Core temperature and maximal oxygen uptake during exercise in the heat: implications for exercise prescription
Arngrimsson, Sigurbjorn Arni
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Previous studies have suggested that attainment of a critical core temperature may limit prolonged, exhaustive exercise in the heat. The aims of this study were to test the hypotheses that large (> 10%) reductions in maximal oxygen uptake (V]O2max) are associated with attainment of a critical core temperature, and that elevation in heart rate during submaximal exercise is related to reduced V]O2max, in the heat. V]O2max and esophageal temperature at exhaustion (TESOmax), and oxygen uptake and heart rate during submaximal exercise, were measured in 11 male and 11 female runners under seven conditions designed to manipulate the TESO at which V]O2max was reached by varying environmental temperature and preheating. The conditions were: 1) 25O C, no preheating (control); 2-4 and 6) 25, 35, 40, and 45O C, with preheating by a 20-min walk at ~33% of control V]O2max; 5) 45O C, no preheating; and 7) 45O C, with passive preheating raising TESO to the same degree as at the end of the 20-min walk in Condition 6. Compared to the control condition, V]O2max was reduced 3-9% under Conditions 2-5 when core temperature at V]O2max was below the apparent critical core temperature (< 39.2O C in men, < 39.1O C in women), but was reduced 17-19% under Conditions 6-7 when a critical core temperature appeared to have been attained (39.6-39.7O C in men, 39.3-39.4O C in women). Reductions in time to exhaustion were strongly related to reductions in V]O2max (r = 0.82-0.84). The effects on heat on TESOmax, V]O2max, and work capacity in men and women in the heat were almost identical. Heart rate and relative metabolic intensity (%V]O2max) during submaximal exercise increased in an identical fashion in the heat, and the elevations in heart rate were significantly related to the reductions in V]O2max (r = 0.79). It was concluded that large reductions in V]O2max andwork capacity in severe, uncompensable heat stress are associated with attainment of a critical core temperature, and that the reductions in the two are strongly related. Men and women do not differ in TESOmax, or in the relation of TESOmax to the reductions in V]O2max and work capacity, in the heat. Elevation in heart rate during submaximal exercise in the heat is proportional to reduced V]O2max, and not indicative of dissociation between heart rate and %V]O2max.