Quantifying head and withers movement asymmetry in sound and naturally forelimb lame horses trotting on a circle on hard and soft surfaces.

Background: Reliable lameness thresholds for vertical head displacement for trotting on a circular path and on different surfaces have yet to be defined. Withers movement in lame horses could help improve the diagnostic accuracy of sensor technology.

Objectives: To define head movement lameness threshold ranges and describe the relationship between head and withers movement during trotting under different circumstances in sound and forelimb lame horses.

Study design: Retrospective analysis of clinical data and comparison with control group.

Methods: Fifty-five sound and thirty-four naturally lame horses were trotted under field conditions on a hard surface on a straight line and on both directions on soft and hard surfaces. Movement asymmetry was analysed by measuring differences in displacement minima for head and withers using an inertial measurement unit sensor-based gait analysis system. Receiver operating characteristic (ROC) analysis was performed to define threshold ranges. Relationships between head and withers data were identified by calculating the correlation with linear regression analysis.

Results: Estimated lameness threshold values for vertical head movement asymmetry ranged from 11.5 to 12.5 mm. On both surfaces, a head movement asymmetry threshold range from 24.5 to 26.5 mm could be found for the circle. Lame horses showed significant correlations between head and withers movement on hard ground straight line (R² 0.714, p < 0.001) and inner leg lame horses on the lunge (R² 0.915, p < 0.001).

Main limitations: Different sources of lameness were not distinguished. Radius and speed during lungeing could not be perfectly controlled. Inclusion in the sound group was based on a subjective examination by one clinician and in the lame group two clinicians subjectively determined inner and outer leg lameness creating an extra amount of uncertainty.

Conclusions: Lameness thresholds for quantifying head movement asymmetry on the lunge were proposed. A significant correlation was observed between head and withers movement asymmetry under several conditions.