FORCE-VELOCITY-POWER AND FORCE-PCA RELATIONSHIPS OF HUMAN SOLEUS FIBERS AFTER 17 DAYS OF BED REST

Soleus muscle fibers from the rat display a reduction in peak power an d Ca2+ sensitivity after hindlimb suspension. To examine human respons es to non-weight bearing, we obtained soleus biopsies from eight adult men before and immediately after 17 days of bed rest (BR). Single che mically skinned fibers were mounted between a force transducer and a s ervo-controlled position motor and activated with maximal (isotonic pr operties) and/or submaximal (Ca2+ sensitivity) levels of free Ca2+. Ge l electrophoresis indicated that all pre- and post-BR fibers expressed type I myosin heavy chain. Post-BR fibers obtained from one subject d isplayed increases in peak power and Ca2+ sensitivity. In contrast, po st-BR fibers obtained from the seven remaining subjects showed an aver age 11% reduction in peak power (P < 0.05), with each individual displ aying a 7-27% reduction in this variable. Post-BR fibers from these su bjects were smaller in diameter and produced 21% less force at the sho rtening velocity associated with peak power. However, the shortening v elocity at peak power output was elevated 13% in the post-BR fibers, w hich partially compensated for their lower force. Post-BR fibers from these same seven subjects also displayed a reduced sensitivity to free Ca2+ (P < 0.05). These results indicate that the reduced functional c apacity of human lower limb extensor muscles after BR may be in part c aused by alterations in the cross-bridge mechanisms of contraction.