ABSTRACT
The purpose of this study was to examine the risk factors of stress fractures in terms of training distance and intensity in young male Japanese high school long-distance runners. Nine hundred and twenty-five runners from high schools, colleges, and work teams responded to our questionnaire. Our analysis of the questionnaire found that the onset rates of stress fractures in males were 25.0%, 40.2%, and 55.3% in high school runners, collegiate runners, and work team runners, respectively, suggesting that young Japanese long-distance runners are more likely to suffer from stress fractures than European and American runners. Stress fractures occurred in male high school and collegiate runners after training that had increased running distance (24.7% and 33.1%, respectively) or running intensity (17.0% and 9.6%), or both increased running distance and increased running intensity (29.8% and 34.6%), suggesting that an increase in running distance at moderate intensity might be a major risk factor in stress fractures in young male long-distance runners. Data from college and work team runners that ran all three years of high school show that stress fractures are most likely to occur in May of the high school freshman year. These results suggested a need to reconsider training programs for freshmen to prevent stress fractures in young runners.
ABSTRACT
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: P1, 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 P1, 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: Pl and Gas-S) showed a significant increase in CS activity at an intensity of 40m/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 <I>1) </I>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, <I>2) </I> Mb synthesis can be enhanced by an increase of training intensity, <I>3) </I> a training duration of 90 min can bring out the Mb adaptation maximally and <I>4) </I> the adaptive response of Mb would need more time as compared with CS activity.
ABSTRACT
The effects of low- and high-intensity endurance training on the capillary network of rat left ventricle were studied morphometrically. Eighteen male albino rats of Wistar strain (4-wk-old) were assigned at random to a sedentary control group (Cont, n=8) and two trained groups which were both subjected to exercise on a motor-driven treadmill for 60 min a day, 5 days/wk for 9 weeks from 7 wks to 16 wks of age with different running speed; the low-intensity trained group (T-20, n=5) ran at 20 m/min and the high-intensity trained group (T-40, n=5) at 40 m/min. All morphometric parameters for the capillary and muscle fiber were determined in perfusion-fixed hearts. After the training period, the average muscle fiber cross-sectional area in the T-20 and the T-40 was not significantly different from the Cont. There were no significant differences in the capillary density and the capillary-to-fiber ratio between any groups, suggesting no significant change in capillary number. On the other hand, the number of capillary with large luminal diameter (8-10 μm) in the T-40 but not the T-20 was significantly greater than the Cont. These results indicate that the high-intensity endurance training causes enlargement of the capillary luminal area, while neither the low-nor the high-intesnity endurance training stimulate the proliferation of capillaries in the left ventriclular myocardium. In conclusion, a structure of the capillary network of rat left ventricle responds to the high-intensity endurance training by enlarging capillary luminal area rather than by increasing capillary number.