Fuel oxidation at the walk-to-run-transition in humans Kathleen J. Ganley a , Anthony Stock b , Richard M. Herman c , Marco Santello b , Wayne T. Willis b, ⁎ a Department of Physical Therapy and Athletic Training, Northern Arizona University, Flagstaff, AZ, USA b Department of Kinesiology, Arizona State University, Tempe, AZ, USA c Harrington Department of Bioengineering, Arizona State University, Tempe, AZ, USA Received 18 September 2009; accepted 7 June 2010 Abstract Multiple factors (including anthropometric, kinetic, mechanical, kinematic, perceptual, and energetic factors) are likely to play a role in the walk-to-run transition in humans. The primary purpose of the present study was to consider an additional factor, the metabolic fuel source. Indirect calorimetry was used to measure fuel oxidation, and perception of effort was recorded as 10 overnight-fasted adults locomoted on a level treadmill at speeds progressing from 1.56 to 2.46 m s -1 in increments of 0.11 m s -1 and 10.0 minutes under 3 conditions: (1) unconstrained choice of gait, (2) walking at all speeds, and (3) running at all speeds. The preferred transition speed was 2.08 ± 0.03 m s -1 . Gait transition from walking to running increased oxygen consumption rate, decreased the perception of effort, and decreased the rate of carbohydrate oxidation. We propose that, in an evolutionary context, gait transition, guided by the perception of effort, can be viewed as a carbohydrate-sparing strategy. © 2011 Elsevier Inc. All rights reserved. 1. Introduction Quadrupeds can adopt multiple terrestrial gait patterns, whereas natural bipedal gait patterns in humans are limited to walking and running. A preferred speed to transition from one gait to another is commonly observed in most mammalian species, including humans. In humans, preferred transition speed (PTS) from walking to running occurs within the delimited range of 1.8 to 2.2 m s -1 [1,2]. Factors other than physical limitations trigger human gait transition because walking at higher speeds and running at lower speeds than the PTS are well tolerated. Previous investigations of gait transition have explored anthropomet- ric [3,4], kinetic [1,5], mechanical [6], kinematic [4,7], perceptual [2,8], and energetic [1,7,9-11] factors. Locomotion across the speed range relevant to the PTS is dependent upon energy derived from the combustion of fat and carbohydrate, 2 fuels stored at vastly different levels. Because of the profound evolutionary implications regarding the need to defend the extremely limited carbohydrate stores of the human body [12-15], the primary purpose of the present study was to consider the potential role of metabolic fuel selection in the walk-to-run transition in humans. From preferred walking speed up to PTS and beyond, the energy cost of walking (in kilocalories per kilometer per kilogram) steadily rises, revealing the right upward leg of this familiar U-shaped curve. In contrast, the energy cost of running remains relatively constant across this range of speed [1]. Prilutsky and Gregor [2] showed that summed lower extremity electromyography (EMG) conformed to this same pattern. Thus, they proposed that the steadily rising neural input to muscle and energy consumption required to increase walking speed toward PTS might elicit the transition to running, where the slope of these changes would flatten. At the PTS, gait transition from walking to running increases the rate of energy expenditure [1,7,10,16], despite the reasonable, and perhaps more intuitive, prediction that it should have the opposite effect [9,11,17,18]. Moreover, this elevated energy cost is curiously attended by decreased perception of effort [2,8,10] and greater neural input into muscle as assessed from the summed EMG of active muscle [2]. These published observations present an interesting paradox: the walk-to-run transition increases energy expen- diture and the apparent neural input into muscle, whereas it decreases the perception of effort. Available online at www.sciencedirect.com Metabolism Clinical and Experimental 60 (2011) 609 – 616 www.metabolismjournal.com This study was approved by the Institutional Review Board at Arizona State University, where the work was performed. ⁎ Corresponding author. E-mail address: waynewillis@asu.edu (W.T. Willis). 0026-0495/$ – see front matter © 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.metabol.2010.06.007