Hypovolemia and hyperosmolality due to thermal dehydration suppress thermoregulatory responses of sweating and cutaneous vasodilation in humans, resulting in increasing a risk of heat illness. Recently, we found in young and older subjects that an ingestion of carbohydrate-protein supplement immediately after a bout of exercise during training accelerated an increase in plasma volume and an improvement of thermoregulatory responses. These results suggest that change in plasma volume alters cutaneous vasodilation and sweat rate through baroreflexes; however, no electrical signals in the efferent path of the reflex loop have not been identified. We have recently successfully recorded skin sympathetic nerve signal components synchronized and non-synchronized with cardiac cycles, separately, in passively heated young subjects, and found that although both components increased with cutaneous vasodilation and sweat rate in hyperthermia, an increase in synchronized component was suppressed by hypovolemia with suppressed cutaneous vasidilation, while an increase in non-synchronized component was not suppressed as sweat rate. On the other hand, we found that hyperosmolality suppressed the increases of both components with suppressed cutaneous vasodilation and sweat rate. These results suggest that a synchronized component controls cutaneous vasodilation while a non-synchronized component controls sweat rate, and also that beat-by-beat changes in atrial pressure due to a fluctuation of venous return to the heart varies cutaneous vasodilation through baroreflexes but not sweat rate.

Muscle strength per unit muscle area related to age was investigated in normal 131 males and 127 females, 7 to 18 years of age. A cybex machine was used to measure the isometric muscle strength of elbow flexion and extension, knee flexion and extension, respectively. The cross-sectional muscle area of extensor and flexor at right upper arm and thigh were determined by using ultrasonic apparatus. The muscle area in male increased with age from 7 years to 17 or 18 years. Females showed increment in muscle area from 7 to 12 years for elbow flexor muscle, and from 7 to 16 years for another muscles. Muscle strength in both males and females increased with age from 7 to 16 or 17 years. Muscle strength per unit muscle area tended to increase with age in males from 7 to 12 years, and in girls 7 to 9 years except for elbow extension. The strength per area under 12 years of age for males and 9 years of age for females except for elbow extension were significantly lower than that of the above age. There were little differences in the strength per unit area with age among males from 13 to 18 years and females from 10 to 18 years, respectively. These results indicate that under 12 years for boys and 9 years for girls the increment in muscle area with age is not accompanied with the development of muscle function.

The maximal isometric strength of the knee extension (KES) and cross-sectionl area of m. quadriceps femoris (MQF) were measured on 89 male Japanese elite athletes and 14 untrained men to evaluate the morphological and functional characteristics of athletes in the knee extensor muscles. Athletes were specialized in seven different sport events; 16 sumo wrestlers, 22 oarsmen, 8 volleyball players, 16 speed skaters, 12 association football plyers, 5 sprinters, 10 middle and long distance runners. KES was measured by using a specially designed straingages dynamometer at knee angle of 110 degrees (180 degrees= fully extended) . Cross-sectional area of MQF at the mid-thigh was determined by ultrasonic apparatus. The following results were obtained.
1. Sumo wrestters showed highest mean values (110.18 cm²) in MQF area among athlete groups. Lower mean values in MQF area were found in sprinters (86.34 cm²) and distance runners (73.86 cm²) whose values were not significantly different from that of untrained (75.32 cm²) .
2. In the cross-sectional area of each muscle bundle of MQF, sumo wrestlers, volleyball players and speed skaters showed higher mean values, and sprinters and distance runners had lower mean values as well as untrained men.
3. Higher mean values in percentage of MQF area to the total muscle area of thigh were observed in volleyball players (58.66%) and oarsmen (57.53%), lower mean value in association football players (53.81%), respectively. However, there were no significant differences in percentage of MQF area between untrained and each athlete group.
4. In the percentage of each muscle bundle area to MQF area, m. rectus femoris of association football players and m. vastus lateralis of speed skaters were significantly higher mean values compared with the untrained and other athlete groups.
5. KES were significantly related to the MQF area at 0.1% level (r=0.657) . There were significant relationships KES and each muscle bundle area in MQF area at 0.1% level. In the relationship between KES and each muscle bundle area, m. vastus lateralis showed the highest correlation coefficient (r=0.603) .
6. Association football players showed highest mean value (8.97 N/cm²) in KES per unit of MQF area among athlete groups. Its value was significantly higher than that of untrained men (8.06 N/cm²), distance runners (7.31 N/cm²), volleyball players (7.10N/cm²), and sumo wrestlers (7.50N/cm²) . There were no significant differences in KES per unit of MQF area intra athlete groups except for association football players.

The purpose of the present study was to determine the effect of isometric strength training in prepubescent boys and girls on muscle strength and cross-sectional area of upper arm muscle. Subjects were ninety-nine healthy elementary school children from the 1 st, 3 rd and 5th grades and who were assigned to either training (TG, n=52) or control (CG, n=47) groups. The TG participated in a strength training program for 12 weeks which consisted of three maximaly sustained isometric contractions of elbow flexion for ten sec, twice a day, three days per week, whereas the CG did not participate in a special training program during this period. The cross-sectional areas of whole tissue, muscle, fat, and bone of the upper arm were measured by ultrasonic methods. Maximum isometric and isokinetic muscle strength of elbow flexion and extension were measured by means of a Cybex II. The hand-wrist X-ray photograph was used to estimate the skeletal age (TW 2 method) . Means (SDs) of skeletal age (yrs) were 6.27 (0.98), 8.48 (0.89), and 10.77 (1.22) for 1 st, 3 rd, and 5 th graders, respectively. After the 12-week training period, the cross-sectional area of the whole tissue increased significantly in both TG (8.9%, 2.52 cm²) and CG (7.3%, 2.11 cm²) . This interment in the TG was due to the significant increases in both muscle and bone areas, while that in CG was due to the significant increase only in fat area. Mean increase in muscle area in the TG was 10.3% (1.29 cm²) with this increase notable in 5th graders (12.5% for males and 12.7% for females) . The increment in cross-sectional area of muscle with training significantly correlated with the skeletal age (r=0.36, p<0.01) . Maximum isometric strength in the TG increased by 5.7% (0.3kg) in flexion and 17.5% (1.2 kg) in extension and in the CG, -0.3 kg (-1.0%) in flexion and 5.7% (0.3 kg) in extension. The increases in the strength of elbow extension were statistically significant in 1st (p<0.01), 3rd (p<0.05), and 5th (p<0.05) grade males. On the other hand, maximum isokinetic strength did not change with training. Muscle strength per cross-sectional area did not show a significant increase except in the training group of 5th grade boys. It was suggested that the effects of strength training on muscle area and strength for prepubescents were similar in its direction to but different in its magnitude from those found in adults.