Relation of heart rate to percent VO2 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 (%Vo(2 max)) utilized due to reduced maximal O(2) uptake (Vo(2 max)) measured after exercise under the same thermal conditions. Peak O(2) uptake (Vo(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 Vo(2 peak). The conditions involved walking for 20 min at approximately 33% of control Vo(2 max) in 25, 35, 40, and 45 degrees C followed immediately by measurement of Vo(2 peak) in the same thermal environment. Vo(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 %Vo(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 %Vo(2 peak) utilized, which is caused by reduced Vo(2 peak) measured during exercise in the heat. At high T(a), the dissociation of HR from %Vo(2 peak) measured after sustained submaximal exercise is less than if Vo(2 max) is assumed to be unchanged during exercise in the heat.

Adaptation of upper airway muscles to chronic endurance exercise.

We tested the hypothesis that chronic endurance exercise is associated with the recruitment of four major upper airway muscles (genioglossus, digastric, sternohyoid, and omohyoid) and results in an increased oxidative capacity and a fast-toward-slow shift in myosin heavy chain (MHC) isoforms of these muscles. Female Sprague-Dawley rats (n = 8; 60 days old) performed treadmill exercises for 12 weeks (4 days/week; 90 minutes/day). Age-matched sedentary female rats (n = 10) served as control animals. Training was associated with an increase (p < 0.05) in the activities of both citrate synthase and superoxide dismutase in the digastric and sternohyoid muscles, as well as in the costal diaphragm. Compared with the control animals, Type I MHC content increased (p < 0.05) and Type IIb MHC content decreased (p < 0.05) in the digastric, sternohyoid, and diaphragm muscles of exercised animals. Training did not alter (p > 0.05) MHC phenotype, oxidative capacity, or antioxidant enzyme activity in the omohyoid or genioglossus muscle. These data indicate that endurance exercise training is associated with a fast-to-slow shift in MHC phenotype together with an increase in both oxidative and antioxidant capacity in selected upper airway muscles. It seems possible that this exercise-mediated adaptation is related to the recruitment of these muscles as stabilizers of the upper airway.