title: Minimizing the cost of locomotion with inclined trunk predicts crouched leg kinematics of small birds at realistic levels of elastic recoil.
abstract: Small birds move with pronograde trunk orientation and crouched legs. Although the pronograde trunk has been suggested to be beneficial for grounded running, the cause(s) of the specific leg kinematics are unknown. Here we show that three charadriiform bird species (northern lapwing, oystercatcher, and avocet; great examples of closely related species that differ remarkably in their hind limb design) move their leg segments during stance in a way that minimizes the cost of locomotion. We imposed measured trunk motions and ground reaction forces on a kinematic model of the birds. The model was used to search for leg configurations that minimize leg work that accounts for two factors: elastic recoil in the intertarsal joint, and cheaper negative muscle work relative to positive muscle work. A physiological level of elasticity (∼ 0.6) yielded segment motions that match the experimental data best, with a root mean square of angular deviations of ∼ 2.1 deg. This finding suggests that the exploitation of elastic recoil shapes the crouched leg kinematics of small birds under the constraint of pronograde trunk motion. Considering that an upright trunk and more extended legs likely decrease the cost of locomotion, our results imply that the cost of locomotion is a secondary movement criterion for small birds. Scaling arguments suggest that our approach may be utilized to provide new insights into the motion of extinct species such as dinosaurs.
authors.affiliation: Department of Motion Science, Institute of Sport Science, Friedrich-Schiller-University Jena, Jena 07749, Germany christian.rode@uni-jena.de.
authors.forename: Christian
authors.initials: C
authors.lastname: Rode
authors.affiliation: Department of Motion Science, Institute of Sport Science, Friedrich-Schiller-University Jena, Jena 07749, Germany.
authors.forename: Yefta
authors.initials: Y
authors.lastname: Sutedja
authors.affiliation: Institute of Systematic Zoology and Evolutionary Biology with Phyletic Museum, Friedrich-Schiller-University Jena, Jena 07743, Germany College for Life Sciences, Wissenschaftskolleg zu Berlin, Wallotstraße 19, Berlin 19143, Germany.
authors.forename: Brandon M
authors.initials: BM
authors.lastname: Kilbourne
authors.affiliation: Department of Motion Science, Institute of Sport Science, Friedrich-Schiller-University Jena, Jena 07749, Germany.
authors.forename: Reinhard
authors.initials: R
authors.lastname: Blickhan
authors.affiliation: Department of Motion Science, Institute of Sport Science, Friedrich-Schiller-University Jena, Jena 07749, Germany Institute of Systematic Zoology and Evolutionary Biology with Phyletic Museum, Friedrich-Schiller-University Jena, Jena 07743, Germany.
authors.forename: Emanuel
authors.initials: E
authors.lastname: Andrada
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena; D046091:Charadriiformes / Q000502:physiology*; D005260:Female; D005684:Gait; D006614:Hindlimb; D008124:Locomotion / Q000502:physiology*; D008297:Male; D008962:Models, Theoretical; D011187:Posture; D060726:Torso
keywords: Avian locomotion; Charadriiform birds; Kinetics; Leg work; Optimization