Abstract
Although the acute effects of exercise on blood rheological variables have been briefly examined, the impact of varying time of day on these responses is not known (El-Sayed 1998). The aim of the present study was to examine the effect of exercise on the main determinants of blood rheology (haematocrit, plasma viscosity, and plasma fibrinogen), and to assess the effects, if any, of varying time of day on these measurements. Ten normal healthy subjects volunteered to participate in the study. Following the determination of VO2max, subjects performed two sub-maximal exercise trials at 09:00 and 17:00 hours to investigate the effect of time of day on blood rheological variables. On two separate occasions, with 7d intervening, subjects performed a cycling exercise test at an intensity corresponding to 60% VO2max for 45 min. Venous blood samples were obtained, with no stasis, at rest, post exercise and also following 30 min recovery. Whole blood was mixed gently with EDTA and a small portion of this was used for the measurement of haematocrit, haemoglobin, and lactate. The remainder of the blood was then centrifuged and platelet rich plasma was separated and used for the measurement plasma viscosity, and plasma fibrinogen concentration. Plasma volume change following exercise was calculated from haematocrit and haemoglobin values (Dill and Costil 1974). Blood lactate concentration, haematocrit, and haemoglobin exhibited similar increase (P<0.05) after morning and evening exercise trials, but returned back to the pre-exercise levels at the end of the recovery period. The mean resting values of the blood rheological variables measured were within the ranges reported in the literature for normal subjects. Post-exercise plasma volume decreased significantly (P<0.05) after both the morning and the evening test trials. Although the contraction of plasma volume after exercise in the morning trial was slightly greater than that observed in the evening trial, this difference was small and did not reach the assigned level of significance (P>0.05). Plasma viscosity and plasma fibrinogen concentration showed a significant (P<0.05) increase after exercise when raw data were not adjusted for plasma volume changes, but these increases were no longer apparent (P>0.05) when raw data were corrected for plasma volume changes. These changes in blood rheological variables post-exercise were similar irrespective of time of day the exercise was performed. In addition, these modifications were transient hence the increase in haematocrit, plasma viscosity, and plasma fibrinogen returned to the baseline values 30-min after exercise. It is concluded therefore that acute exercise induces significant changes in blood rheological variables mainly due to haemoconcentration and these changes do not appear be influenced significantly by the two different times of day chosen. REFERENCES El-Sayed. M.S (1998) Effect of exercise and training on blood rheology. Sports Medicine, 26 (5) 281-292. Dill D.B and Costill D.L (1974) Calculation of percentage change in volumes of blood, plasma, and red blood cells in hydration. Journal of Applied Physiology, 37, 247-284.