Correlations between the metabolic costs of level and graded running: A secondary analysis of the literature metabolic cost of graded

Background: The metabolic cost of running (Cr) is one of the three main determinants of performance in distance running, along with maximum oxygen consumption and lactate threshold. However, level Cr is only weakly correlated with performance in trail running, which almost always involves uphill and downhill sections. Objective: Through a secondary analysis of published data, the primary aim of this study was to characterize the correlations between individuals' level and graded (downhill and uphill) Cr. We took advantage of this database to also re-evaluate the pattern and critical points (i.e., optimum slope) of the relationship between Cr and slope. Methods: We analyzed 23 studies evaluating level and graded Cr, published before August 2024. Both Cr and Pearson correlation coefficients between level and graded Cr were plotted as functions of slope. Results: Using third-order polynomial functions, a correlation between level and graded Cr was a continuous function of slope in which correlation coefficients declined with progressively steep uphill or downhill slopes. Downhill Cr was minimized at - 18.8%, and a linear relationship was observed for uphill Cr with slope across the range of slopes analyzed (up to + 26.8%). Conclusions: The present secondary analysis extends our knowledge on the influence of slope on Cr, showing that graded Cr is weakly correlated with level Cr when slope is steeper than ± 10%. Consequently, testing and training protocols for trail runners should incorporate steeper slopes (i.e., ± 15%) to provide more relevant assessments and preparation for mountainous trail races (including steep slopes).