2007, Piran, Slovenia

Non-thermal factors
THE DIFFERENCE OF HEMATOLOGICAL STATUS AND
CARDIOVASCULAR RESPONSE DURING BASEBALL PRACTICE IN
SCHOOLCHILDREN.
Yoko Kajiwara 1 , Masami Hirashita 2 , Saburo Yokokura 3
1 Faculty of Education, Bunkyo University, Japan
2 Department of Social Welfare, Kinjyo University, Japan
3 School of Science and Engineering, Meisei University, Japan
Contact person: kajiwara@koshigaya.bunkyo.ac.jp
INTRODUCTION
Since a large amount of heat is generated by skeletal muscles during exercise, the deep body
temperature elevates, and evaporative (sweating) or non-evaporative heat dissipation is
enhanced. With the elevation of ambient temperature, heat loss facilitated by non-evaporative
heat dissipation via blood circulation in the skin decreases, while the dependence on
evaporative heat dissipation (sweating) increases. Sweating alone is effective in heat
dissipation at a temperature above skin temperature. In Japan, the rainy season generally ends
in late July, and subsequently, the temperature is higher than 32°C over several weeks.
Although the density of active sweat glands is higher in children than in young adults, the
amount of sweating is smaller due to the smaller size of sweat glands [2, 3, 4]. To
compensate, heat dissipation is enhanced in children by further dilation of cutaneous blood
vessels [2, 4,]. Therefore, to prevent heat transfer from the environment to the body at an
ambient temperature higher than skin temperature, the contraction of skin blood vessels is
considered to be induced [5]. Since the ratio of the body-surface area to body weight is higher
in children, heat transfer can be larger, suggesting that the contraction of skin blood vessels is
stronger in children.
Spontaneous dehydration increases with exercise duration, and it is estimated that it is high in
schoolchildren during baseball. On the other hand, it has been reported that muscle cells are
damaged by resistance exercise, and lactic acid and myoglobin flow into the blood. It has also
been reported that blood concentrations of such myoglobin and uric acid generated by protein
degradation after eating or during exercise are high in athletes. It has been indicated that
further increases in these compounds in the blood during dehydration reduce renal function.
This is named skeletal myolysis, which is considered to be a form of heat injury. In this study,
we examined the relationship between dehydration and the blood concentration of myoglobin
or uric acid in schoolchildren during baseball exercise at a high temperature in summer.
METHODS
The subjects were 8 schoolchildren (age 10.5 ± 1.2 yr) in the fifth or sixth grade in 2
randomly selected baseball teams in Ishikawa Prefecture, and consent was obtained from the
children and their parents. Table 1 summarizes the physical characteristics of the children.
Procedures: The children gathered 30 min before the start of practice, and rested. After blood
collection, the body weight, blood pressure, and tympanic temperature were measured. A
cardiotachometer and an energy metabolimeter were fixed to the children before the start of
practice, and they started routine practice. A time study was performed during the practice
period. After the completion, the children rested, sitting in the shade, and blood pressure and
tympanic temperature were measured, and blood collected and body weight measured.
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