The aim of this study was first to determine if level, uphill, and downhill energy cost of running (ECR) values were correlated at different slopes and for different running speeds, and second, to determine the influence of lower limb strength on ECR. Twenty-nine healthy subjects completed a randomized series of 4-min running bouts on an instrumented treadmill to determine their cardiorespiratory and mechanical (i.e., ground reaction forces) responses at different constant speeds (8, 10, 12, and 14 km·h−1) and different slopes (−20, −10, −5, 0, +5, +10, +15, and +20%). The subjects also performed a knee extensor (KE) strength assessment. Oxygen and energy costs of running values were correlated between all slopes by pooling all running speeds (all r2 ≥ 0.27; p ≤ 0.021), except between the steepest uphill vs. level and the steepest downhill slope (i.e., +20% vs. 0% and −20% slopes; both p ≥ 0.214). When pooled across all running speeds, the ECR was inversely correlated with KE isometric maximal torque for the level and downhill running conditions (all r2 ≥ 0.24; p ≤ 0.049) except for the steepest downhill slope (−20%), but not for any uphill slopes. The optimal downhill grade (i.e., lowest oxygen cost) varied between running speeds and ranged from −14% and −20% (all p < 0.001). The present results suggest that compared to level and shallow slopes, on steep slopes ~±20%, running energetics are determined by different factors (i.e., reduced bouncing mechanism, greater muscle strength for negative slopes, and cardiopulmonary fitness for positive slopes). On shallow negative slopes and during level running, ECR is related to KE strength.

Purpose The purpose of this study was twofold: (i) determine if well-trained athletes can achieve similar peak oxygen uptake (V˙O2peak) in downhill running (DR) versus level running (LR) or uphill running (UR) and (ii) investigate if lower limb extensor muscle strength is related to the velocity at V˙O2peak (vV˙O2peak) in DR, LR, and UR. Methods Eight athletes (V˙O2max = 68 ± 2 mL·min−1·kg−1) completed maximal incremental tests in LR, DR (−15% slope), and UR (+15% slope) on a treadmill (+1, +1.5, and +0.5 km·h−1 every 2 min, respectively) while cardiorespiratory responses and spatiotemporal running parameters were continuously measured. They were also tested for maximal voluntary isometric strength of hip and knee extensors and plantar flexors. Results Oxygen uptake at maximal effort was approximately 16% to 18% lower in DR versus LR and UR (~57 ± 2 mL·min−1·kg−1, 68 ± 2 mL·min−1·kg−1, and 70 ± 3 mL·min−1·kg−1, respectively) despite much greater vV˙O2peak (22.7 ± 0.6 km·h−1 vs 18.7 ± 0.5 km·h−1 and 9.3 ± 0.3 km·h−1, respectively). At vV˙O2peak, longer stride length and shorter contact time occurred in DR versus LR and UR (+12%, +119%, −38%, and −61%, respectively). Contrary to knee extensor and plantar flexor, hip extensor isometric strength correlated to vV˙O2peak in DR, LR, and UR (r = −0.86 to −0.96, P < 0.05). At similar V˙O2, higher heart rate and ventilation emerged in DR versus LR and UR, associated with a more superficial ventilation pattern. Conclusions This study demonstrates that well-trained endurance athletes, accustomed to DR, achieved lower V˙O2peak despite higher vV˙O2peak during DR versus LR or UR maximal incremental tests. The specific heart rate and ventilation responses in DR might originate from altered running gait and increased lower-limb musculotendinous mechanical loading, furthering our understanding of the particular physiology of DR, ultimately contributing to optimize trail race running performance.

Objectives: Recent studies investigated the determinants of trail running performance (i.e., combining uphill (UR) and downhill running sections (DR)), while the possible specific physiological factors specifically determining UR vs DR performances (i.e., isolating UR and DR) remain presently unknown. This study aims to determine the cardiorespiratory responses to outdoor DR vs UR time-trial and explore the determinants of DR and UR performance in highly trained runners. Design: Randomized controlled trial. Methods: Ten male highly-trained endurance athletes completed 5-km DR and UR time-trials (average grade: ±8%) and were tested for maximal oxygen uptake, lower limb extensor maximal strength, local muscle endurance, leg musculotendinous stiffness, vertical jump ability, explosivity/agility and sprint velocity. Predictors of DR and UR performance were investigated using correlation and commonality regression analyses. Results: Running velocity was higher in DR vs UR time-trial (20.4 ± 1.0 vs 12.0 ± 0.5 km•h −1 , p < 0.05) with similar average heart rate (95 ± 2% vs 94 ± 2% maximal heart rate; p > 0.05) despite lower averageVO 2 (85 ± 8% vs 89 ± 7%VO 2max ; p < 0.05). Velocity atVO 2max (vVO 2max ) body mass index (BMI) and maximal extensor strength were significant predictors of UR performance (r 2 = 0.94) whereas vVO 2max , leg musculotendinous stiffness and maximal extensor strength were significant predictors of DR performance (r 2 = 0.84). Conclusions: Five-km UR and DR running performances are both well explained by three independent predictors. If two predictors are shared between UR and DR performances (vVO 2max and maximal strength), their relative contribution is different and, importantly, the third predictor appears very specific to the exercise modality (BMI for UR vs leg musculotendinous stiffness for DR).

These results emphasize the cardiorespiratory responses to DR and highlight the need for cautious interpretation of [Formula: see text]O, HR, and [Formula: see text] patterns as markers of exercise intensity for training load prescription and management.