Overground running incurs a higher energetic cost than treadmill running at a 1% grade: A comparison of running economy, oxygen cost of transport, and energy cost in endurance athletes

This study compared the physiological and metabolic demands of treadmill and overground running by examining running economy (RE), oxygen cost of transport (O2-COT), and energy cost (EC) across ventilatory threshold-scaled intensities in trained endurance athletes. Twelve male runners (21.3‍±‍2.4 years; 176.2‍±‍6.8 cm; 67.8‍±‍5.9 kg) completed a ramp test to determine V̇O2max, followed by identical stepwise protocols (8-15 km·h‍1‍¹; 3-min stages) performed indoors on a treadmill and outdoors on a 400 m track. Breath-by-breath gas exchange was averaged over the final 60-90 s of each stage to determine steady-state V̇O2. Within-subject comparisons were used to evaluate differences between field and treadmill running across exercise intensities. Two-way repeated-measures ANOVA revealed significant main effects of Condition and Intensity domain for all three primary outcomes (all p‍≤‍0.012). RE was significantly higher during overground running (51.3‍±‍2.2 vs. 49.6‍±‍3.1 mL·kg‍1‍¹·min‍1‍¹; p-FDR‍=‍0.01), with divergence occurring primarily at intensities between 70% and 100% of VT2. O2-COT was consistently greater during overground running across all intensity domains (204.2‍±‍11.3 vs. 197.1‍±‍12.3 mL·kg‍1‍¹·km‍1‍¹; p-FDR‍≤‍0.01), with no significant Condition‍×‍Intensity interaction, indicating a consistent between-condition difference across all intensity domains. Energy cost remained consistently higher during overground running (3.87‍±‍0.21 to 4.60‍±‍0.27 vs. 3.65‍±‍0.22 to 4.10‍±‍0.18 J·kg‍1‍¹·m‍1‍¹; p-FDR‍<‍0.001), with a significant Condition‍×‍Intensity interaction (p‍=‍0.045). Carbohydrate oxidation was significantly higher during overground running at intensities approaching VT2 (p-FDR‍=‍0.02). These findings suggest that the commonly used 1% treadmill grade correction does not fully replicate the energetic demands of overground running, with implications for laboratory-based performance evaluation.