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First published online December 22, 2003
Journal of Experimental Biology 207, 399-410 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00761

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Walking and running in the red-legged running frog, Kassina maculata

A. N. Ahn^*, E. Furrow and A. A. Biewener

Concord Field Station, MCZ, Harvard University, Old Causeway Road, Bedford, MA 01730, USA

View larger version (63K): [in a new window]   Fig. 1. Slow (A) and fast (B) locomotion in Kassina. The numbers represent time (s) for both columns of video images. The vertical columns of images show a typical slower speed (0.12 m s-1) trial and a typical faster speed (0.21 m s-1) trial of an individual. The widths of all frames are identical. Each column shows one complete stride, beginning and ending with the right forelimb contacting the ground. A mirror placed at an angle shows a simultaneous dorsal view to allow visibility of all four limbs simultaneously. The asterisk (*) and plus (+) symbols represent corresponding stages of the slow and fast strides. Supplementary movies are available on-line.

View larger version (12K): [in a new window]   Fig. 2. Speed range observed in the trials used for the present study. The points represent the speeds measured as the frogs moved across the force platform. The shaded region represents the range of speeds observed when the animals ran on a treadmill. Only faster runs were accepted for the treadmill trials. Therefore, the shaded region indicates the upper range of the steady-state speeds attained by the animals in the laboratory.

View larger version (16K): [in a new window]   Fig. 3. Stride frequency and stride length as a function of speed. Both stride frequency (open symbols) and stride length (filled symbols) increased with speed, but to different degrees. The animals primarily moved faster by increasing stride frequency.

View larger version (12K): [in a new window]   Fig. 4. Duty factor versus forward speed. As the animals moved faster, the duty factor, or the portion of the stride cycle during which a single limb was on the ground, decreased but was never less than 0.5.

View larger version (13K): [in a new window]   Fig. 5. Footfall patterns as a function of speed. Both diagonal limb (DL; squares) phase and same-side limb (SSL; diamonds) phase remained constant or varied little with speed.

View larger version (25K): [in a new window]   Fig. 6. Representative ground forces, center of mass (COM) velocites, and COM mechanical energies during a `mechanical walk' (A) and a `mechanical run' (B). For each trial, a single complete stride is shown. For the walking trial, animal weight=0.083 N; speed=0.10 m s-1; stride frequency=2.2 Hz; phase shift=147°; recovery=24.3%; duty factor=0.74; same-side limb (SSL) phase=36.2%; diagonal limb (DL) phase=13.0%; congruity=39.9%; external mechanical power=0.11 W kg-1. For the running trial, animal weight=0.085 N; horizontal velocity=0.19 m s-1; stride frequency=3.0 Hz; phase shift=3.4°; recovery=14.5%; duty factor=0.73; SSL phase=38.1%; DL phase=14.3%; congruity=61.1%; external mechanical power=0.21 W kg-1. Ek, horizontal kinetic energy; Ep, gravitational potential energy.

View larger version (12K): [in a new window]   Fig. 7. Phase shift (A), %Recovery (B) and %Congruity (C) as a function of forward speed. Walking (open circles) was considered to occur when the phase shift between horizontal kinetic energy (Ek) and gravitational potential energy (Ep) of the center of mass (COM) was out-of-phase or near 180°; running (filled circles) was considered to occur when the phase shift between the Ek and Ep of the COM was in-phase or near 0°. Those trials during which the phase shift was neither in- nor out-of-phase were categorized into an intermediate gait (crosses). (B) %Recovery generally decreased with speed. (C) %Congruity increased with forward speed and generally showed a similar separation as %Recovery with respect to walking and running mechanical trials.

View larger version (15K): [in a new window]   Fig. 8. Representative ground forces, center of mass (COM) velocities, and COM mechanical energies during a gait that combined vaulting and bouncing mechanics. Similar to a mammalian gallop, Kassina often used a gait that vaulted for half of the stride and bounced for half of the stride, resulting in an intermediate %Congruity. A single complete stride is shown, for which animal weight=0.094 N, horizontal velocity=0.13 m s-1, stride frequency=2.1 Hz, phase shift=114°, recovery=23.1%, duty factor=0.72, same-side limb (SSL) phase=33.9%, diagonal limb (DL) phase=11.9%, congruity=50.5%, and external mechanical power=0.13 W kg-1. Ek, horizontal kinetic energy; Ep, gravitational potential energy.

View larger version (26K): [in a new window]   Fig. 9. Hildebrand plot. Using solely kinematics, the footfall patterns indicate that Kassina only walked at all speeds. Duty cycle always exceeded 50%. Adapted from Hildebrand (1985).

View larger version (11K): [in a new window]   Fig. 10. Mechanical determinants of walking and running. For the present study, phase shifts near 180° (greater than 135°) defined walking (open circles), phase shifts between 45° and 135° defined the intermediate gait (crosses), and phase shifts near 0° (less than 45°) defined running (filled circles). The two variables generally agreed (upper left and lower right quadrants), as demonstrated by their inverse relationship.

View larger version (12K): [in a new window]   Fig. 11. Mass-specific external mechanical power required to lift and accelerate the center of mass as a function of forward speed. Mechanical power increased with speed during terrestrial locomotion in Kassina (walking- open circles; intermediate gait- crosses; running- filled circles; power=1.5v-0.05; r2=0.45; P<0.0001; solid lines represent 95% confidence intervals). The slope of the linear regression (broken line) represents the mechanical work required to move the center of mass (COM) by 1 m (1.5 J kg-1 m-1).

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