A simple method reveals minimum time required to quantify steady-rate metabolism and net cost of transport for human walking

The U-shaped net cost of transport (COT) curve of walking has helped
scientists understand the biomechanical basis that underlies energy
minimization during walking. However, to produce an individual’s net COT
curve, data must be analyzed during periods of steady-rate metabolism.
Traditionally, studies analyze the last few minutes of a 6–10 min trial,
assuming that steady-rate metabolism has been achieved. Yet, it is
possible that an individual achieves steady rates of metabolism much
earlier. However, there is no consensus on how to objectively quantify
steady-rate metabolism across a range of walking speeds. Therefore, we
developed a simple slope method to determine the minimum time needed for
humans to achieve steady rates of metabolism across slow to fast walking
speeds.We hypothesized that a shorter time window could be used to produce
a net COT curve that is comparable to the net COT curve created using
traditional methods. We analyzed metabolic data from 21 subjects who
completed several 7 min walking trials ranging from 0.50 to 2.00 m s−1. We
partitioned the metabolic data for each trial into moving 1, 2 and 3 min
intervals and calculated their slopes. We statistically compared these
slope values with values derived from the last 3 min of the 7 min trial,
our ‘gold’ standard comparison. We found that a minimum of 2 min is
required to achieve steady-rate metabolism and that data from 2–4 min
yields a net COT curve that is not statistically different from the one
derived from experimental protocols that are generally accepted in the
field.