Introduction Running economy (RE), Maximal Lactate Steady State (MLSS), and maximal oxygen uptake (VO2max) are pivotal factors influencing endurance running performance. Purpose: This study aimed to evaluate RE, MLSS, and VO2max across varying submaximal and maximal speeds, employing both oxygen cost (ml/kg/km) and caloric unit cost (kcal/kg/km) metrics in a cohort of highly trained male distance runners. It was hypothesized that caloric unit cost would afford greater sensitivity to speed-induced changes in running economy than traditional oxygen cost measurements, owing to its capacity to account for substrate utilization disparities. Methods Five highly trained male distance runners (mean ± SD: VO2max 64.5 ± 5.6 ml/kg/min, body mass 66.9 ± 7.3 kg, height 176.6 ± 7.0 cm, age 23.6 ± 5.0 yr) participated in treadmill running sessions lasting 3 minutes each, with a 1% gradient, starting at 12 km/h, and subsequent incremental speeds, with 1-minute rest intervals between stages for blood lactate collection. Oxygen uptake was quantified using open-circuit calorimetry. Results Analysis revealed no significant difference in maximal VO2 among subjects (P > 0.001). However, discernible variations were observed in running economy and maximal lactate steady state (P > 0.001). Notably, the expression of running economy and MLSS in terms of caloric unit cost exhibited heightened sensitivity to speed alterations, suggesting its superior utility in characterizing running economy dynamics relative to oxygen uptake alone. Conclusions In conclusion, our findings underscore the importance of caloric unit cost as a more sensitive indicator of running economy and MLSS, offering valuable insights into the physiological underpinnings of endurance running performance
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