Natural variability can increase human walking metabolic costs and its implications to simulation-based metabolic estimation

TL;DR

This study shows that natural variability in human walking increases metabolic costs and that ignoring this variability in simulations leads to inaccurate metabolic cost estimates.

Contribution

The paper demonstrates that stride-to-stride variability increases metabolic costs and that simulations using averaged strides underestimate these costs.

Findings

Using an averaged stride underestimates metabolic cost by about 2.5%.

Random stride selection can mis-estimate metabolic cost by up to 15%.

Increased sensory or motor noise raises both metabolic cost and variability in a feedback-controlled model.

Abstract

Human walking contains variability due to small intrinsic perturbations arising from sensory or motor noise, or to promote motor learning. We hypothesize that such stride-to-stride variability may increase the metabolic cost of walking over and above a perfectly periodic motion, and that neglecting such variability in simulations may mis-estimate the metabolic cost. Here, we quantify the metabolic estimation errors accrued by neglecting the stride-to-stride variability using human data and a musculoskeletal model by comparing the cost of multiple strides of walking and the cost of a perfectly periodic stride with averaged kinematics and kinetics. We find that using an averaged stride underestimates the cost by approximately 2.5%, whereas using a random stride may mis-estimate the cost positively or negatively by up to 15%, ignoring the contribution of measurement errors to the observed stride-to-stride variability. As a further illustration of the cost increase in a simpler dynamical context, we use a feedback-controlled inverted pendulum walking model to show that increasing the sensory or motor noise increases the overall metabolic cost, as well as the variability of stride-to-stride metabolic costs, as seen with the musculoskeletal simulations. Our work establishes the importance of accounting for stride-to-stride variability when estimating metabolic costs from motion.

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