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Effects of speed on the hindlimb kinematics of the lizard dipsosaurus dorsalis

Lizards are the most diverse extant lineage of vertebrates believed to have retained the sprawling limb posture of ancestral tetrapods and, although the sprinting speeds of lizards are well documented, quantitative analyses of limb movements are lacking. We therefore examined a wide range of steady running speeds to quantify variation in the kinematics of a morphologically generalized hindlimb in the lizard Dipsosaurus dorsalis. From speeds of 50 to 350 cm s-1, stride length, step length and stride frequency all had significant curvilinear increases from 13.3 to 39.4 cm, from 8.9 to 12.8 cm and from 3.9 to 8.6 Hz, respectively, whereas duty factor (percentage of cycle when foot is on ground) decreased from 63 to 34 % with increased speed. For five standardized speeds ranging from 50 to 250 cm s-1, analyses of variance detected highly significant (P<0.001) effects of speed on 27 out of a total of 46 kinematic variables. The height of the hip at the time of footfall increased significantly with increased speed, whereas the amount of pelvic rotation in the horizontal plane (mean 36 degrees ) per stride was unaffected by speed. The orientation of the long axis of the foot changed by nearly 50 degrees with increased speed such that the foot was nearly straight forward at 250 cm s-1, and at high speeds the heel often did not touch the ground. The effective length of the limb at footfall nearly doubled with increased speed as a combined result of increased plantar flexion and knee extension and a more erect femur. The pervasive effects of speed on hindlimb kinematics observed for this species do not conform simply with previous generalizations of lizards as having plantigrade foot posture and sprawling limbs.