Multi-objective control in human walking: insight gained through simultaneous degradation of energetic and motor regulation systems

Abstract

Minimization of metabolic energy is considered a fundamental principle of human locomotion, as demonstrated by an alignment between the preferred walking speed (PWS) and the speed incurring the lowest metabolic cost of transport. We aimed to (i) simultaneously disrupt metabolic cost and an alternate acute task requirement, namely speed error regulation, and (ii) assess whether the PWS could be explained on the basis of either optimality criterion in this new performance and energetic landscape. Healthy adults ( N = 21) walked on an instrumented treadmill under normal conditions and, while negotiating a continuous gait perturbation, imposed leg-length asymmetry. Oxygen consumption, motion capture data and ground reaction forces were continuously recorded for each condition at speeds ranging from 0.6 to 1.8 m s −1 , including the PWS. Both metabolic and speed regulation measures were disrupted by the perturbation ( p < 0.05). Perturbed PWS selection did not exhibit energetic prioritization (although we find some indication of energy minimization after motor adaptation). Similarly, PWS selection did not support prioritization of speed error regulation, which was found to be independent of speed in both conditions. It appears that, during acute exposure to a mechanical gait perturbation of imposed leg-length asymmetry, humans minimize neither energetic cost nor speed regulation errors. Despite the abundance of evidence pointing to energy minimization during normal, steady-state gait, this may not extend acutely to perturbed gait. Understanding how the nervous system acutely controls gait perturbations requires further research that embraces multi-objective control paradigms.

Humans trade off whole-body energy cost to avoid overburdening muscles while walking

“…Humans have been observed to expend more energy than minimally required for both upper and lower limb movement tasks [40,[50][51][52][53][54][55][56]. These studies suggest that energy minimization may be task dependent.…”

Humans trade-off whole-body energy cost to avoid overburdening muscles while walking

“…The authors propose this may be related to the discomfort/pain associated with an unshod gait (56). Some evidence also suggests humans do not prioritize metabolic cost minimization when moving in less stable environments, e.g., walking downhill (50) (but this remains inconclusive (57)) or walking with experimentally imposed asymmetrical leg lengths (40). Humans have also been observed to perform walking tasks in more effortful ways to eliminate a cognitive burden (51).…”

“…All participants achieved the one-minute benchmark and progressed to the next phase of data collection, in which their crouch preferred walking speed (PWS C/W ; m s -1 ) was assessed in accordance with the method outlined by Dingwell and Marin (39). All crouch and incline walking trials described below were administered at the PWSC/W so as not to disadvantage metabolic energy use in the crouch condition: i.e., it was assumed that preferred walking speed selection would minimize metabolic cost of transport for the crouch condition (2,40).…”

“…There are a handful of previous studies in which humans have been observed to expend more energy than minimally required for both upper and lower limb movement tasks (40,(50)(51)(52)(53)(54)(55)(56). These studies suggest that energy minimization may be task-dependent.…”

Perspective on musculoskeletal modelling and predictive simulations of human movement to assess the neuromechanics of gait

“…While trade-offs between criteria might be subtle for steady-state level-ground walking, they might be more pronounced in other conditions [59]. Predictive simulations could help identify experimental conditions that would allow distinguishing between optimality criteria.…”