Despite the tremendous range of size and shape, terrestrial animals engaged in straight-line running can be modelled successfully. "Using simple mathematical models such as the SLIP model to describe movement allows for design and function to be understood in terms of overall mechanical task constraints." Add to this manoeuvring, and the story gets much more complicated. Stops, starts and changes in direction are much more of a challenge.
Past work in this area has involved cockroaches and humans. Newly reported research concerns ostriches. At first glance, these large birds might seem unlikely runners, but they are actually capable of taking on racehorses and certainly outmanoeuvring predators. Compared with humans, the centre of mass is higher, they have long spindly legs and long necks, and their heads are small. Not only can they run much faster than humans, they look remarkably graceful, even when changing direction. They do run turn differently: "When humans execute 30 degreees sidestep and crossover cuts, braking forces are 26% of Fpmax compared to 6-11% for ostriches executing 15-20 degree turns. Moreover, whereas humans generated almost exclusively braking forces during sidesteps and crossovers, 40% of the net forces observed during turns for ostriches were acceleratory." How do they achieve this remarkable performance?
"Most of the differences between ostriches and humans were explained by differences in body morphology. Ostrich morphology is appropriate for effective maneuvers that require minimal acceleratory or braking forces." "These results suggest that, with an appropriately designed morphological system, maneuvers can be executed with minimal changes to running dynamics." "In summary, ostrich morphology is appropriate for maneuvering without requiring large braking or acceleratory forces."
What stands out in these studies is the effectiveness of design thinking. Some will attribute this design to the powers of natural selection acting on genetic variations, with little or no direct evidence to support this hypothesis. However, there is another alternative, which is to be open to the possibility of this design being real, involving intelligent (rather than natural) agency. Ostriches have traditionally been considered the epitome of foolish behaviour, supposedly burying their heads in the sand. However, their running skills are outstanding and demonstrate superb design. One wonders whether heads are being buried in the sand when design inferences are excluded on ideological grounds from science.
Mechanics of cutting maneuvers by ostriches (Struthio camelus)
Devin L. Jindrich, Nicola C. Smith, Karin Jespers, and Alan M. Wilson
Journal of Experimental Biology, 2007 210: 1378-1390.
Abstract: We studied the strategies used by cursorial bipeds (ostriches) to maneuver during running. Eight ostriches were induced to run along a trackway and execute turns. Ground reaction forces and three-dimensional kinematics of the body and leg joints were simultaneously recorded, allowing calculation of joint angles and quasi-static net joint torques. Sidesteps, where the leg on the outside of the turn changes the movement direction, and crossovers using the inside leg, occurred with nearly equal frequency. Ostriches executed maneuvers using a simple control strategy that required minimal changes to leg kinematics or net torque production at individual joints. Although ostriches did use acceleration or braking forces to control body rotation, their morphology allowed for both crossovers and sidesteps to be accomplished with minimal net acceleratory/braking force production. Moreover, body roll and ab/adduction of the leg shifted the foot position away from the turn direction, reducing the acceleratory/braking forces required to prevent under- or over-rotation and aligning the leg with the ground reaction force.
Clare, S. BUILT TO RUN, Journal of Experimental Biology, 210, 0i (March 31, 2007)
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