We investigated how hindlimb immobilization (HI) affected contractile functions of single skinned soleus fibers from the rat. HI (6 weeks) resulted in reduced wet weight of soleus muscle (〜40%). The area of single skinned fibers was 〜30% less in immobilized muscle than in control muscle. We found that in immobilized fibers, maximal Ca²⁺-actiavated force was reduced by 〜30%. We then induced Ca²⁺-independent active force under the relaxing condition by lowering the MgATP concentration to investigate whether the decrease in Ca²⁺-activated force results from reduced likelihood of crossbridge formation. We found that in immobilized fibers, the Ca-independent active force was reduced by 〜40%. These results suggest that cross-bridge recruitment is suppressed in immobilized muscle via, probably, structural changes of the sarcomere, resulting in reduced active force production.

DESCENTE SPORTS SCIENCE Vol.27/THE DESCENTE AND ISHIMOTO MEMORIAL FOUNDATION FOR THE PROMOTION SPORTS SCIENCE