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First published online October 27, 2003

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Propulsive impulse as a covarying performance measure in the comparison of the kinematics of swimming and jumping in frogs

Sandra Nauwelaerts^* and Peter Aerts

Department of Biology, University of Antwerp (UIA), Universiteitsplein 1, B-2610 Wilrijk (Antwerpen), Belgium

View larger version (52K): [in a new window]   Fig. 1. The theoretical sense and orientation of the external forces for jumping and swimming frogs, indicated by green arrows. W, weight; GRF, ground reaction force; L, lift; WRF, water reaction force; D, drag. The red broken line shows the direction of motion. During jumping, the external forces exerted on the frog are at an angle to the direction of motion, whereas in swimming, the external forces are either parallel to or perpendicular to the direction of motion.

View larger version (70K): [in a new window]   Fig. 2. 3-D joint angles (Hip, Ankle, Knee) and the local coordinate system in a jumping frog. To compare the posture of the different segments, the coordinates of the digitized markers were transformed from a global coordinate system to a new relative coordinate system that moves (and rotates) with the animal. See text for a description. The 3-D projection angles in the XY (the coronal plane), the XZ (the sagittal plane) and the YZ plane (the transverse plane) were calculated from these new coordinates. The red lines represent the local coordinate system XYZ, which is also called the relative coordinate system, while the blue lines indicate the body segments defined by the digitalization points (the blue circles).

View larger version (12K): [in a new window]   Fig. 3. Differences in impulse between swimming and jumping. There is no relationship between the impulse of the propulsion force and the duration of the propulsive phase within a locomotor mode. The range of the duration of the propulsive phase is greater in swimming, but there is a considerable overlap in the duration of both modes. The impulse gained during jumping is greater, and there is a small overlap area with swimming impulse.

View larger version (54K): [in a new window]   Fig. 4. Surface plots of 3-D hip (A,B), knee (C,D), ankle (E,F) joint angle profiles against impulse during the propulsive phase in both jumping (A,C,E) and swimming (B,D,F). The angle axis is scaled between zero (fully flexed) and 180° (fully extended). Time is set to zero at the end of the propulsive phase, when maximal velocity is reached. An interactive effect with impulse becomes visible when the colour scheme does not follow the axes of the plot. The key indicates the colour codes for the angles (in degrees) on the Z-axis.

View larger version (16K): [in a new window]   Fig. 5. Mean kinematic profiles of hip (black), knee (red) and ankle (blue) joints during jumping and swimming. The thick lines represent the mean profile, the thin lines indicate ± S.D. The total time of each propulsive phase is set at 100%.

View larger version (12K): [in a new window]   Fig. 6. The first derivatives of the kinematic profiles show that the coordination between the two locomotor modes differs slightly. The colour codes are the same as for Fig. 5. The hip action is earlier in the movement during jumping. Although a proximo–distal succession of the joints is optimal during jumping, the timing of the knee and ankle action is similar. During swimming, all joints are synchronously active.

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