1890 AJVR, Vol 73, No. 12, December 2012 M ovement of terrestrial animals in a natural en- vironment often deviates from the steady-state, straight-line, hard-surface condition that can most eas- ily be assessed in a gait evaluation laboratory, with or without a treadmill. For example, to navigate obstacles or avoid being caught by a predator, animals often change movement direction, which requires increasing mediolateral forces for turning. a During locomotion on a circular path, the centripetal force is directed toward the center of the circle, and horses lean inward. 1,2 If an animal’s posture were to remain the same during lung- Effect of trotting speed and circle radius on movement symmetry in horses during lunging on a soft surface Thilo Pfau, Dr.-Ing; Narelle C. Stubbs, PhD; LeeAnn J. Kaiser, MS; Lucy E. A. Brown, BVetMed; Hilary M. Clayton, BVMS, PhD Objective—To determine whether body lean angle could be predicted from circle radius and speed in horses during lunging and whether an increase in that angle would decrease the degree of movement symmetry (MS). Animals—11 medium- to high-level dressage horses in competition training. Procedures—Body lean angle, head MS, and trunk MS were quantified during trotting while horses were instrumented with a 5-sensor global positioning system–enhanced in- ertial sensor system and lunged on a soft surface. Speed and circle radius were varied and used to calculate predicted body lean angle. Agreement between observed and predicted values was assessed, and the association between lean angle and MS was determined via least squares linear regression. Results—162 trials totaling 3,368 strides (mean, 21 strides/trial) representing trotting speeds of 1.5 to 4.7 m/s and circle radii of 1.8 to 11.2 m were conducted in both lunging directions. Differences between observed and predicted lean angles were small (mean ± SD difference, –1.2 ± 2.4°) but significantly greater for circling to the right versus left. Move- ment symmetry values had a larger spread for the head than for the pelvis, and values of all but 1 MS variable changed with body lean angle. Conclusions and Clinical Relevance—Body lean angle agreed well with predictions from gravitational and centripetal forces, but differences observed between lunging directions emphasize the need to investigate other factors that might influence this variable. For a fair comparison of MS between directions, body lean angle needs to be controlled for or cor- rected with the regression equations. Whether the regression equations need to be adapt- ed for lame horses requires additional investigation. (Am J Vet Res 2012;73:1890–1899) ing as during straight-line movement, an increase in ground reaction force moment arms would be observed with respect to the limb joints. Thus, it is not surprising that heavy animals, like horses, adapt their posture to Received January 12, 2012. Accepted March 28, 2012. From the Department of Veterinary Clinical Sciences, Royal Veterinary College, University of London, North Mymms, Hatfield, AL9 7TA, Hertfordshire, England (Pfau, Brown); and Mary Anne McPhail Equine Performance Center, Depart- ment of Large Animal Clinical Sciences, Veterinary Medical Cen- ter, College of Veterinary Medicine, Michigan State University, East Lansing, MI 48824 (Stubbs, Kaiser, Clayton). Dr. Brown was a fourth-year veterinary student at the time of manuscript submission. Supported by the McPhail endowment. Address correspondence to Dr. Pfau (tpfau@rvc.ac.uk). ABBREVIATIONS CI Confidence interval Δ obs,pred Difference between the absolute values of the observed and predicted body lean angles GPS Global positioning system HipHikeDiff Difference in vertical upward movement between left and right tuber coxae IMU Inertial measurement unit LTC Left tuber coxae MaxDiff Difference between first and second maximum values in vertical displacement MinDiff Difference between first and second minimum values in vertical displacement RTC Right tuber coxae