Comparison between the kinematics for kangaroo rat hopping on a solid versus sand surface.

Joseph K. Hall,David C. Lin,Craig P. McGowan

doi:10.1098/rsos.211491

Abstract

In their natural habitats, animals move on a variety of substrates, ranging from solid surfaces to those that yield and flow (e.g. sand). These substrates impose different mechanical demands on the musculoskeletal system and may therefore elicit different locomotion patterns. The goal of this study is to compare bipedal hopping by desert kangaroo rats (Dipodomys deserti) on a solid versus granular substrate under speed-controlled conditions. To accomplish this goal, we developed a rotary treadmill, which is able to have different substrates or uneven surfaces. We video recorded six kangaroo rats hopping on a solid surface versus sand at the same speed (1.8 m s−1) and quantified the differences in the hopping kinematics between the two substrates. We found no significant differences in the hop period, hop length or duty cycle, showing that the gross kinematics on the two substrates were similar. This similarity was surprising given that sand is a substrate that absorbs mechanical energy. Measurements of the penetration resistance of the sand showed that the combination of the sand properties, toe-print area and kangaroo rat weight was probably the reason for the similarity.

Highlights

Animals locomote on a variety of terrains in their habitats, moving on surfaces that may have substantially different mechanical properties
The actual centre of mass (COM) lies approximately 2–3 cm anterior to the hip in animals with a crouched posture, the COM position probably does not shift substantially relative to the hip position during hopping, we assumed that the velocities and accelerations should be similar for the hip and COM
The overall kinematics of the COM was similar for hopping on sand versus solid substrates

Summary

Introduction

Animals locomote on a variety of terrains in their habitats, moving on surfaces that may have substantially different mechanical properties. A solid substrate, such as hard-packed dirt, is a relatively rigid surface that animals lose minimal energy to during impact or during push-off, while granular substrates, royalsocietypublishing.org/journal/rsos R. Such as sand, are flowable media which absorb energy during penetration. Walking on sand 2 requires 1.6 to 2.5 times more total work to be produced for humans versus walking on concrete [1]. In running zebra-tailed lizards, it was estimated that the hindlimb muscles performed three times more mechanical work on a granular substrate than on a solid surface [2]. Different substrates impose different mechanical demands on the musculoskeletal system, and animals may adapt their motor strategies to meet the changes in terrain substrate

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