Energetic cost of locomotion closely aligns with the preferred uphill walk–run transition at constant vertical speed in runners

PURPOSE: The determinants of the preferred walk-to-run transition speed (PTS) during uphill locomotion remain unclear when the rate of vertical ascent is controlled rather than allowed to vary with treadmill speed. This study examined whether spatiotemporal, biomechanical, cardiorespiratory and energetic variables provide crossover-based optimal transition speed (OTS) that align with PTS during incline locomotion at constant vertical speed. METHODS: 17 Endurance-trained runners performed randomized six-minute walking and running bouts at speeds from 0.55 to 2.50 m·s⁻¹ for walking and from 0.55 to 3.88 m·s⁻¹ for running, all conducted at a constant vertical speed of 800 m·h⁻¹. The PTS assessment protocol was designed to maintain a constant rate of vertical ascent throughout the test. RESULTS: cost of transport as well as metabolic power exhibited optimal transition speed that did not differ significantly from PTS, indicating a strong association between these energetic variables and the choice of gait. In contrast, heart rate, ventilation, and internal mechanical work displayed optimal transition speeds that were significantly different from PTS, while external and total mechanical work, step frequency, and step length showed no crossover between gaits. CONCLUSION: These findings indicate that, under conditions of constant vertical speed, uphill PTS aligns more closely with energetic crossover criteria than with cardiorespiratory or mechanical-work-based crossovers.