Click name to view affiliation
The effects of chronic stretching or shortening of the soleus muscle of adult rats during hindlimb suspension on muscle mass and contractile properties 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 was 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 than 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 shortening of the muscle. The rate of tension development during a twitch (dp/dt), expressed as g/s, of Susp-Free group was decreased, but that expressed as g/s/g Pt was greater than controls. That in Susp-DF was subnormal. The fatigue 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 maximum tension production and the shift toward a fast-twitch type following hindlimb suspension are prevented by chronic stretching of muscle, although detrimental effect was induced for the fatigue resistance.
The authors are with the Department of Physiology and Biomechanics at the National Institute of Fitness and Sport, Kanoya City, Kagoshima Prefecture 891-2393, Japan.