This study quantified the interaction of electromyography (EMG) obtained from the vastus lateralis and metabolic energy cost of running (C_r; mL·[mass+load]⁻¹·meter⁻¹), an index of running economy, during submaximal treadmill running. Experiments were conducted with and without load on the back on a motor-driven treadmill on the downhill, level and uphill slopes. The obtained EMG was full-wave rectified and integrated (iEMG). The iEMG was divided into eccentric (ECC) and concentric (CON) phases with a foot sensor and a knee-joint goniometer. The ratio of ECC to CON (ECC/CON ratio) was regarded as the muscle elastic capacity during running on each slope. The C_r was determined as the ratio of the 2-min steady-state VO₂ to the running speed. We found a significant decrease in the C_r when carrying the load at all slopes. The ECC/CON ratio was significantly higher in the load condition at the downhill and level slopes, but not at the uphill slope. A significant gradient difference was observed in the C_r (down<level<up) and ECC/CON ratio (down=level>uphill). Thus, an alteration of Cr by the gradient and load was almost consistent with that of the ECC/CON ratio. The ECC/CON ratio, but not the rotative torque (T) functioning around the center of body mass, significantly correlated with C_r (r=−0.41, p<0.05). These results indicated that the ECC/CON ratio, rather than T, contributed to one of the energy-saving mechanisms during running with load.