Quantifying the Net Cost of Transport Curve During Human Walking: How Much Time Is Required?

The U-shaped net cost of transport (COT) curve of walking, a highly conserved feature in humans, helps us understand the biomechanical basis that underlies energy minimization during walking. However, to produce a subject’s net COT curve, their metabolic data must be analyzed during a period of steady-rate metabolism. Traditionally, studies analyze the last 3 minutes of a 6-10 min trial, assuming that steady-rate metabolism has been achieved. However, it's possible that long trial periods are not necessary to achieve steady rates of metabolism. Therefore, we sought to determine the minimum time needed for humans to achieve steady rates of metabolism across slow to fast walking speeds. We reasoned that if shorter time windows are possible, these data should elicit a net COT curve that is comparable to the net COT curve created using traditional, less objective methods. Twenty-one subjects completed a series of 7-minute walking trials across speeds ranging from 0.50-2.00m/s while we collected their metabolic data. We partitioned the metabolic data for each trial into 2-minute intervals and calculated the slope for each interval. We found that a minimum of 2.50 minutes is required to achieve steady-rates metabolism, which yielded a net COT curve similar to the one derived by traditional methods. Overall, we find that a simple slope method can serve as an objective criterion for defining steady rates of metabolism. These analyses demonstrate that shorter trials can be used to decrease protocol times, while still maintaining the integrity of the net COT curve.