On the mechanisms of changes in skeletal muscles in the weightless environment

Some characteristics of muscle contraction and mechanical properties of two muscles (M. soleus and M. extensor digitorum longus) in Wistar rats after 22 days of weightlessness have been investigated. On the second day after return to earth, the following changes were evident: slowing of twitch responses of the muscles studied; shortening half tetanic contraction time (defined by point of interactions of the increasing curve with 50% level of the peak value) in soleus; a rise of tension in both muscles as shown by the curve "length-force"; an increase of twitch/tetanus ratio and fatigability in both muscles. During repeated study of muscle properties, on the 26th day after return to earth, there were not any significant changes in values of most the above mentioned indices except the diminished strength of soleus. Shortening of contraction time in slow antigravitary soleus muscle is believed to be a sign of adaptive change in its characteristics resulting from unloading under the conditioning of weightlessness. Increased stiffness and diminished strength of muscles are considered as functional signs of atrophic processes, developing in the experiment. Dynamics of functional changes, conditioned, as we believe, mainly by weightlessness and correlated with morphological and biochemical data obtained on analogous material, allows us to postulate a reversible character of the changes described.