Proximal hindlimb flexion in the horse: effect on movement symmetry and implications for defining soundness.

REASONS FOR PERFORMING THE STUDY: Flexion tests are a common tool during the prepurchase and clinical lameness examination, yet studies quantifying the effect of flexion, apart from distal forelimb flexion in sound horses, are sparse.
OBJECTIVES: To investigate the effect of proximal hindlimb flexion on perceivable and measurable changes in movement symmetry in horses with objective movement symmetry falling within the margins of 'sound'.
METHODS: Thirteen horses, selected based on objective movement symmetry, were instrumented with inertial sensors on left and right tuber coxae and over the os sacrum. Vertical movement symmetry was quantified at trot before and after proximal hindlimb flexion, with a repeat of flexion after 5 min. Video recordings of the horses were assessed visually.
RESULTS: Proximal hindlimb flexion introduced additive changes in movement symmetry to the individual baseline movement, with a tendency towards smaller effects with increasing stride number. The main systematic effect was a decrease in upward movement of the os sacrum following mid-stance of the flexed limb and an increase in upward movement following mid-stance of the nonflexed limb, also manifesting in a 'hip hike' of the flexed limb; these findings reflected increased movement asymmetry following flexion. Depending on individual baseline asymmetry, flexion can also increase movement symmetry.
CONCLUSIONS: Proximal hindlimb flexion can exacerbate subtle asymmetry when performed carefully. Variation in measured symmetry following flexion within and between horses showed that the individual response to flexion is highly variable. POTENTIAL RELEVANCE: Proximal hindlimb flexion may elevate the asymmetry of a slightly lame limb above the threshold for visibility, thus assisting in the clinical gait examination. Further work is needed to examine the causes for a positive response to flexion and possible differences between sound and lame horses as well as horses of different athletic disciplines.