Human skeletal muscle responses to prolonged spaceflight: morphological and fiber type changes

TL;DRAbstract

Soleus and gastrocnemius biopsies from 5 astronauts and 5 cosmonauts were obtained ~45 days preflight and on landing day after 6 mo aboard the International Space Station. Crewmembers self-distributed into those performing high (>200 min/wk) treadmill exercise (HT) and low treadmill exercise (<100 min/wk) (LT). The intersubject variation in the percentage of fiber cross-sectional atrophy in alkaline myofibrillar ATPase sections was striking (3–64% in soleus and 3–48% in gastrocnemius). Postflight in both muscles 3 to 48% of slow fibers transitioned into slow/fast (I/IIA) and fast (IIA) fibers. Following spaceflight central myonuclei indicative of fiber degeneration/regeneration (soleus 4.62±.02%, gastrocnemius 2.01±.01%) were not significantly different from preflight (soleus 3.76±.01%, gastrocnemius 1.75±.01%). Thick to thin filament ratios were normal postflight. Increased glycogen occurred in all muscles. Lipid droplets remained high only in the HT group, indicating availability of

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