We examined age-related changes in functional, morphological and metabolic adaptations of the rat diaphragm muscle to compensatory increased activity. Forty-two male Wistar rats (6, 48 and 100 weeks after birth) were randomly divided into control (CTL) and compensatory activation (CAC) groups (n=7 in each group). In the CAC group, denervation surgery in the hemidiaphragm was performed to induce compensatory activation on the contralateral hemi-diaphragm muscle. Four weeks later (10 weeks, 1 year and 2 years after birth), isometric contraction properties by in-vitro physiological measurement, fiber type composition and metabolic properties (succinic dehydrogenase (SDH), 3-hydroxyacyl-CoA dehydrogenase (HAD) and phosphofructkinase (PFK)) were analyzed by histochemical and biochemical procedures.In the CTL groups, significant prolongations in the contraction and half-relaxation times were found in 2-year group, as compared to 10-week group. This age-related changes were attributed to changes in muscle fiber type composition; selective hypertrophy in Slow-twitch Oxidative fiber. In 2-year CAC group, as compared to age matched CTL group, significant improves were found in the half-relaxation time, SDH activities in each fiber type, HAD and PFK activities of the diaphragm muscle.These results demonstrated that the aged diaphragm muscle has a tendency toward slow muscle in functional and morphological properties, and that the muscle has a high adaptability to compensatory increased activity, at least in metabolic properties.

The thoroughbred horse is a remarkable animal, with both speed and endurance abilities during running. Muscle adaptation associated with training and growth has been examined using various physiological, histochemical and biochemical measurements. We have emphasized the importance of type IIX fiber recruitment in training program for thoroughbred. Because type IIX fiber provides the greatest contribution to total muscle volume in the gluteus medius muscle and increases in oxidative enzymes of the fibers would markedly improve the whole-muscle oxidative capacity. Based on these concepts, we examined the adaptations of type IIX fiber including hybrid fiber to various training programs.It is considered that scientific information about optimal training of these excellent abilities can provide significant insights into exercise physiology and health science not only for thoroughbred horses, but also for human beings.