THE ENERGY-COST OF RUNNING INCREASES WITH THE DISTANCE COVERED

The net energy cost of running per unit of body mass and distance (C(r), 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 C(r) was determined on a treadmill at the same speed and each run was performed twice. The average value of C(r), as determined before the runs, amounted to 174.9 ml O2.kg-1.km-1, SD 13.7. After 15 km, C(r) 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, C(r) 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 C(r) increased linearly with D, whereas H decreased exponentially with increasing N, i.e. C(r) = C(r0) + aD + He(-bN). The C(r0), the "true" energy cost of running in nonfatigued subjects accustomed to treadmill running, was assumed to be equal to the average value of C(r) 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 C(r) on N and D in the form of the above equation showed firstly that C(r) 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 C(r) 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 C(r0) for the first trial and became negligible after three to four trials.