Background <p>Running economy is influenced by both anthropometric and biomechanical factors. Among these, body mass (BM) has a particularly significant impact on the energy cost of running.</p> Purpose <p>This study investigated how BM affects the correlation between RE and lower limb kinematic variables, including segmental movement coordination during submaximal running.</p> Methods <p>Twenty recreationally active (male and female; 26.5 ± 5.8&#xa0;years; 64.0 ± 10.9&#xa0;kg) participated. Running economy was assessed by measuring oxygen uptake (VO₂) during a submaximal treadmill running. Kinematic data, including continuous relative phase for thigh-leg and leg-foot couplings, were collected via video analysis. VO₂ was subsequently adjusted for BM using the ratio standard (VO2.BM⁻¹) and the allometric method (VO2.BM^−0.75).</p> Results <p>Moderate to large correlation (<i>r</i> = 0.429–0.909; <i>p</i> ≤ 0.05) were observed between VO₂ and VO₂.BM⁻¹ with contact time, flight time, step frequency, step length, and maximal ground reaction force. When VO₂.BM^−0.75 was considered, no significant correlation was found with these variables. A moderate correlation (<i>r</i> = 0.493; <i>p</i> ≤ 0.05) was observed between VO2.BM^−0.75 and the continuous relative phase of thigh-leg coupling.</p> Conclusion <p>Adjusting VO₂ for BM using the allometric method alters its relationship with biomechanical variables, emphasizing the need for appropriate scaling to avoid misleading interpretations in correlation analyses. In addition, the findings suggest that improved intersegmental coordination of the thigh–leg coupling may contribute to a lower energy cost during running.</p>