Magnetic resonance imaging (MRI) measured the cross-sectional area (CSA) of knee extensor, knee flexor, and gluteal muscles.

Main Results:

• Astronauts experienced a significant 10% decrease in knee extensor strength across all tested actions post-flight (P<0.05).
• Muscle CSA decreased by 8% in knee extensors and gluteal muscles (P<0.05), while knee flexors remained unchanged.
• A control group of eight men showed no significant changes in muscle strength over 20 days.

Conclusions:

• Short-duration spaceflight leads to measurable decreases in muscle strength and size, consistent with ground-based studies.
• The observed muscle atrophy and strength loss in astronauts highlight the physiological challenges of space travel.
• Findings contrast with prior reports of no strength decline in ankle plantar flexors within the same crew, suggesting specific muscle group vulnerabilities.