Data from: Trunk orientation causes asymmetries in leg function in small bird terrestrial locomotion

In contrast to the upright trunk in humans, trunk orientation in most
birds is almost horizontal (pronograde). It is conceivable that the
orientation of the heavy trunk strongly influences the dynamics of bipedal
terrestrial locomotion. Here we analyze for the first time the effects of
a pronograde trunk orientation on leg function and stability during
bipedal locomotion. For this, we first inferred the leg function and trunk
control strategy applied by a generalized small bird during terrestrial
locomotion by analyzing synchronously recorded kinematic (3D x-ray
videography) and kinetic (3D force measurement) quail locomotion data.
Then, by simulating quail gaits using a simplistic bio-inspired numerical
model which made use of parameters obtained in in vivo experiments with
real quail, we show that the observed asymmetric leg function (reflected
in left-skewed ground reaction force and touchdown lift-off limb
asymmetry) is necessary for pronograde steady-state locomotion. In
addition, steady-state locomotion becomes stable for specific
morphological parameters. For quail-like parameters, the most common
stable solution is grounded running, a gait preferred by the quail and
most of the other small birds. We hypothesize that stability of bipedal
locomotion is a functional demand that, depending on trunk orientation,
constrains basic hind limb morphology and function.