Influence of exercise duration on cardiorespiratory responses, energy cost and tissue oxygenation within a 6 hour treadmill run

PURPOSE: The physiological mechanisms for alterations in oxygen utilization ([Image: see text] ) and the energy cost of running (C(r)) during prolonged running are not completely understood, and could be linked with alterations in muscle and cerebral tissue oxygenation. METHODS: Eight trained ultramarathon runners (three women; mean ± SD; age 37 ± 7 yr; maximum [Image: see text] 60 ± 15 mL min(−1) kg(−1)) completed a 6 hr treadmill run (6TR), which consisted of four modules, including periods of moderate (3 min at 10 km h(−1), 10-CR) and heavy exercise intensities (6 min at 70% of maximum [Image: see text] , HILL), separated by three, 100 min periods of self-paced running (SP). We measured [Image: see text] , minute ventilation ([Image: see text] ), ventilatory efficiency ([Image: see text] ), respiratory exchange ratio (RER), C(r), muscle and cerebral tissue saturation index (TSI) during the modules, and heart rate (HR) and perceived exertion (RPE) during the modules and SP. RESULTS: Participants ran 58.3 ± 10.5 km during 6TR. Speed decreased and HR and RPE increased during SP. Across the modules, HR and [Image: see text] increased (10-CR), and RER decreased (10-CR and HILL). There were no significant changes in [Image: see text] , [Image: see text] , C(r), TSI and RPE across the modules. CONCLUSIONS: In the context of positive pacing (decreasing speed), increased cardiac drift and perceived exertion over the 6TR, we observed increased RER and increased HR at moderate and heavy exercise intensity, increased [Image: see text] at moderate intensity, and no effect of exercise duration on ventilatory efficiency, energy cost of running and tissue oxygenation.