Muscle contraction induced by 30 min of continuous nerve stimulation at 50 Hz resulted in sarcomere changes of the soleus muscle in the rat in our previous study. To further investigate the cause of sarcomere alterations, the sciatic nerve was electrically stimulated intermittently for 30 min. Nerve stimulation was also conducted after cutting the tendons of the soleus, gastrocnemius and plantaris muscles in order to prevent imposing tension on these muscles as a result to their own contractions. In addition, the muscles were pulled by weights via their tendons to load high tension for 30 min without nerve stimulation. Sarcomere alterations immediately after treatments were quantified by electron microscopy. The percentages of aberrant sarcomere areas of the soleus muscle were 25.7 +/- 16.4% (mean +/- SD) in the group of intermittent nerve stimulation with intact tendons and 21.1 +/- 35.4% in the group of tenotomy and continuous nerve stimulation, which were roughly equal to or more severe than the group of continuous nerve stimulation with intact tendons (18.8 +/- 15.8%) in our previous study. Sarcomere alterations consisted mainly of hypercontraction in these groups. Almost all sarcomere changes in the tension-loaded (pulled) soleus muscles were scarce myofilaments (1.7 +/- 1.0% by 600 g; 4.5 +/- 2.9% by 1200 g), and hypercontraction was not observed. These findings indicate that neither high tension nor a decrease of muscle blood flow during continuous contraction seems to be the primary cause of sarcomere alterations in the present study. There are probably other causes that produce aberrant sarcomeres.
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