Effects of short spaceflights on mechanical characteristics of rat muscles. The aim of this study was to investigate the contractile protein characteristics after 5-day (Cosmos 1514) and 7-day (Cosmos 1667) spaceflights. The experiments were performed on skinned fibers from the soleus, gastrocnemius lateralis, and plantaris muscles isolated from Wistar rats. A reduction in fiber diameter might explain the decrease in the maximal tension in the soleus, whereas this tension was unaltered in the gastrocnemius and the plantaris. Moreover the calcium sensitivity of the myofilament appeared modified in the soleus and in the gastrocnemius: The tension/pCa relationships were shifted toward higher calcium concentrations, indicating a decrease in the apparent calcium binding constant of the troponin C. The tension/pCa relationship appeared unaltered in the plantaris after spaceflight. Finally, the studies of the time to reach a steady tension indicated an increase in the rate of force development in the soleus and, on the contrary, a slowing down in the plantaris. No change in the gastrocnemius was found. The results were analyzed with references to the different muscle functions in disuse atrophy. 