Quadrupedal locomotor simulation: producing more realistic gaits using dual-objective optimization.

William Irvin Sellers,Eishi Hirasaki

doi:10.1098/rsos.171836

Abstract

In evolutionary biomechanics it is often considered that gaits should evolve to minimize the energetic cost of travelling a given distance. In gait simulation this goal often leads to convincing gait generation. However, as the musculoskeletal models used get increasingly sophisticated, it becomes apparent that such a single goal can lead to extremely unrealistic gait patterns. In this paper, we explore the effects of requiring adequate lateral stability and show how this increases both energetic cost and the realism of the generated walking gait in a high biofidelity chimpanzee musculoskeletal model. We also explore the effects of changing the footfall sequences in the simulation so it mimics both the diagonal sequence walking gaits that primates typically use and also the lateral sequence walking gaits that are much more widespread among mammals. It is apparent that adding a lateral stability criterion has an important effect on the footfall phase relationship, suggesting that lateral stability may be one of the key drivers behind the observed footfall sequences in quadrupedal gaits. The observation that single optimization goals are no longer adequate for generating gait in current models has important implications for the use of biomimetic virtual robots to predict the locomotor patterns in fossil animals.

Highlights

Symmetrical quadrupedal gaits are a fundamental component of the locomotor repertoire of almost all terrestrial quadrupeds
Quadrupedal footfall sequences have been measured for a large range of animals in at least 143 tetrapod genera [7] but it still remains difficult to distinguish the importance of gait choice criteria
Energetics have been implicated in dog walking [8]; peak limb forces and metabolic factors are found not to be important for gait selection in horses [9]; balance is suggested as an important driver among primates [10] but there is some disagreement [11,12]; and neural control [13], skeletal loading [4] and limb interference avoiding [14] have all been suggested as alternatives

Summary

Introduction

Symmetrical quadrupedal gaits are a fundamental component of the locomotor repertoire of almost all terrestrial quadrupeds. Primates are a interesting case because they have a very unusual diagonal footfall sequence at walking speeds, and generally they transition from walking directly to galloping without an intermediate trotting phase [15]. This diagonal gait pattern is preserved in other locomotor modes such as climbing [16] and even in human bipedal locomotion where arm swinging is diagonally out of phase with footfalls [17] and where diagonal gaits predominate in the short post-natal quadrupedal crawling stage [18]

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Conclusion