Relation of heart rate to percentV˙ o 2 peak during submaximal exercise in the heat

We tested the hypothesis that elevation in heart rate (HR) during submaximal exercise in the heat is related, in part, to increased percentage of maximal O 2 uptake (%V˙o 2 max ) utilized due to reduced maximal O 2 uptake (V˙o 2 max ) measured after exercise under the same thermal conditions. Peak O 2 uptake (V˙o 2 peak ), O 2 uptake, and HR during submaximal exercise were measured in 22 male and female runners under four environmental conditions designed to manipulate HR during submaximal exercise and V˙o 2 peak . The conditions involved walking for 20 min at ∼33% of controlV˙o 2 max in 25, 35, 40, and 45°C followed immediately by measurement ofV˙o 2 peak in the same thermal environment. V˙o 2 peak decreased progressively (3.77 ± 0.19, 3.61 ± 0.18, 3.44 ± 0.17, and 3.13 ± 0.16 l/min) and HR at the end of the submaximal exercise increased progressively (107 ± 2, 112 ± 2, 120 ± 2, and 137 ± 2 beats/min) with increasing ambient temperature (T a ). HR and %V˙o 2 peak increased in an identical fashion with increasing T a . We conclude that elevation in HR during submaximal exercise in the heat is related, in part, to the increase in %V˙o 2 peak utilized, which is caused by reduced V˙o 2 peak measured during exercise in the heat. At high T a , the dissociation of HR from %V˙o 2 peak measured after sustained submaximal exercise is less than ifV˙o 2 max is assumed to be unchanged during exercise in the heat.