Does a crouched leg posture enhance running stability and robustness?

Yvonne Blum,Aleksandra Birn-Jeffery,Monica A Daley,Andre Seyfarth

doi:10.1016/j.jtbi.2011.04.029

Yvonne Blum, Aleksandra Birn-Jeffery + Show 2 more

Open Access

https://doi.org/10.1016/j.jtbi.2011.04.029

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Abstract

Humans and birds both walk and run bipedally on compliant legs. However, differences in leg architecture may result in species-specific leg control strategies as indicated by the observed gait patterns. In this work, control strategies for stable running are derived based on a conceptual model and compared with experimental data on running humans and pheasants ( Phasianus colchicus). From a model perspective, running with compliant legs can be represented by the planar spring mass model and stabilized by applying swing leg control. Here, linear adaptations of the three leg parameters, leg angle, leg length and leg stiffness during late swing phase are assumed. Experimentally observed kinematic control parameters (leg rotation and leg length change) of human and avian running are compared, and interpreted within the context of this model, with specific focus on stability and robustness characteristics. The results suggest differences in stability characteristics and applied control strategies of human and avian running, which may relate to differences in leg posture (straight leg posture in humans, and crouched leg posture in birds). It has been suggested that crouched leg postures may improve stability. However, as the system of control strategies is overdetermined, our model findings suggest that a crouched leg posture does not necessarily enhance running stability. The model also predicts different leg stiffness adaptation rates for human and avian running, and suggests that a crouched avian leg posture, which is capable of both leg shortening and lengthening, allows for stable running without adjusting leg stiffness. In contrast, in straight-legged human running, the preparation of the ground contact seems to be more critical, requiring leg stiffness adjustment to remain stable. Finally, analysis of a simple robustness measure, the normalized maximum drop, suggests that the crouched leg posture may provide greater robustness to changes in terrain height.

Highlights

The great majority of living terrestrial vertebrates are quadrupeds
Birds are digitigrade, whereby their elongated tarsometatarsals keep their ankles clearly off the ground during walking and running (Alexander, 2004). This avian leg geometry in combination with the crouched leg posture allows for leg lengthening before touching the ground and thereby coping with large ground disturbances, as it was impressively demonstrated by experiments on running guinea fowl (Numida meleagris) (Daley et al, 2006, 2007)
Humans touch the ground with steeper angles of attack αTD, which decrease with increasing running speed

Summary

Introduction

The great majority of living terrestrial vertebrates are quadrupeds. bipedalism can be found within a few families of mammals, reptiles and within all birds. We concentrate on bipedal running and compare two types of leg architecture: The straight leg posture, represented by the human leg, and the crouched leg posture, represented by the avian leg. Birds are digitigrade, whereby their elongated tarsometatarsals keep their ankles clearly off the ground during walking and running (Alexander, 2004) This avian leg geometry in combination with the crouched leg posture allows for leg lengthening before touching the ground and thereby coping with large ground disturbances, as it was impressively demonstrated by experiments on running guinea fowl (Numida meleagris) (Daley et al, 2006, 2007)

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