DOI: 10.5061/dryad.00000002r
title: Does the preferred walk-run transition speed on steep inclines minimize energetic cost, heart rate or neither?

description: Humans prefer to walk at slow speeds and to run at fast speeds. In
 between, there is a speed at which people choose to transition between
 gaits, the Preferred Transition Speed (PTS). At slow speeds, it is
 energetically cheaper to walk and at faster speeds, it is cheaper to run.
 Thus, there is an intermediate speed, the Energetically Optimal Transition
 Speed (EOTS). Our goals were to determine: 1) how PTS and EOTS compare
 across a wide range of inclines and 2) if the EOTS can be predicted by the
 heart rate optimal transition speed (HROTS). Ten healthy, high-caliber,
 male trail/mountain runners participated. On day 1, subjects completed
 0&[deg] and 15&[deg] trials and on day 2, 5&[deg]
 and 10&[deg]. We calculated PTS as the average of the walk-to-run
 transition speed (WRTS) and the run-to-walk transition speed (RWTS)
 determined with an incremental protocol. We calculated EOTS and HROTS from
 energetic cost and heart rate data for walking and running near the
 expected EOTS for each incline. The intersection of the walking and
 running linear regression equations defined EOTS and HROTS. We found that
 PTS, EOTS, and HROTS all were slower on steeper inclines. PTS was slower
 than EOTS at 0&[deg], 5&[deg], and 10&[deg], but
 the two converged at 15&[deg]. Across all inclines, PTS and EOTS
 were only moderately correlated. Although EOTS correlated with HROTS, EOTS
 was not predicted accurately by heart rate on an individual basis.
description: Subjects walked and ran on a classic Quinton
 18-60 motorized treadmill with a rigid steel deck (Quinton Instrument
 Company, Bothell, WA). <b>Determination of
 PTS: </b>The average of the walk-to-run transition speed (WRTS) and
 run-to-walk transition speed (RWTS) defined the PTS as per Hreljac et. al.
 (2007). We first determined the WRTS in the walk-first group
 and then their RWTS and <i>vice versa</i> for the
 run-first group. Based on pilot experiments, we selected starting speeds
 such that there was no doubt which gait would be preferred at the initial
 speed. Once the speed of the treadmill was correctly set, subjects mounted
 the treadmill and chose their gait <i>ad
 libitum</i>. After we determined the preferred gait at
 the particular speed, the subject straddled the treadmill belt while we
 changed the speed by 0.1 m/s (increased during WRTS trials, decreased
 during RWTS trials). The process repeated until a gait transition occurred
 and was sustained for 30 seconds.
 <b>Determination of
 EOTS and HROTS: </b>For the energetics and heart rate trials, we set
 the initial speed based on pilot experiments that indicated it would be
 near the EOTS. Subjects in the walk-first group walked at the
 incline-specific initial speed for 5 min, rested for ∼5 min and then ran
 at that speed for 5 min. Subjects in the run-first group did the opposite.
 During the rest periods, we re-weighed the subject and they drank just
 enough water to compensate for the weight loss due mostly to sweating.
 Thus, each subject maintained a nearly constant weight throughout all the
 trials. To measure metabolic rate
 during walking and running, we used an open-circuit, expired gas analysis
 system (TrueOne 2400; ParvoMedics, Sandy, UT). Subjects wore a mouthpiece
 with a one-way breathing valve and a nose clip allowing us to collect
 their expired air. The ParvoMedics software calculated the STPD rates of
 oxygen consumption (V□O<sub>2</sub>) and carbon dioxide
 production (V□CO<sub>2</sub>) and we averaged
 the last 2 minutes of each 5-minute trial. We then calculated metabolic
 power using the equation of Péronnet and Massicotte (1991) equation, as
 clarified by Kipp et al. (2018). We only included trials with respiratory
 exchange ratios (RER) &lt;1.0 to ensure that metabolic energy was
 predominantly being provided from oxidative pathways. We used an R7 Polar
 iWL (Polar Electro Oy, Kempele, Finland) to measure heart rate in beats
 per minute (bpm) and averaged the values for the last 2 min of each
 trial. Immediately after both gait
 trials were completed for the initial speed, we calculated and compared
 the metabolic power required for walking and running. If walking was the
 more economical gait, we increased the treadmill speed by 0.1 m/s, and the
 process repeated. If running was the more economical gait, we decreased
 the treadmill speed by 0.1 m/s, and the process repeated. Each subject
 performed three speeds, both walking and running at each incline. However,
 some subjects needed to complete walking and running trials at a fourth
 speed so that we could obtain energetics data for one speed faster and one
 speed slower than their EOTS. For the three speeds at which the differences
 between metabolic rates between walking and running were least, we
 calculated linear regression equations for both metabolic power and heart
 rate as functions of speed for both walking and running for each subject
 and incline. The speeds at which the two equations intersected defined the
 EOTS and HROTS for each subject. Overall, we analyzed ten subjects at four different
 inclines, i.e. 40 determinations of EOTS and HROTS. Of those 80 linear
 regression analyses, the walking vs. running regressions intersected at a
 speed &lt; 3 m/sec for all but two subjects (one subject for EOTS at
 15° and a different subject for HROTS at 10°). Essentially, those
 individuals’ regression lines were nearly parallel. We chose to exclude
 those two conditions from further statistical analysis and aggregate data
 compilation.
description: There are two missing values, as noted in the methods: HROTS
 for subject 5 at 10 degrees and EOTS for subject 4 at 15
 degrees.