Overground gait transitions are not sharp but involve gradually changing walk-run mixtures even over long distances

Humans typically walk at low speeds and run at higher speeds. Previous studies of transitions between walking and running were mostly on treadmills, but real-world locomotion allows more flexibility. Here, we study overground locomotion over long distances (800 or 2400 m) under time constraints, simulating everyday scenarios like traveling to an appointment. Unlike on treadmills, participants can vary both speed and gait during this task. Gait transition in this overground task occurs over a broad 'gait transition regime' spanning average speeds from 1.9 to 3.0 m s^-1. In this regime, people use mixtures of walking and running on each travel bout: mostly walking at low average speeds (around 1.9 m s^-1) and mostly running at high average speeds (3.0 m s^-1). The walk-run fraction changes gradually between these speed limits and is 50% at about 2.5 m s^-1. Within each walk-run mixture, walking is slower than running, with an unused gap between the two gait speeds. These gait mixtures and their speed dependence are predicted by energy optimality. These findings extend earlier results for shorter distances, showing that similar energetic principles govern longer, more physically and cognitively demanding tasks. Our results highlight the role of whole-task energy minimization including transients in shaping human locomotion and gait choice.