Interaction effects of time of day and sub-maximal treadmill exercise on the main determinants of blood fluidity.

The purpose of this study was to investigate the effects of time of day on responses of the main determinants of blood rheology to acute endurance exercise. Ten healthy male subjects (age, 26.9 +/- 5.5 yr) performed two bouts of running at 65% of VO2peak for 45 min on a motorised treadmill in the morning (08:00 h) and evening (20:00 h), which were followed by 30 min recovery. The two exercise trials were performed in two separate days with 7 days intervening. Haemorheological variables were measured before, immediately after exercise and after recovery. Haematocrit, haemoglobin and RBC count were increased significantly (p < 0.01) after 45 min running in both morning and evening trials and normalised following recovery, irrespective of time of day. Plasma viscosity increased significantly (F2,18 = 12.4, p < 0.01) following sub-maximal exercise in both trials and returned to pre-exercise level at the end of recovery. Baseline values (p < 0.05) but not responses of plasma viscosity to exercise were significantly affected by time of day. Neither a significant main effect of exercise nor a significant (p > 0.05) time-of-day effect was found for plasma proteins. It was concluded that sub-maximal running at 08:00 or 20:00 h does not induce different responses in the main determinant of blood rheology.