Speeding up, not slowing down, decreases the metabolic energy needed to stabilize walking in the sagittal plane

There is a metabolic cost associated with stabilizing walking, but it remains unclear to what extent stabilizing walking in the sagittal plane contributes to this cost. Furthermore, strategies for stabilizing walking in the sagittal plane vary with speed, but it is unclear whether this also leads to a speed-dependent metabolic cost of stabilizing walking. Here, we explored the metabolic cost of stabilizing walking in the sagittal plane across speeds and its relationship with control strategies. To this aim, we applied continuous treadmill belt speed perturbations (a standard deviation of 0.13 ms^-1) to 22 healthy individuals walking at 0.8, 1.2, and 1.6 ms^-1. We evaluated changes in metabolic energy consumption and control strategies between perturbed and unperturbed walking and explored relationships between energy consumption and control strategies. Perturbations induced larger increases in metabolic rate and changes in control strategies at slower than faster walking speeds, suggesting that walking is more robust against perturbations at faster speeds. Perturbations increased the metabolic rate by 16.7% at the slowest vs. 4.6% at the fastest walking speed. When perturbed, subjects took shorter, wider, and more variable steps and variability in ankle muscle activation increased, but most changes were larger at slower speeds. Metabolic rate increased more due to perturbations in individuals who reduced step length more, that is, relied more on anticipatory adjustments of the walking pattern. Our findings are especially relevant to explain the increased metabolic cost of individuals with mobility impairments, who often walk slower and have altered walking control.<b>NEW & NOTEWORTHY</b> Stabilizing walking requires more active control in the frontal than in the sagittal plane. Nevertheless, we demonstrated that there is a considerable energetic cost associated with stabilizing walking in the sagittal plane, especially at slower speeds. This cost is higher in individuals who adjusted their average walking pattern more when walking was perturbed. Diseases that affect both walking speed and control might therefore have a disproportionately large effect on the metabolic cost of walking.