# Sprawling locomotion in the lizard Sceloporus clarkii: the effects of speed on gait, hindlimb kinematics, and axial bending during walking > How does a sprawling lizard speed up? Filming lizards on a treadmill at three walking speeds, researchers found they kept the same trotting gait, the same leg angles and the same wave of body bending, and went faster mainly by pulling the thigh back more quickly. Their strides grew from 80 to 143 mm while their legs stayed just as sprawled. Summary by Kersey Lawrence, Original Wisdom. Updated 9 November 2026. Page: https://www.originalwisdom.com/tracking-literature/1997-reilly-delancey-sprawling-locomotion-lizard-sceloporus/ ## Citation and access - **Authors:** S. M. Reilly and M. J. DeLancey - **Year published:** 1997 - **Journal or source:** Journal of Zoology, London - **Full citation:** Reilly, S. M. and DeLancey, M. J. 1997. Sprawling locomotion in the lizard Sceloporus clarkii: the effects of speed on gait, hindlimb kinematics, and axial bending during walking. Journal of Zoology, London 243(2): 417–433. - **DOI:** 10.1111/j.1469-7998.1997.tb02791.x - **Access status:** Publisher link only - **Where to find it:** https://doi.org/10.1111/j.1469-7998.1997.tb02791.x ## Tags - **Species:** Reptiles > Clark’s spiny lizard - **Region:** North America > United States - **Behaviour and ecology:** Behaviour; Movement - **Methods and technology:** Descriptive statistics; Linear regression - **Data type:** New field data - **Observer qualification:** Not needed - **Discipline:** Behaviour and ecology - **Year published:** 1997 ## Detailed keywords - **Species:** Studied: the lizard Sceloporus clarkii (the paper gives no common name), three adults with snout-vent lengths (SVL) of 90, 91 and 92 mm. Compared from the literature: the gecko Teratoscincus scincus, the salamander Dicamptodon tenebrosus, the American alligator (Alligator mississippiensis), the vervet monkey (Cercopithecus aethiops), a marsupial (Monodelphis domestica), cats and dogs. - **Location and habitat:** Lizards collected in Molino Basin, Santa Catalina Mountains, 10 miles north of Tucson, Arizona, United States. Filmed in the laboratory at Ohio University, Athens, Ohio, on a 70 cm canvas treadmill; housed at 25 °C, body temperature during runs 27 to 30 °C. - **Topics:** sprawling locomotion; gait (walking trot); speed effects; hindlimb kinematics (hip, knee, ankle); femoral retraction; knee flexion; plantar flexion; limb adduction and posture (sprawling versus erect); pelvic rotation; axial bending as a standing wave with nodes at the girdles; stride length and stride frequency; duty factor (proportion of the stride a foot is on the ground); caudifemoralis muscle; ecomorphology (how body shape relates to performance); evolution of bipedalism and erect posture. - **Statistics or models:** Linear regression of individual means against speed (angular variables, timing variables in real time and scaled to stride duration, axial displacements); means with standard errors or standard deviations; Hildebrand’s gait model. Inconsistencies in the paper: (1) the text says axial data are in Fig. 3, but the axial bending figure is Fig. 4 and Fig. 3 is the gait diagram; (2) Table V’s column heading reads “Toe (lateral view)” while its caption and the methods say the toe was measured in dorsal view; (3) the appendix defines AMAXEXT as the “minimum ankle angle just prior to foot down”, while the methods call it the maximal extension of the ankle; (4) the text says the time to maximum knee flexion fell from about 32% to 13% of the stride, while Table II gives 0.32 to 0.11 and 0.15; (5) the significance level is given as P = 0.01 for angular variables and 0.02 for scaled timing variables, and the real-time timing results are reported at P ≤ 0.036; (6) the methods call HRETDUR “hip protraction duration” (written “THRETDUR”), while the appendix and results treat it as hip retraction duration. - **Data type:** New field data (laboratory high-speed video of wild-caught lizards). Dates of data collection not given. ## Synopsis ### 1. What they did Lizards are thought to push themselves forward mainly with their hind legs, but until this study nobody had measured how the hind leg moves over several strides by the same animal, or looked at leg and body movements together. The authors asked how a lizard with a sprawling posture (legs held out to the side) changes its leg and body movements to go faster. They used Sceloporus clarkii, a lizard thought to move in a generalized sprawling way and with a body shape typical of its family (Phrynosomatidae). - Filming. Lizards collected near Tucson, Arizona, were filmed on a treadmill with high-speed video (200 fields per second), from the side and, with mirrors, from above. Painted dots marked the backbone, hip, knee, ankle and the tip of the longest (fourth) toe. - Speeds. Three preset speeds covered a threefold range, from about 1 to 3 km/h (0.27, 0.48 and 0.83 m per second). They used 22 strides of the right hind leg in which the lizard exactly matched the treadmill speed: 2 strides from one lizard at the slowest speed and 5 strides each from two other lizards at the two faster speeds. - Measurements. From the video, they measured angles of the pelvis, hip, knee and ankle, the timing of each movement within the stride, the timing of all four footfalls (gait), how far the knee and toe swung sideways (a sign of how sprawled the leg was), and the side-to-side bending of the body from snout to tail. - Analysis. They tested which measurements changed with speed using regression. ### 2. What they found Gait - At all three speeds the lizards used a trot: diagonally opposite front and hind feet moved together. Their gait went from a moderate walking trot to a fast walking trot, and at the fastest speed was close to becoming a run. - From the slowest to the fastest speed, stride, stance and swing durations fell by about 40%. Stride frequency rose from 3.4 to 5.8 strides per second, and stride length from 80 to 143 mm. - The feet spent a smaller share of each stride on the ground as speed rose (hind feet 63.9% to 52.4%; front feet 60.7% to 47.9%), and the hind feet were always on the ground slightly longer than the front feet. Leg movements - None of the joint angles or ranges of movement changed with speed. - In real time, 13 of 17 timing measures got faster as speed increased, as expected. - When timing was scaled to stride length, only three measures changed: the femur (thigh bone) was pulled back faster, reaching full retraction earlier (from about 64% to 36% of the stride, about 44% faster), and the knee bent to its minimum sooner during the stance. - The ankle movements that push the foot off the ground did not change with speed. - The sideways swing of the knee and toe did not change, so the lizards kept the same sprawling posture and did not stand more upright when going faster. Pelvis and body bending - Maximum pelvic rotation stayed between 16 and 25 degrees at all speeds and peaked just before the foot lifted. - The body bent in a standing wave with two still points (nodes): one at the shoulder girdle and one just in front of the pelvis. These did not shift with speed. - Only the snout’s side-to-side movement changed, decreasing as speed increased (from 20.8 to 11.0 mm), so the head was held more steadily forward. ### 3. What they concluded - Over this range of walking speeds, the lizards went faster almost entirely by pulling the thigh back faster, while everything else stayed the same. - A similar pattern in a salamander suggests this may be a general way that sprawling animals speed up at walking speeds. - Of three ideas about how lizards run faster (longer strides through more body bending, switching to a travelling wave of body bending, or more force from the legs), their results support only the third, and point to femoral retraction as the main mechanism. - This supports the idea that leg length, especially femur length, and the size of the caudifemoralis muscle (the main muscle that pulls the thigh back) matter for lizard running performance, and that this muscle was important in the evolution of walking on land and of walking on two legs. - The speeds studied were only about 14% to 44% of the lizards’ maximum sprint speed. At higher speeds, body bending, stride length, push-off force or posture may change too, and this still needs study. - Sprawling and upright walking may work more alike than has been thought. ### 6. Other technologies, skills or innovations - NAC HSV-400 high-speed video system at 200 fields per second, under strobe lights, with mirrors to film side and top views at once. - Motorized canvas treadmill with a background marked every 10 cm. - Measurement TV software for digitizing the painted landmarks frame by frame. ### 8. Implications for tracking (Original Wisdom's observations) _These are Original Wisdom's observations, not the authors'._ - For a lizard walking with a trot, faster movement meant longer strides (80 to 143 mm) without any change in how sprawled the legs were, so the sideways spread of the trail may stay about the same at walking speeds while stride length grows. ## Cite this summary Lawrence, K. 2026. Summary of: Sprawling locomotion in the lizard Sceloporus clarkii: the effects of speed on gait, hindlimb kinematics, and axial bending during walking (Reilly and DeLancey, 1997). Original Wisdom, Tracking Literature. https://www.originalwisdom.com/tracking-literature/1997-reilly-delancey-sprawling-locomotion-lizard-sceloporus/