Effects of hindlimb suspension with stretched or shortened muscle length on contractile properties of rat soleus

The effects of chronic stretching or shortening of the soleus muscle of adu lt rats during hindlimb suspension on muscle mass and contractile propertie s were studied. Rats suspended with the ankle joint immobilized in either a dorsiflexed (Susp-DF soleus stretched), a plantarflexed position (Susp-PF, soleus shortened), or without immobilization (Susp-Free, soleus shortened) were compared with cage control rats. Suspension-related muscle atrophy wa s prevented in Susp-DF The relative muscle weight in Susp-PF was also less than in cage control and Susp-DF Both isometric maximum twitch tension (Pt) and maximum tetanic tension (Po) in the Susp-Free and Susp-PF were less th an control. Both Pt and Po in Susp-DF were normal. The twitch time-to-peak tension and one-half relaxation time tended to be reduced by chronic shorte ning of the muscle. The rate of tension development during a twitch (dp/dt) , expressed as gis, of Susp-Free group was decreased, hut that expressed as g/s/g Pt was greater than controls. That in Susp-DF was subnormal. The fat igue resistance in Susp-Free was normal but was reduced in Susp-DF and Susp -PF These data suggest that the decreases in the rat soleus mass and maximu m tension production and the shift toward a fast-twitch type following hind limb suspension are prevented by chronic stretching of muscle, although det rimental effect was induced for the fatigue resistance.