Changes in the Energy Cost of Running during a 24-h Treadmill Exercise

Purpose: Although fatigue generally increases the energy cost of running (Cr), the changes of Cr and associated variables during an ultramarathon are not known. This study aimed to determine the changes of metabolic and cardiovascular adjustments during an ultraendurance exercise. Methods: Twelve healthy males ran 24 h on a motorized treadmill (24TR). Overall oxygen consumption ((V)over dotO(2) mL.min(-1).kg(-1)), net energy cost (Cr J.kg(-1).m(-1)), and respiratory exchange ratio (RER) were determined before, every 2 h, and after the 24TR at 8 km.h(-1). Running speed and heart rate (HR) were continuously measured during the 24TR. Results: (V)over dotO(2) increased (+7.6%, P < 0.001) during the 24TR, principally in the first 8 h of exercise. The RER mirrored changes in (V)over dotO(2), that is, decreased significantly until the eighth hour and remained constant thereafter. As a consequence of RER decrease, the increased Cr was markedly attenuated but was still significantly higher at the 8th and 12th hour compared with pre-24TR. Speed was constant over the first 6 h then significantly decreased during the 24TR. HR increased until the sixth hour (i.e., HR drift), then decreased until post-24TR. Furthermore, a significant positive correlation (R = 0.75, P < 0.01) was observed between the velocity sustained during the 24TR (expressed in percentage of the velocity attained at (V)over dotO(2max): %V-(V)over dotO2max) and the pre- to postchanges in Cr. Conclusions: The present study characterized accurately the changes of energy cost and substrate use during an extreme run, showing a plateau after 8 h of exercise. It is also concluded that the participants who maintained the highest %V-(V)over dotO2max were also those having most deteriorated their Cr over the 24TR, supporting the notion of a trade-off between running speed (relative to V-(V)over dotO2max) and Cr.