First published online November 10, 2003
Mechanics of wing-assisted incline running (WAIR)
Matthew W. Bundle* and
Kenneth P. Dial
Flight Laboratory, Division of Biological Sciences, The University of
Montana, Missoula, MT, 59812, USA
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Fig. 1. The relationship between the frames of reference of the instruments during
the experiments (A, level; B, inclined; C, vertical) and the global frame of
reference. The force plate is sensitive in the normal plane (perpendicular to
the plate) and in the parallel plane (plane of motion). The accelerometers are
sensitive to acceleration in the dorso-ventral (D-V) plane and the
anterior-posterior (A-P) plane, relative to the bird's back. During inclined
WAIR (B), weight support is no longer measured from a single axis. Both during
a single wing beat and across different inclines, the changing orientation of
the bird's back requires that the accelerometers be analyzed from within the
bird's frame of reference (i.e. the A-P and D-V planes).
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Fig. 2. Representative and simultaneous acceleration and force-plate data, from a
chukar (mass 683.7 g) engaged in wing-assisted incline running, on a 70°
incline. The D-V and A-P accelerometer signals have been conditioned and
calibrated in multiples of the acceleration due to gravity
(g). The force-plate ground reaction signals (N) have also
been conditioned and converted.
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Fig. 3. The average instantaneous acceleration of the center of mass measured
during WAIR (A-F) and during free flight at a similar angle (G-L). Red arrows
represent the magnitude and direction of the instantaneous acceleration,
calculated as the resultant vector from the A-P and D-V component
accelerometers. The orientation of the acceleration vector during the late
stages of downstroke (C,D and I,J) differs between WAIR and free flight. We
propose the acceleration during WAIR acts to push the animal against the
substrate in order for the legs to contribute a portion of the forces required
for movement. The black arrows in A and G denote the cable connecting the
accelerometers to the recording equipment.
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Fig. 4. During ascents (body postures shown above), both the normal (squares) and
parallel (circles) components of the ground reaction force (GRF) show large
increases over level walking values (A). The relative importance of the
parallel axes to the peak GRF increases with the angle of ascent (B). The high
forces in the normal plane during WAIR are consistent with the development of
high frictional forces, which permit hindlimb propulsion. The magnitude of the
GRF decreases during climbs when the wings are used to assist locomotion,
generally angles greater than 60°. In B the sum of the percentage of peak
GRF for each component does not equal 100%; rather vector addition requires
the squares of the components to sum to the square of the GRF.
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Fig. 5. The magnitude and orientation (length and direction of the blue arrows) of
the peak ground reaction force (GRF) from representative runs of different
inclination. (A) Peak GRF during fast walking on the level. During this trial
the animal was decelerating slightly, and not maintaining a steady velocity.
(B-E) Peak GRF for the illustrated runs at 60°, 70°, 80° and
90°, respectively. During wing-assisted running (C-E) the magnitude of the
GRF is less than at 60°, where wings are generally not used. Despite the
pattern shown between wing position and the instance of peak GRF shown in C-E,
during sustained bouts of WAIR on an inclined treadmill we found no
relationship between footfall and wing position. Mb, body
weight.
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Fig. 6. The calculated fraction of vertical mechanical work done on the center of
mass (COM) during locomotion across different inclines by the legs (A) and
wings (B). At 60° chukars rarely use their wings, and the magnitude of the
vertical component of the ground reaction force is sufficient to provide the
motion in this plane. Over steeper inclines the vertical component of the GRF
is no longer sufficient to provide the energy required of vertical motion,
thus the difference must be attributed to the wings.
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© The Company of Biologists Ltd 2003
