A study was conducted to clarify the effects of running intensity and duration of endurance training on myoglobin concentration ([Mb]) in rat skeletal muscles, and to clarify its temporal changes during the training. One hundred five male Wistar rats were divided into a training group and an untrained group. The training was carried out at 5 times a week for 12 weeks when the animals were 4 to 16 weeks of age. The training intensities were set at 20, 30 and 40 m/min with a duration of 60 min. The training duration was varied to 30, 60, 90 and 120 min when the rats were trained at 30 m/min. The temporal changes in the [Mb] were examined after the first, third and ninth week of training, during which the rats were trained at 40 m/min for 60 min per session. Three muscles (soleus:Sol, plantaris:PI, gastrocnemius-surface/deep : Gas-S, Gas-D) were analyzed for the [Mb] and citrate synthase activity (CS activity). With regard to the intensity of training, the [Mb] increased with exercise intensity in Sol, Gas-D and PI, but not in Gas-S. P1 showed a greater increase of the [Mb] than Sol or Gas-D. On the other hand, CS activity in red muscle (Sol and Gas-D) increased even at low intensity, whereas white muscle (fast-twitch muscle:P1 and Gas-S) showed a significant increase in CS activity at an intensity of 40 m/min. As to the duration of training, the [Mb] increased with the duration of running at 30 m/min of intensity, and showed the maximal adaptation with 90-min duration in all muscles except for Gas-S. Changes in CS activity according to the duration of running were similar to those for the [Mb] in all muscles. Finally, the [Mb] increased significantly with prolongation of the training period (after the 1 st, 3 rd and 9 th weeks training) in all muscles except Gas-S. However, the adaptive response of Mb tended to be delayed as compared with CS activity. These results suggest that 2) the response of Mb to training stimuli can depend on the muscle specificity (fiber type composition or the initial [Mb]), and level of motor unit recruitment in usual, 2) Mb synthesis can be enhanced by an increase of training intensity, 3) a training duration of 90 min can bring out the Mb adaptation maximally and 4) the adaptive response of Mb would need more time as compared with CS activity.
