Muscle degeneration after exercise in rats.

To search for morphological changes in muscle, related to overuse syndromes of muscle due to exercise, groups of untrained rats ran on a treadmill for 1 h at submaximal intensity. Each group was sacrificed at a different interval after the end of the exercise. To evaluate the physiologic load, the colonic temperature and blood lactate level were determined. The right hindlimb was fixated with buffered glutaraldehyde, injected into the femoral artery, and different muscles were dissected and prepared for electron and light microscopy. The muscles of the left limb were frozen in liquid Freon and used for histochemistry. Signs of degeneration were noted in the soleus, rectus femoris, and vastus lateralis muscles, but were absent in the gastrocnemius, tibialis anterior, extensor digitorum, and biceps femoris muscles. Immediately after exercise, only minor signs of degeneration were observed at the ultrastructural level, while after 2-3 h degeneration became clearly visible at the light microscopic level. The most pronounced changes were observed 24-48 h after exercise, whereafter regeneration occurred. Only 2%-5% of all fibers in the soleus muscle showed signs of degeneration, while in the vastus lateralis and rectus femoralis muscle less than 0.5% of the fibers were affected. The affected fibers showed degeneration only in segments with a length between 150-1250 micrometers. The affected fibers in the soleus and vastus lateralis muscles belong to the type I population, while in the rectus femoris type I as well as type II fibers were affected.