Intensity related changes of running economy in recreational level distance runners

BACKGROUND: Running economy (RE) is often described as a key demand of running performance. The variety of currently used assessment methodswith different running intensities and outcomes restricts interindividual comparability of RE in recreational level runners. The purpose of this study was to compare the influence of RE, assessed as oxygen cost (OC) and caloric unit cost (CUC), on running speed at individual physiological thresholds. METHODS: Eighteen recreational runners performed: 1) a graded exercise test to estimate first ventilatory threshold (VT1), respiratory compensation point (RCP) and maximal oxygen uptake (VO2max); 2) discontinuous RE assessment to determine relative OC in milliliters per kilogram per kilometer (mL/kg/km) and CUC in kilocalories per kilogram per kilometer (kcal/kg/km) at three different running intensities: VT1, RCP and at a third standardized reference point (TP) in between. RESULTS: OC (mL/kg/km; at VT1: 235.4 +/- 26.2; at TP: 227.8 +/- 23.4; at RCP: 224.9 +/- 21.9) and CUC (kcal/kg/km at VT1: 1.18 +/- 0.13; at TP: 1.14 +/- 0.12; at RCP: 1.13 +/- 0.11) decreased with increasing intensities (P=0.01). Controlling for the influence of sex OC and CUC linearly correlated with running speed at RCP and VO2max (P=0.01). CONCLUSIONS: RE, even assessed at low intensity, is strongly related to running performance in recreational athletes. Both calculation methods used (OC and CUC) are sensitive for monitoring intensity related changes of substrate utilization. RE values decreased with higher running intensity indicating an increase of anaerobic and subsequent decrease of aerobic substrate utilization.