The purpose of this study was to compare running economy across three submaximal speeds expressed as both oxygen cost (ml ⅐ kg Ϫ1 ⅐ km Ϫ1 ) and the energy required to cover a given distance (kcal ⅐ kg Ϫ1 ⅐ km Ϫ1 ) in a group of trained male distance runners. It was hypothesized that expressing running economy in terms of caloric unit cost would be more sensitive to changes in speed than oxygen cost by accounting for differences associated with substrate utilization. Sixteen highly trained male

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... istance runners [maximal oxygen uptake (V O2max) 66.5 Ϯ 5.6 ml ⅐ kg Ϫ1 ⅐ min Ϫ1 , body mass 67.9 Ϯ 7.3 kg, height 177.6 Ϯ 7.0 cm, age 24.6 Ϯ 5.0 yr] ran on a motorized treadmill for 5 min with a gradient of 0% at speeds corresponding to 75%, 85%, and 95% of speed at lactate threshold with 5-min rest between stages. Oxygen uptake was measured via open-circuit calorimetry. Average oxygen cost was 221 Ϯ 19, 217 Ϯ 15, and 221 Ϯ 13 ml ⅐ kg Ϫ1 ⅐ km Ϫ1 , respectively. Caloric unit cost was 1.05 Ϯ 0.09, 1.07 Ϯ 0.08, and 1.11 Ϯ 0.07 kcal ⅐ kg Ϫ1 ⅐ km Ϫ1 at the three trial speeds, respectively. There was no difference in oxygen cost with respect to speed (P ϭ 0.657); however, caloric unit cost significantly increased with speed (P Ͻ 0.001). It was concluded that expression of running economy in terms of caloric unit cost is more sensitive to changes in speed and is a more valuable expression of running economy than oxygen uptake, even when normalized per distance traveled. caloric cost; distance running; respiratory exchange ratio; running performance Address for reprint requests and other correspondence: B. R. MacIntosh, Downloaded from Fig. 4 . Caloric unit cost for each subject measured at his absolute speeds. Each group represents 1 subject's caloric unit cost at the 3 measured speeds corresponding to 75%, 85%, and 95% sLT for that subject. CALORIC COST AND RUNNING ECONOMY