Human physical performance and cardio-circulatory responses to hot environments during sub-maximal upright cycling

Abstract

Four well-trained men performed a bout of moderate cycling exercise (66% &Vdot;O₂max) in an upright posture until exhaustion in environmental temperatures of 0°C, 20°C, or 40°C. Exhaustion was estimated as a score of 18 or higher on the Borg scale for each of the subjects. The performance time was not different at 0°C or 20°C, but at 40°C was markedly decreased (mean value: 91 min at 0°C, 87min at 20°C, 19minat 40°C). Compared with the results of measurements al exhaustion al 0°or 20°C, at 40°C there were significant decreases in 0₂-uptake (mean 2?69 ±0?131 min at 0°C vs. 2?29 ± 0-28 1 min ^-1 at 40°C, p <0?001), cardiac output (CO₂ rebreathing technique) (mean 19 42 ± 0 861min^-1at 0°C vs. 14 59± 1 01min^-1 at 40°C.p < 0001) and mean arterial blood pressure (mean 107 ± 3 1 mm Hgat0°C vs. 99 ± 3 3mm Hg at 40°C, p<0 001), and increases in arteriovenous O₂ difference (mean 136 7± 11 9mll^-1 at 0°C vs. 158 2 ± 25 5ml l^-1 at 40°C, p<0 001)and total peripheral resistance (TPR) (mean 5 49 ± 0 27 at 0°C us. 6 79 ± 0 67 at 40°C, p <0 001). Systolic time intervals during exercise were correlated with heart rate (HR) regardless of the temperatures. At 0°C left ventricular ejection time (LVET) and pre-ejection period (PEP) were slightly shortened at exhaustion (e.g. mean LVET 184 ± 12 ms at the 10th min to 177 ± 4 ms at exhaustion) along with an increase in HR but unchanged stroke volume (SV). At exhaustion al 40°C, although there were the same decrements (6ms) in LVET with slightly rising HR (mean HR 167 ± 9 beats min^-1 at the 10th min to 174 ± 4 beats min^-1 al exhaustion) as at 0°C, stroke volume (SV) declined significantly (mean SV 118 ± 9 ml at the 10th min to 84 ± 5 ml at exhaustion, p < 0 001). Left ventricular ejection fraction (LVEF; PEP/LVET)and rectal temperature (T_re) at exhaustion were almost the same regardless of ambient temperatures. The present results suggest that the decrease in physical performance with moderate exercise at 40°C is due to the failure to maintain the systemic circulation at a level adequate to continue exercise, and is not due to an increase in T_re. The failure may be caused by the alteration of peripheral circulation during exercise in hot environments, whereas myocardial contractility and LVEF may adapt to the new balance established between the decrease in left ventricular filling (SV) and the increase in peripheral resistance indicated by TPR.