Abstract
Larger terrestrial animals tend to support their weight with more upright limbs. This makes structural sense, reducing the loading on muscles and bones, which is disproportionately challenging in larger animals. However, it does not account for why smaller animals are more crouched; instead, they could enjoy relatively more slender supporting structures or higher safety factors. Here, an alternative account for the scaling of posture is proposed, with close parallels to the scaling of jump performance. If the costs of locomotion are related to the volume of active muscle, and the active muscle volume required depends on both the work and the power demanded during the push-off phase of each step (not just the net positive work), then the disproportional scaling of requirements for work and push-off power are revealing. Larger animals require relatively greater active muscle volumes for dynamically similar gaits (e.g. top walking speed)—which may present an ultimate constraint to the size of running animals. Further, just as for jumping, animals with shorter legs and briefer push-off periods are challenged to provide the power (not the work) required for push-off. This can be ameliorated by having relatively long push-off periods, potentially accounting for the crouched stance of small animals.
Highlights
Larger birds and quadrupeds tend to support their weight during locomotion with more upright, relatively stiffer limbs [1,2]
There has been no mechanical or energetic account for smaller animals benefitting from more crouched postures; some benefits relating to stability, manoeuvrability or control are generally suggested [1,2,3]
Might a similar power constraint for small animals undergoing steady locomotion account for their high duty factors, higher mechanical cost of transport and crouched postures?
Summary
Larger birds and quadrupeds tend to support their weight during locomotion with more upright, relatively stiffer limbs [1,2]. Consider running: an exceedingly stiff, upright leg operating at a very low duty factor b (proportion of the stride period with the foot in contact with the ground) allows nearly vertical (albeit very high) forces, thereby avoiding the costly fore– aft fluctuations in speed. Some compromise prevents such gaits from being realized in biology. Might a similar power constraint for small animals undergoing steady locomotion account for their high duty factors (figure 1; [2,8,9,10,11,12]), higher mechanical cost of transport (see the electronic supplementary material based on Heglund et al [13], but contrasting with the conclusions of Biewener [1] and Heglund et al [13]) and crouched postures?
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