Pubblicazioni

nic tread Energetics of vertical kilometer foot races; is steeper cheaper?

Nicola Giovanelli, Amanda Louise Ryan Ortiz, Keely Henninger and Rodger Kram

ABSTRACT: Vertical kilometer foot races consist of a 1,000 m elevation gain in less than 5,000 m of overall distance and the inclines of the fastest courses are ~30°. Previous uphill locomotion studies have focused on much shallower angles. We aimed to quantify the metabolic costs of walking and running on very steep angles and to biomechanically distinguish walking from running. Fifteen runners (10 M, 5 F, 32.9±7.5 years, 1.75±0.09 m, 64.3±9.1 kg) walked and ran for 5 minutes at 7 different angles (9.4°, 15.8°, 20.4°, 24.8°, 30.0°, 35.0° and 39.2°) all at a fixed vertical velocity (0.35 m/s). We measured the metabolic rates and calculated the vertical costs of walking (Cwvert) and running (Crvert). Using video analysis, we determined stride frequency, stride length and duty factor (fraction of stride that each foot is in ground contact). At all angles other than 9.4°, Cwvert was cheaper than Crvert (average -8.45%±1.05%; p<0.001). Further, broad minima for both Cwvert and Crvert existed between 20.4° and 35° (average Cwvert 44.17±0.41 J・kg-1・m-1 and average Crvert 48.46±0.35 J・kg-1・m-1). At all angles and speeds tested, both walking and running involved having at least one foot on the ground at all times. But, in walking, stride frequency and stride length were ~28% slower and longer, respectively than in running. In conclusion, we found that there is a range of angles for which energy expenditure is minimized. At the vertical velocity tested, on inclines steeper than 15.8°, athletes can reduce their energy expenditure by walking rather than running.