Ontogenetic scaling of locomotor kinetics and kinematics of the ostrich (Struthio camelus)

Nicola C Smith,Alan M Wilson,Karin J Jespers

doi:10.1242/jeb.020271

Nicola C Smith, Alan M Wilson + Show 1 more

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https://doi.org/10.1242/jeb.020271

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Abstract

Kinematic and kinetic parameters of running gait were investigated through growth in the ostrich, from two weeks up to 10 months of age, in order to investigate the effects of increasing body size. Ontogenetic scaling relationships were compared with published scaling relationships found to exist with increasing body size between species to determine whether dynamic similarity is maintained during growth. During the study, ostrich mass (M(b)) ranged from 0.7 kg to 108.8 kg. Morphological measurements showed that lengths scaled with positive allometry during growth (hip height proportional to M(b)(0.40); foot segment length proportional to M(b)(0.40); tarsometatarsus length proportional to M(b)(0.41); tibiotarsus length proportional to M(b)(0.38); femur length proportional to M(b)(0.37)), significantly exceeding the close to geometric scaling observed between mammalian and avian species of increasing body size. Scaling of kinematic variables largely agreed with predicted scaling for increasing size and demonstrated relationships close to dynamic similarity and, as such, ontogenetic scaling of locomotor parameters was similar to that observed with increasing body mass between species. However, the ways in which these scaling trends were achieved were very different, with ontogenetic scaling of locomotor mechanics largely resulting from simple scaling of the limb segments rather than postural changes, likely to be due to developmental constraints. Small deviations from dynamic similarity of kinematic parameters and a reduction in the predicted scaling of limb stiffness (proportional to M(b)(0.59)) were found to be accounted for by the positive allometric scaling of the limb during growth.

Highlights

Studies between species of different sizes have identified musculoskeletal and biomechanical variations that occur with increasing body size to enable an animal to achieve the desired level of athletic performance, whilst maintaining essential safety factors
For two systems to truly move in a dynamically similar way, they should be geometrically similar, where lengths would be identical if multiplied by a scaling factor, such that masses are directly proportional to body mass (Mb1), lengths scale proportional to Mb1/3 and areas scale proportional to Mb2/3, as has been shown to be the case for many species, both bipedal and quadrupedal, over a large size range (Alexander et al, 1979; Alexander et al, 1981; Biewener, 1982; Christiansen, 2002; Maloiy et al, 1979; Olmos et al, 1996)
All segment lengths increased with increasing mass and hip height but showed differing scaling exponents, with the foot and tarsometatarsus showing greater positive allometry with body mass than the femur and tibiotarsus

Summary

Introduction

Studies between species of different sizes have identified musculoskeletal and biomechanical variations that occur with increasing body size to enable an animal to achieve the desired level of athletic performance, whilst maintaining essential safety factors. For dynamically similar locomotion at a similar relative speed (equal Froude number), relative stride length, relative stride frequency, relative peak force and duty factor (defining the proportion of the stride time that an individual foot is in contact with the ground) should remain constant with increasing body size. Factors such as the stresses in bone, muscle and tendon limit the forces that an animal can withstand during locomotion and the forces applied to the musculoskeletal system are dependent on size, speed and limb orientation. Maintaining similar stress within the body tissues necessitates appropriate changes in specific segment geometry or postural arrangement (Biewener, 1989; Main and Biewener, 2007)

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