Effect of exercise on age-related muscle atrophy.

Fifty-five-week-old male Wistar rats exercised voluntarily in running wheels for 10 weeks. The number and cross-sectional area of fibers in the slow-twitch soleus (SOL) and fast-twitch tibialis anterior (TA) muscles were recorded and compared with those of 20-week-old rats, and of age-matched rats not exercised. Muscle fibers were classified as fast-twitch oxidative glycolytic (FOG), fast-twitch glycolytic (FG), or slow-twitch oxidative (SO). After injection of horseradish peroxidase into the SOL and TA for retrograde neuronal labeling, oxidative enzyme activity of labeled motoneurons in the spinal cord was measured by microspectrophotometry. There were fewer FOG fibers in the SOL, and fewer FG fibers in the TA, at 65 weeks than at 20 weeks of age. The cross-sectional area of FOG and FG fibers in the TA was lower at 65 weeks than at 20 weeks of age. Exercise prevented the atrophy of FOG fibers in the TA. There were no age- or exercise-related differences in the number or oxidative enzyme activity of motoneurons in the SOL or the TA. These findings suggest that exercise can prevent the atrophy of FOG fibers by restoring their decreased metabolic capacity, and by inhibiting the degeneration of neuromuscular junctions.