Oxygen delivery does not limit peak running speed during incremental downhill running to exhaustion

Oxygen consumption ( \(\dot VO_2 \) ), ventilation ( \(\dot V_I \) ), respiratory exchange ratio (R), stride frequency and blood lactate concentrations were measured continuously in nine trained athletes during two continuous incremental treadmill runs to exhaustion on gradients of either 0‡ or −3°. Compared to the run at 0° gradient, the athletes reached significantly higher maximal treadmill velocities but significantly lower \(\dot VO_2 \) , \(\dot V_I \) ,R and peak blood lactate concentrations (P<0.001) during downhill running. These lower \(\dot VO_2 \) and blood lactate concentrations at exhaustion indicated that factors other than oxygen delivery limited maximal performance during the downhill run. In contrast, stride frequencies were similar at each treadmill velocity; the higher maximal speed during the downhill run was achieved with a significantly longer stride length (P<0.001); maximal stride frequency was the same between tests. Equivalent maximal stride frequencies suggested that factors determining the rate of lower limb stride recovery may have limited maximal running speed during downhill running and, possibly, also during horizontal running.