Decreased Thin Filament Density and Length in Human Atrophic Soleus Muscle Fibers After Spaceflight

Document Type




Format of Original

6 p.

Publication Date



American Physiological Society

Source Publication

Journal of Applied Physiology

Source ISSN


Original Item ID

DOI: 10.1152/jappl.2000.88.2.567


Soleus muscle fibers were examined electron microscopically from pre- and postflight biopsies of four astronauts orbited for 17 days during the Life and Microgravity Sciences Spacelab Mission (June 1996). Myofilament density and spacing were normalized to a 2.4-μm sarcomere length. Thick filament density (~1,062 filaments/μm2) and spacing (~32.5 nm) were unchanged by spaceflight. Preflight thin filament density (2,976/μm2) decreased significantly (P < 0.01) to 2,215/μm2 in the overlap A band region as a result of a 17% filament loss and a 9% increase in short filaments. Normal fibers had 13% short thin filaments. The 26% decrease in thin filaments is consistent with preliminary findings of a 14% increase in the myosin-to-actin ratio. Lower thin filament density was calculated to increase thick-to-thin filament spacing in vivo from 17 to 23 nm. Decreased density is postulated to promote earlier cross-bridge detachment and faster contraction velocity. Atrophic fibers may be more susceptible to sarcomere reloading damage, because force per thin filament is estimated to increase by 23%.


Journal of Applied Physiology, Volume 88, No. 2 (February 2000): 567-572. DOI.