Effects of immobilization by 60 days of experimental bed rest on endomysium of the soleus muscle in humans

Abstract

Movements result from contracting skeletal muscles. Muscles adapt to changing loads through hypertrophy (enlargement of muscle mass due to increased load) and atrophy (loss of muscle mass due to decreased load). Muscles are known to atrophy during immobilization and disuse. The two primary components of muscle are the muscle fibers (muscle cells) and intramuscular connective tissue. Intramuscular connective tissue can be categorized into endomysium (the smallest component which engulfs each individual muscle fiber), perimysium (which engulfs muscle bundles), and epimysium (which engulfs the muscle as a whole). It has been suggested that endomysium plays a key role for intramuscular force transmission. Animal data suggests that long-term immobilization results in increased endomysium content in the muscle while the muscle fibers themselves atrophy. This result has to date not been demonstrated in humans. Our objective was therefore to examine the endomysium content of the human soleus muscle during 60 days of bed rest. Bed rest is an established model to investigate changes occurring in microgravity conditions.