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Bipedal bird range of limb motion: long-axis rotation

From: Ben Creisler

A recent paper that may be of interest and not yet mentioned:

Robert E. Kambic, Thomas J. Roberts and Stephen M. Gatesy (2014)
Long-axis rotation: a missing degree of freedom in avian bipedal locomotion.
Journal of Experimental Biology (advance online publication)
doi: 10.1242/jeb.101428

Ground-dwelling birds are typically characterized as erect bipeds
having hind limbs that operate parasagittally. Consequently, most
previous research has emphasized flexion/extension angles and moments
as calculated from a lateral perspective. Three-dimensional motion
analyses have documented non-planar limb movements, but the skeletal
kinematics underlying changes in foot orientation and transverse
position remain unclear. In particular, long-axis rotation of the
proximal limb segments is extremely difficult to measure with topical
markers. Here we present six degree of freedom skeletal kinematic data
from maneuvering guineafowl acquired by marker-based XROMM (X-ray
Reconstruction of Moving Morphology). Translations and rotations of
the hips, knees, ankles, and pelvis were derived from animated bone
models using explicit joint coordinate systems. We distinguished
sidesteps, sidestep yaws, crossover yaws, sidestep turns, and
crossover turns, but birds often performed a sequence of blended
partial maneuvers. Long-axis rotation of the femur (up to 38°)
modulated the foot's transverse position. Long-axis rotation of the
tibiotarsus (up to 65°) also affected medio-lateral positioning, but
primarily served to either reorient a swing phase foot or yaw the body
about a stance phase foot. Tarsometatarsal long-axis rotation was
minimal, as was hip, knee, and ankle abduction/adduction. Despite
having superficially hinge-like joints, birds coordinate substantial
long-axis rotations of the hips and knees to execute complex 3-D
maneuvers while striking a diversity of non-planar poses.