The Smooth Transition From Many-Legged to Bipedal Locomotion-Gradual Leg Force Reduction and its Impact on Total Ground Reaction Forces, Body Dynamics and Gait Transitions.

Tom Weihmann

doi:10.3389/fbioe.2021.769684

Abstract

Most terrestrial animals move with a specific number of propulsive legs, which differs between clades. The reasons for these differences are often unknown and rarely queried, despite the underlying mechanisms being indispensable for understanding the evolution of multilegged locomotor systems in the animal kingdom and the development of swiftly moving robots. Moreover, when speeding up, a range of species change their number of propulsive legs. The reasons for this behaviour have proven equally elusive. In animals and robots, the number of propulsive legs also has a decisive impact on the movement dynamics of the centre of mass. Here, I use the leg force interference model to elucidate these issues by introducing gradually declining ground reaction forces in locomotor apparatuses with varying numbers of leg pairs in a first numeric approach dealing with these measures’ impact on locomotion dynamics. The effects caused by the examined changes in ground reaction forces and timing thereof follow a continuum. However, the transition from quadrupedal to a bipedal locomotor system deviates from those between multilegged systems with different numbers of leg pairs. Only in quadrupeds do reduced ground reaction forces beneath one leg pair result in increased reliability of vertical body oscillations and therefore increased energy efficiency and dynamic stability of locomotion.

Highlights

As well as legged machines moving on level ground, body dynamics are characterised by the ground reaction forces (GRF) applied by the single legs and the temporal coordination between them (i.e., leg coordination patterns (Weihmann et al, 2016))
By using a simple numerical approach to study the interaction of single leg ground forces in polypedal locomotor apparatuses, it has been shown previously that the number of walking legs involved significantly affects the impact of ipsilateral phase shifts onto overall ground force oscillations and body-dynamics (Weihmann, 2018)
When all legs generate the full amount of GRF, the results of the present approach cover those presented by Weihmann (2018); when the forces beneath one of the leg pairs diminish, the results can deviate significantly

Summary

Introduction

As well as legged machines moving on level ground, body dynamics are characterised by the ground reaction forces (GRF) applied by the single legs and the temporal coordination between them (i.e., leg coordination patterns (Weihmann et al, 2016)). When accelerating from low to medium speeds, many few legged animals (i.e. those with less than or equal to four pairs of walking legs like vertebrates, insects and arachnids) shift ipsilateral phase relations (θ) from intermediate values towards values close to 0.5. Deviations from strictly alternating sets of legs result in reduced oscillations of the total vertical forces (Figure 1) and vertical oscillations of the animals’ bodies . They are advantageous when leg elasticities cannot be used owing to anatomical constraints or specific environmental conditions (Li et al, 2013; Gravish et al, 2014; Li et al, 2015; Weihmann, 2018)

Methods

Results

Discussion

Conclusion