Relationships between fitness and blood viscosity in untrained normal short children

Abstract

18 healthy untrained children (13 boys, 5 girls) performed a 15 min submaximal incremental exercise on cycloergometer rising heart rate (HR) up to a final step (5min) at 90% of theoretical maximal heart rate. Whole blood viscosity (at high shear rate), plasma viscosity and hematocrit increased after exercise (p < 0.01). The index of fitness Ẇ 170 was negatively correlated with blood viscosity at rest (r = 0.752, p < 0.001). Plasma viscosity and hematocrit were also correlated with Ẇ 170 while relative blood viscosity at corrected hematocrit 45% did not show such a correlation. Therefore (a) exercising children undergo the same hemorheologic modification as adults; (b) Ẇ 170, a very classical index of fitness, is strongly related to blood fluidity; (c) the viscosity of blood at very high shear rate (i.e. reflecting the newtonian behavior of blood) is correlated with fitness; (c) the factors of blood viscosity involved in this relationship appear to be plasma viscosity and hematocrit rather than red cell flexibility.