2007, Piran, Slovenia

Environmental Ergonomics XII
Igor B. Mekjavic, Stelios N. Kounalakis & Nigel A.S. Taylor (Eds.), © BIOMED, Ljubljana 2007
DISCUSSION
It has been reported that since the milking action of skeletal muscles is lower during baseball
practice or games than during soccer or athletics practice, a strong orthostatic circulatory
regulation reflex occurs. Furthermore, to prevent the transfer of heat radiation from the
environment to the body at an ambient temperature higher than the skin temperature,
contraction of the skin blood vessels is considered to be stronger in children than in adults,
because the size of sweat glands is smaller in the former. Since non-evaporative heat
dissipation via skin blood flow is suppressed by the strong contraction of skin blood vessels at
a high temperature, heat dissipation by sweating is considered to compensate for nonevaporative
heat dissipation. In this study, the contraction of skin blood vessels was
considered to occur through 2 different causes in the schoolchildren during baseball practice.
On the day of testing, the ambient temperature was very high, as shown in Fig. 1, and the
WBGT was 36.5°C, suggesting the occurrence of contraction of the skin blood vessels.
Figure 2 shows the relationship between the amount of sweating and water intake, indicating
that water intake was clearly insufficient for sweating (voluntary dehydration). The osmotic
pressure, and myoglobin, uric acid, and BUN levels were significantly higher after than
before practice, suggesting hemoconcentration (Figure 3). Exercise damages skeletal muscles,
and rhabdomyolysis induces acute renal failure. Hyperuricemia induced by metabolizing
purine precursors derived from muscular cells into uric acid in the liver, and myoglobin
generated by the destruction of skeletal muscles are considered to cause acute renal failure,
and dehydration is reported to be strongly involved in this disorder. In the baseball teams
examined in this study, muscle training was performed over about 30 min, and it is suggested
that myoglobin in the blood was increased by this training, and hemoconcentration occurred
with dehydration, resulting in the high blood concentration of myoglobin.
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