Unique musculoskeletal characteristics explain why emus and other birds must rely on a running style in which one foot is always touching the ground, according to a new investigation.
For most other bipeds, this “ground-running” style consumes more energy, presenting a long-standing paradox in the case of birds.
Using the emu, which cannot fly, as a model, the anatomical simulations showed that the leg muscles of birds are better suited for crouching, which prevents them from fully straightening their legs, as occurs with most other bipeds.
When humans run, for example, there is a moment in each stride when neither foot touches the ground. Birds, however, always have one foot on the ground when they run.
To resolve this apparent paradox, Pasha van Bijlert and colleagues asked: is there something biomechanically specific to birds that makes ground-running the best option? They performed detailed simulations of the emu’s gait, examining energy expenditure and muscular movements.
In doing so, they were able to study the dynamics of tendon elasticity and the independent postures of the limbs during ground-running — something that cannot be studied in live birds.
They found that the emu’s body plan prevents birds from fully straightening their legs. Instead, the emu’s muscles work best in crouched positions, which predispose the birds to ground-running. Emus and all birds still share certain morphologies with their ancestors, the non-avian theropod dinosaurs.
Thus, van Bijlert et al. suggest that these dinosaurs may have been ground runners before the evolution of birds. “The fossil footprints of dinosaurs demonstrate a smooth distribution of stride lengths, which provides an independent line of evidence for ground-running in non-avian theropods,” they note.
