Endurance exercise is traditionally considered to be a function of three factors.
1. Cardiac output which is , according to the Fick equation, the product of cardiac output and a-v oxygen difference. Cardiac output is the product of stroke volume and heart rate.
2.Ability to perform at a high percentage of the O2 Max for prolonged periods.This can be measured or approximated by measurement of the lactate threshold.
The lactate threshold is lowered during exercise in hot and humid conditions.
Pappadopoulos et al (1) demonstrated in healthy young men that exercise at a temperature of 98 degrees that the lactate threshold was increased at a lower oxygen uptake ( from 2.86 +/- 0.13 to 2.38 +/- 0.09) while hypohydration was not shown to alter the lactate threshold.
Tanaka and Seals (2) state that there is evidence that the lactate threshold is lowered in older athletes but not when expressed relative to te percentage of 02 max.This suggests that decreases in lactate threshold may be secondary to decreases in 02 max. but however you look at it is lower.
)Pappadopoulos,c et al "Effect of hypohydration on the lactate threshold in a hot and humid environment." J Sports Med Phys Fitness. 2008 48 (3) 293-298
2)Tanaka,H and Seals,DR "Endurance exercise performance in Masters athletes:age associated changes and underlying physiological mechanisms. J Physiol 2008, 586 55-63