The energy cost of running increases with the distance covered

SummaryThe net energy cost of running per unit of body mass and distance (Cr, ml O2·kg−1·km−1) was determined on ten amateur runners before and immediately after running 15, 32 or 42 km on an indoor track at a constant speed. The Cr was determined on a treadmill at the same speed and each run was performed twice. The average value of Cr, as determined before the runs, amounted to 174.9 ml O2·kg−1·km−1 SD 13.7. After 15 km, Cr was not significantly different, whereas it had increased significantly after 32 or 42 km, the increase ranging from 0.20 to 0.31 ml O2·kg−1·km−1 per km of distance (D). However, Cr before the runs decreased, albeit at a progressively smaller rate, with the number of trials (N), indicating an habituation effect (H) to treadmill running. The effects of D alone were determined assuming that Cr increased linearly with D, whereas H decreased exponentially with increasing N, i.e.Cr =Cr0+aD+He−bN. The Cro, the “true” energy cost of running in nonfatigued subjects accustomed to treadmill running, was assumed to be equal to the average value of Cr before the run for N equal to or greater than 7 (171.1 ml O2·kg−1·km−1, SD 12.7;n = 30). A multiple regression of Cr on N and D in the form of the above equation showed firstly that Cr increased with the D covered by 0.123%·km−1, SEM 0.006 (i.e. about 0.22 ml O2·kg−1·km−1 per km,P<0.001); secondly, that in terms of energy consumption (obtained from oxygen consumption and the respiratory quotient), the increase of Cr with D was smaller, amounting on average to 0.08%·km−1 (0.0029 J·kg−1·m−1,P<0.001) and thirdly that the effects of H amounted to about 16% of Cr0 for the first trial and became negligible after three to four trials.