Assessing the metabolic cost of walking: the influence of baseline subtractions

Partitioning locomotor metabolic rates into resting and locomotor components is a common practice that has both basic and applied value. Here, we evaluated the quantitative influence of the specific baseline value subtracted (quiet standing vs. resting metabolic rates) from the gross metabolic rates measured during walking. We quantified resting, standing and gross metabolic rates during horizontal treadmill walking at six speeds from 0.2 through 1.9 m*s(-1) in 6 healthy, adult subjects. We found that standing metabolic rates were significantly greater than resting values (1.25 +/- 0.03 vs. 1.08 +/- 0.02 W*kg(-1)) and that both constituted large fractions of the gross metabolic rate while walking at all speeds examined (range 16-58%). Differences in the respective net metabolic rates obtained by subtracting standing vs. resting values differed most at the slowest speed measured (16.0% at 0.2 m*s(-1)) and least at the fastest one (2.9% at 1.9 m*s(-1)). Standing metabolic rates, like walking metabolic rates, include the metabolic cost of muscular activation for balance and maintaining an upright posture. Therefore, the net metabolic rates determined by subtracting standing from gross rates underestimate the total muscular costs that walking requires. We suggest that the net walking metabolic rates obtained by subtracting resting metabolic rate values are more representative of the total metabolic energy that walking requires.