EFFECTS OF AGING AND EXERCISE ON SOLEUS AND EXTENSOR DIGITORUM LONGUSMUSCLES OF FEMALE RATS

The effects of ageing and of exercise on muscle mass, fiber cross-sect ional area, and fiber type composition of a weight-bearing muscle, the soleus and a non-weight-bearing muscle, the extensor digitorum longus (EDL) were investigated in female Long-Evans rats. The animals were e xercised by means of voluntary wheel running beginning at 4 months. Ru nners and sedentary controls were studied at 9 months and 27 months of age. In sedentary rats, the soleus muscle weighed 26% less, and the E DL weighed 19% less at age 27 months, than at 9 months. This decline i n muscle mass was accounted for by a similar decrease in muscle fiber cross-sectional area. The wheel running resulted in significant hypert rophy of the soleus in both 9- and 27-month-old rats; as a consequence the 27-month-old runners had larger soleus muscles than the 9-month-o ld sedentary rats. The running did not prevent atrophy of the EDL in t he old rats, but did increase the proportion of type IIa fibers. The e xercise also increased the number of capillaries per fiber in the sole us muscles of both young and old rats. In conclusion, the finding that wheel running prevented atrophy with aging of the weight-bearing sole us but not that exercise-induced muscle hypertrophy can be maintained in old age by appropriate exercise.