A slow V̇O2 on-response allows to comfortably adopt aerobically unaffordable walking and running speeds in short stairs ascending.

The aim of this paper is to investigate the mechanical and metabolic reasons of the spontaneous gait/speed choice of ascending short flight of stairs, where walking on every step or running on every other step are frequently interchangeable options. Twenty-four subjects' kinematics, oxygen uptake (V̇O2), ventilation and heart rate were sampled during climbing one and two flights of stairs while using the two gaits. Although motor acts were very short in time (5-22 s), metabolic kinetics, extending in the successive 250 s after the end of climbing, consistently reflected the (equivalent of the) needed mechanical energy and allowed to compare the two ascent choices: despite a 250% higher mechanical power associated to running, measured V̇O2, ventilation and heart rate peaked only at +25% with respect to walking, and in both gaits at a much lower values than V̇O2max despite of predictions based on previous gradient locomotion studies. Mechanical work and metabolic cost of transport, as expected, showed similar increase (+25%) in running. For stairs up to 4.8 m tall (30 steps at 53% gradient), running makes us consuming slightly more calories than walking, and in both gaits at no discomfort at all. The cardio-respiratory-metabolic responses similarly delay and damp the replenishing of phosphocreatine stores, which were much faster depleted during the impulsive, highly powered mechanical event, with almost overlapping time courses. Such a discrepancy between mechanical and metabolic dynamics allows to afford almost-to-very anaerobic climbs and to interchangeably decide whether to walk or run up a short flight of stairs.