Skeletal muscle fiber of certain mammals consists of three types, namely, red muscle fiber, white muscle fiber and medium muscle fiber.
The purpose of this study is to inquire histologically the effect of training upon medium muscle fibers by comparing with red and white muscle fibers.
First, the form, size and distribution of red muscle fibers, white muscle fibers and medium muscle fibers of M. tibialis ant. of rats and mice were studied, and then the effect of training on those muscles was investigated.
Wistar-strain rats and dd-strain mice were devided into training and control groups. One training group of Wistar-strain rats was made to run a treadmill with a speed of 40 m/min., a distance of 250 m and for a period of 90 days. Another training group of dd-strain mice was compelled to run on the treadmill with a speed of 20 m/min., a distance of 250 m and for a period of 60 days.
After the training period, the anterior tibial muscle was removed and the histological method, using Sudan black B, was adopted in this experiment.
The results obtained were as follows:
1) Form and size ; Red muscle fibers are more or less roundish, and white muscle fibers are polygonal. In gereral red muscle fibers are thinner than white muscle fibers. Medium muscle fibers are intermediate between the two in all respects. Red muscle fibers, white muscle fibers and medium muscle fibers are all thick at the peripheral part of the muscle, and become thinner towards the central part.
2) Number ; The ratio of number of medium muscle fibers involved in the anterior tibial muscles is less than that of red and white muscle fibers in rats and mice.
3) Distribution ; The outer layer of the peripheral region of the muscle is composed mostly white muscle fibers, and red muscle fibers are more abundant in the central part of the muscle and near the tibia. Medium muscle fibers reveal a gradual increase in the central part.
4) In rats, increasing rate of the entire cross sectional area of medium muscle fibers was 19.1%, that of red muscle fibers was 18.0%, and that of white muscle fibers was 33.0%. In mice, each value of medium, red and white muscle fibers was 13.4%, 10.6% and 31.5%.
5) The effect of training on the average cross sectional area per fiber showed the same tendency as the effect of training on the entire area of cross section in medium, red and white muscle fibers.

To clarify changes in body temperature during endurance exercise in patients with spinal cord injury (SCI), we measured tympanic temperature (Tty) and skin temperature in the head, arm, chest, thigh, shin and calf in 5 patients with SCI (T6-T 12) and 7 normal controls during 30 minutes arm cranking exercise (20 watts) from 10 minutes before the initiation of exercise until 10 minutes after the termination of exercise in an artificial climate room at a temperature of about 25°C with a relative humidity of about 50%. The Tty in the SCI group was lower than that in the control group from 10 minutes before the initiation of exercise to 10 minutes after the termination of exercise with a significant difference only at the initiation of exercise. The difference in Tty slightly decreased with continuation of exercise. The Tty in the SCI group at rest was 36.05-37.15°C. Four patients in this group showed a decrease of 0.04-0.12°C in the early stage and an increase of 0.66°C±0.19 (mean±SD) at the end of exercise over the value at the initiation of exercise.
The skin temperature was lower in the SCI group than in the control group in all sites excluding the arm. Significant differences were observed in the head in the early stage of exercise and after exercise, in the chest from 10 minutes before the initiation of exercise to 5 minutes after the termination of exercise, in the thigh from 10 minutes before the initiation of exercise to 10 minutes after the termination of exercise, in the shin 10 minutes and 5 minutes before the initiation of exercise, and in the calf from before to 15 minutes after the initiation of exercise. In the SCI group, marked individual differences were observed in the skin temperatures in the thigh, shin, and calf, suggesting specificity of the skin temperature response in and near the paralysis area.
Results in Tty in this study suggested no heat retention in the SCI patients. Therefore, the risk for heat disorders seems to be low during moderate or mild exercise under moderate temperature environment at a temperature of about 25°C with a relative humidity of about 50% even when the skin temperature is low, and thermolysis is not marked.

Using several electrophoretic techniques, this study examined the effects of 3 weeks hindlimb suspension on the patterns of isomyosins, myosin heavy chain (HC) isoforms and myosin light chain (LC) isoforms in the soleus muscle of the rat. The suspended soleus showed a shift in the HC isoform distribution with a marked increase in fast HC isoforms and a commensurate decrease in HCI. In addition, the change in the fast HC isoforms consisted of the expression of HCIId and HC IIb absent in the normal soleus. In contrast to HC isoforms, suspension did not lead to appreciable changes in LC isoform distribution. Analyses of electrophoresis under nondenaturing conditions demonstrated that the normal soleus expressing HCI and HCIIa isoforms contained two isomyooins. Although, of the two isomyosins observed in the normal soleus, the faster migrating band most likely represented the HCIIa-based one (FMas), its mobility was not identical with that of the HCIIa-based isomyosin (FMaf) found in fast-twitch muscles, migrating in the order FMaf>FMas. FMas was designated as intermediate isomyosin (IM) . Some of the suspended soleus contained slow isomyosin (SM) and IM whereas the others comprised FM 3 and/or FM 2 as well as SM and IM. In spite of the expression of HCIIb and HCIId in the suspended soleus, FM 3 and FM 2 observed in these muscles exhibited distinct mobilities from either HCIId-based or HCIIb-based isomyosins comprised in fast-twitch muscles. These results suggest that some of newly expressed HCIId and/or HCIIb isoforms in the suspended soleus are associated with not only fast but also slow LC isoforms and function as a constitutive element of the myosin molecule.