The Effect of Hyperthermia on Cerebral Hemodynamics, Biological Fatigue and Cardiovascular Strain During 40km Time- Trial Cycling in Trained Triathletes
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The 40 kilometer cycling time-trial (40kmTT) is a sanctioned event in the summer Olympics and is an integral part of the olympic distance triathlon. It is often performed in hot humid climates. Thus, both the exercising muscle and the brain are subject to significant biological fatigue and task failure. This study examined the influence of a 40kmTT performed in the heat on central (brain) and peripheral (muscle) variables. It was hypothesized that middle cerebral artery velocity (MCAv) and the degree of muscle activation ( integrated electromyography - iEMG) of the vastus lateralis would decrease during 40kmTT cycling in hyperthermia (HYP) when compared to thermoneutrality (TN). Cardiovascular strain (stroke volume – SV, mean arterial pressure - MAP, cardiac output – Q and heart rate – HR) was expected to be higher in the HYP versus the TN 40kmTT. Twelve triathletes completed a 40kmTT in HYP (40ºC) and TN (18ºC) on a Velotron cycle ergometer. iEMG (normalized to signal intensity at VO2max - %PP), MCAv (cm·s-1), performance measures, cardiovascular strain parameters, ventilatory measures were recorded and averaged over a one minute period immediately prior to the completion of every five kilometers. Results showed that iEMG was significantly lower at 30, 35 and 40km in HYP compared to TN (26.2 ± 7.7, 26.2 ± 5.4 and 26.7 ± 9.2 %PP versus 35.1± 10.2, 36.1 ± 10.1 and 40.7 ± 11.3 %PP respectively, p < 0.05). This coincided with slower 5km splits during HYP over these intervals (10.06 ± 1.4, 9.81 ± 1.2 and 10.03 ± 1.1 minutes versus 8.92 ± 1.0, 8.81 ± 0.9 and 8.51 ± 0.9 minutes, p < 0.05). No change in MCAv between and the HYP and TN 40kmTT (p < 0.05) were found, despite higher cardiovascular strain (i.e., reduced MAP and SV and increased HR). Protective mechanism of the cerebral circulatory system maintains cerebral blood flow over a wide range of physiological conditions. In this investigation the degree of muscle activation of the vastus lateralis was found to be significantly influenced by heat, likely contributing to the reduced 40kmTT performance.