SKELETAL-MUSCLE ADAPTATIONS TO ENDURANCE TRAINING IN 60-YR-OLD TO 70-YR-OLD MEN AND WOMEN

Previous studies of endurance exercise training in older men and women generally have found only minimal skeletal muscle adaptations to training. To evaluate the possibility that this may have been due to an inadequate training stimulus, we studied 23 healthy older (64 +/- 3 yr) men and women before and after they had trained by walking/jogging at 80% of maximal heart rate for 45 min/day 4 days/wk for 9-12 mo. This training program resulted in a 23% increase in maximal O2 consumption. Needle biopsy samples of the lateral gastrocnemius muscle were obtained before and after training and analyzed for selected histochemical and enzymatic characteristics. The percentage of type I muscle fibers did not change with training. The percentage of type IIb fibers, however, decreased from 19.1 +/- 9.1 to 15.1 +/- 8.1% (P < 0.001), whereas the percentage of type IIa fibers increased from 22.1 +/- 7.7 to 29.6 +/- 9.1% (P < 0.05). Training also induced increases in the cross-sectional area of both type I (12%; P < 0.001) and type IIa fibers (10%; P < 0.05). Capillary density increased from 257 +/- 43 capillaries/mm2 before training to 310 +/- 48 capillaries/mm2 after training (P < 0.001) because of increases in the capillary-to-fiber ratio and in the number of capillaries in contact with each fiber. Lactate dehydrogenase activity decreased by 21% (P < 0.001), whereas the activities of the mitochondrial enzymes succinate dehydrogenase, citrate synthase, and beta-hydroxyacyl-CoA dehydrogenase increased by 24-55% in response to training (P < 0.001-0.05). We conclude that, given an adequate training stimulus, the skeletal muscles of older men and women undergo adaptations to endurance exercise training similar to those observed in young people.