The manuscript “Move less, spend more: The metabolic demands of short walking bouts” is a thorough investigation into the measurements of short-bout (non-steady state) gait, and the mechanisms that characterize the higher metabolic cost associated with shorter bouts of gait.

I highly commend the authors on very thorough experiments and excellent clarity in writing. This was a pleasure to read.

I have only a few comments and suggestions (that are meant to be helpful to an already strong body of work). Below I list my main thoughts on the manuscript followed by minor comments/edits:

Major Comments:

1. The authors introduction is very good. But I wonder if they missed one important area where this work will also prove valuable. That is, the large body of literature on gait/movement optimization. Energy cost of walking is among the most common criteria used to interpret gait selection and optimization. Considering that most walking indeed consists of short bouts, this study will be highly relevant to this field also and I think will be a valuable technique resource.

2. I have no problem with the efficiency calculations for the stair climbing, where the major mechanical objective is to do mechanical work to raise the center of mass. Using a constant mechanical work value 0.88J/kg/m for level walking is, however, only a very crude estimate. Calculating the mechanical work (especially of the metabolically responsible muscles) for level walking is non-trivial and highly assumption-dependent. There are a multitude of factors that could affect the mechanical work of level walking in individuals (e.g. tendon elastic contributions, energy transfer between limbs, simultaneous positive and negative work at joints, to name a few). I feel that, at the very least, some amount of warning is required for interpreting the efficiency data for level walking.

3. In a few spots the authors raise some very interesting information and interpretations around non-metabolic oxygen consumption (lines 55-74; lines 310 – 321). The authors cite some key papers, but I am wondering if they can spend just a smidge (a few lines) to go into a little more depth. Especially for the mechanisms of non-metabolic EEOC. It could be helpful. I realize there are length constraints that might not permit this, but at least this reviewer thought even just a little bit of background could be helpful to the non-informed.

4. Incorporating changes in EqO2 in different phases of the oxygen kinetics (on- off-transients) is a strength of the study. I wonder, however, if there are approaches in calculating energy use that can help overcome limitations in measuring accurately VCO2 and VO2, or making assumptions of EqO2 (lines 181 – 182). In the work by Koteja (1996; Functional Ecology, Vol 10, 675-677) (and later by Phil Withers) it is shown that when using simple calculations using only fractional concentrations of oxygen and assuming an RER (RQ) that the calculation of the rate of energy expenditure is nearly insensitive to RER. “The error of estimating the energy metabolism rate is below 0.6% for the entire range of reasonable RQ values”. This might be a useful approach also, in parallel with what the authors have done.

Minor Comments

Line 189: is there missing text here? Or do the authors write “as for [66]” to mean, “as for the work cited in [66]”? Or “as in [66]”. This was a bit confusing.

Line 290: There are a lot of abbreviations for variables that the reader needs to follow. I wonder for some of the variables that are not used often if it could be clearer to also include the full text description of the variable.

Line 419: Do the authors mean “hard to measure in a reliable way…”?