Comparison of kinematic symmetry index calculations and the effects of straight and circular trotting A. M. WALKER*, A. M. WILSON † and T. PFAU ‡ Structure and Motion Laboratory; † Department of Veterinary Basic Sciences; and ‡ Department of Veterinary Clinical Sciences; Royal Veterinary College, Hatfield, UK. Keywords: horse; locomotion; kinematics; symmetry; circle; trot Summary Reasons for performing study: When assessing lameness in horses, left to right ratios of kinematic parameters are often used to quantify movement symmetry. Different methods of symmetry related measures have been proposed and inertial sensor data was used to evaluate the application of 3 methods of symmetry calculation during straight and circular trotting. Objectives: To compare 3 sensor based methods of symmetry index calculation to assess; tuber coxae vs. sacrum motion, the effects of circular trotting and effect of using whole trials in place of individual stride calculations. Methods: Inertial sensors were attached to the sacrum, left and right tuber coxae (LTC/RTC) of 21 non-lame horses. Straight and circular trotting data were collected. Symmetry indices based on vertical movement were calculated for each stride using 3 previously published methods. Results: Method 2 (Fourier analysis) had significantly higher ratio values than method 1 (displacement amplitude ratio; tuber coxae) and methods 1 (displacement amplitude ratio; sacrum) and 3 (difference between LTC/RTC displacement). The effect of circular trotting within methods was highly variable, but was not significantly different between methods. No significant differences were found between whole trial and individual stride calculations. Method 2 when compared to method 1 underestimates the asymmetry present in a non- uniform way due to the use of squared amplitudes. Methods 2 and 3 become less accurate during circular trotting due to changes in amplitude timing. Conclusions: On the same data substantial differences in symmetry indices are found when using different methods and locations. Further differences exist in accuracy when used on circular data. Interpretation and direct comparison of different symmetry indices should be approached with caution. Introduction Unilateral lameness is characterised by asymmetry during the left and right stance phases of a stride. Many methods have been developed to enable objective analysis of kinematic asymmetry (Buchner et al. 1996; Peham et al. 1996, 1999; Uhlir et al. 1997; Keegan et al. 2001; Kramer et al. 2004). Quantitative analysis of lameness usually involves the calculation of symmetry ratios. A symmetry ratio refers to the calculation of a ratio aimed at describing parameter asymmetry between the left and right phases of a stride. Kinematic studies have calculated symmetry ratios to enable asymmetry to be quantified numerically (Peham et al. 1996; Pourcelot et al. 1997; Keegan et al. 2001, 2004; Audigie et al. 2002; Church et al. 2009). Trotting is a symmetrical gait (Hildebrand 1965) with each of the diagonal pairs being dynamically coupled. Most methods of calculating kinematic symmetry ratios produce a ratio of 1 (100% symmetry) if the parameter under investigation is symmetrical (Pourcelot et al. 1997; Audigie et al. 2002; Church et al. 2009). If the ratio is ‘non-directional’ then an increase in asymmetry corresponds to a reduction in ratio towards zero regardless of the side of the lameness. Three published methods used for analysis of kinematic symmetry utilise different parameters or use a slightly different approach while maintaining similar overall principles (i.e. quantifying left/right symmetry). To date, no comparison between these methods has been performed; however, many studies utilise only one of the methods to define changes in symmetry during investigations, therefore limiting comparisons between studies. The aim of this study was to quantify differences between 3 published methods of sensor-based kinematic symmetry ratio calculations. This was approached with 3 objectives: 1) Compare 3 methods of ratio calculation to assess the use of tuber coxae against os sacrum displacement data for calculation of symmetry ratios; 2) determine the effects of circular trotting on symmetry ratios and 3) investigate the use of whole trials in place of individual stride calculations. Hypotheses We hypothesise that significant differences exist between symmetry ratios calculated using each method, that ratios calculated using the os sacrum differ significantly from ratios calculated using tuber coxae. We also hypothesise that no significant differences exist between symmetry indices calculated using individual strides or whole trials and that changes in symmetry ratio with circular trotting differ between methods (with circular trotting causing a reduction in symmetry ratios calculated using each method). *Corresponding author email: amwalker@rvc.ac.uk. [Paper received for publication 09.01.10; Accepted 02.07.10] © 2010 EVJ Ltd 482 EQUINE VETERINARY JOURNAL Equine vet. J. (2010) 42 (Suppl. 38) 482-487 doi: 10.1111/j.2042-3306.2010.00195.x