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 ([Formula: see text]) and the energy cost of running (Cr ) 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 [Formula: 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 [Formula: see text], HILL), separated by three, 100 min periods of self-paced running (SP). We measured [Formula: see text], minute ventilation ([Formula: see text]), ventilatory efficiency ([Formula: see text]), respiratory exchange ratio (RER), Cr , 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 [Formula: see text] increased (10-CR), and RER decreased (10-CR and HILL). There were no significant changes in [Formula: see text], [Formula: see text], Cr , 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 [Formula: see text] at moderate intensity, and no effect of exercise duration on ventilatory efficiency, energy cost of running and tissue oxygenation.