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.